TOP 200+ JAVA Interview Questions And Answers

JAVA Interview Questions: More than 200 JAVA Interview Questions with detailed answers for freshers and experienced persons:

What is Java?

Java is the high-level, object-oriented, robust, secure programming language, platform-independent, high performance, Multithreaded, and portable programming language. It was developed by James Gosling in June 1991. It can also be known as the platform as it provides its own JRE and API.

What are the differences between C++ and Java?

The differences between C++ and Java are given in the following table.

Comparison Index	C++	Java
Platform-independent	C++ is platform-dependent.	Java is platform-independent.
Mainly used for	C++ is mainly used for system programming.	Java is mainly used for application programming. It is widely used in window, web-based, enterprise and mobile applications.
Design Goal	C++ was designed for systems and applications programming. It was an extension of C programming language.	Java was designed and created as an interpreter for printing systems but later extended as a support network computing. It was designed with a goal of being easy to use and accessible to a broader audience.
Goto	C++ supports the goto statement.	Java doesn’t support the goto statement.
Multiple inheritance	C++ supports multiple inheritance.	Java doesn’t support multiple inheritance through class. It can be achieved by interfaces in java.
Operator Overloading	C++ supports operator overloading.	Java doesn’t support operator overloading.
Pointers	C++ supports pointers. You can write pointer program in C++.	Java supports pointer internally. However, you can’t write the pointer program in java. It means java has restricted pointer support in Java.
Compiler and Interpreter	C++ uses compiler only. C++ is compiled and run using the compiler which converts source code into machine code so, C++ is platform dependent.	Java uses compiler and interpreter both. Java source code is converted into bytecode at compilation time. The interpreter executes this bytecode at runtime and produces output. Java is interpreted that is why it is platform independent.
Call by Value and Call by reference	C++ supports both call by value and call by reference.	Java supports call by value only. There is no call by reference in java.
Structure and Union	C++ supports structures and unions.	Java doesn’t support structures and unions.
Thread Support	C++ doesn’t have built-in support for threads. It relies on third-party libraries for thread support.	Java has built-in thread support.
Documentation comment	C++ doesn’t support documentation comment.	Java supports documentation comment (/** … */) to create documentation for java source code.
Virtual Keyword	C++ supports virtual keyword so that we can decide whether or not override a function.	Java has no virtual keyword. We can override all non-static methods by default. In other words, non-static methods are virtual by default.
unsigned right shift >>>	C++ doesn’t support >>> operator.	Java supports unsigned right shift >>> operator that fills zero at the top for the negative numbers. For positive numbers, it works same like >> operator.
Inheritance Tree	C++ creates a new inheritance tree always.	Java uses a single inheritance tree always because all classes are the child of Object class in java. The object class is the root of the inheritance tree in java.
Hardware	C++ is nearer to hardware.	Java is not so interactive with hardware.
Object-oriented	C++ is an object-oriented language. However, in C language, single root hierarchy is not possible.	Java is also an object-oriented language. However, everything (except fundamental types) is an object in Java. It is a single root hierarchy as everything gets derived from java.lang.Object.

List the features of Java Programming language.

There are the following features in Java Programming Language.

Simple: Java is easy to learn. The syntax of Java is based on C++ which makes easier to write the program in it.
Object-Oriented: Java follows the object-oriented paradigm which allows us to maintain our code as the combination of different type of objects that incorporates both data and behavior.
Portable: Java supports read-once-write-anywhere approach. We can execute the Java program on every machine. Java program (.java) is converted to bytecode (.class) which can be easily run on every machine.
Platform Independent: Java is a platform independent programming language. It is different from other programming languages like C and C++ which needs a platform to be executed. Java comes with its platform on which its code is executed. Java doesn’t depend upon the operating system to be executed.
Secured: Java is secured because it doesn’t use explicit pointers. Java also provides the concept of ByteCode and Exception handling which makes it more secured.
Robust: Java is a strong programming language as it uses strong memory management. The concepts like Automatic garbage collection, Exception handling, etc. make it more robust.
Architecture Neutral: Java is architectural neutral as it is not dependent on the architecture. In C, the size of data types may vary according to the architecture (32 bit or 64 bit) which doesn’t exist in Java.
Interpreted: Java uses the Just-in-time (JIT) interpreter along with the compiler for the program execution.
High Performance: Java is faster than other traditional interpreted programming languages because Java bytecode is “close” to native code. It is still a little bit slower than a compiled language (e.g., C++).
Multithreaded: We can write Java programs that deal with many tasks at once by defining multiple threads. The main advantage of multi-threading is that it doesn’t occupy memory for each thread. It shares a common memory area. Threads are important for multi-media, Web applications, etc.
Distributed: Java is distributed because it facilitates users to create distributed applications in Java. RMI and EJB are used for creating distributed applications. This feature of Java makes us able to access files by calling the methods from any machine on the internet.
Dynamic: Java is a dynamic language. It supports dynamic loading of classes. It means classes are loaded on demand. It also supports functions from its native languages, i.e., C and C++.

What do you understand by Java virtual machine?

Java Virtual Machine is a virtual machine that enables the computer to run the Java program. JVM acts like a run-time engine which calls the main method present in the Java code. JVM is the specification which must be implemented in the computer system. The Java code is compiled by JVM to be a Bytecode which is machine independent and close to the native code.

How many types of memory areas are allocated by JVM?

Class(Method) Area: Class Area stores per-class structures such as the runtime constant pool, field, method data, and the code for methods.

Heap: It is the runtime data area in which the memory is allocated to the objects

Stack: Java Stack stores frames. It holds local variables and partial results, and plays a part in method invocation and return. Each thread has a private JVM stack, created at the same time as the thread. A new frame is created each time a method is invoked. A frame is destroyed when its method invocation completes.

Program Counter Register: PC (program counter) register contains the address of the Java virtual machine instruction currently being executed.

Native Method Stack: It contains all the native methods used in the application.

What is JIT compiler?

Just-In-Time(JIT) compiler: It is used to improve the performance. JIT compiles parts of the bytecode that have similar functionality at the same time, and hence reduces the amount of time needed for compilation. Here the term “compiler” refers to a translator from the instruction set of a Java virtual machine (JVM) to the instruction set of a specific CPU.

What is the platform?

A platform is the hardware or software environment in which a piece of software is executed. There are two types of platforms, software-based and hardware-based. Java provides the software-based platform.

What are the main differences between the Java platform and other platforms?

There are the following differences between the Java platform and other platforms.

Java is the software-based platform whereas other platforms may be the hardware platforms or software-based platforms.
Java is executed on the top of other hardware platforms whereas other platforms can only have the hardware components.

What gives Java its ‘write once and run anywhere’ nature?

The bytecode. Java compiler converts the Java programs into the class file (Byte Code) which is the intermediate language between source code and machine code. This bytecode is not platform specific and can be executed on any computer.

What is classloader?

Classloader is a subsystem of JVM which is used to load class files. Whenever we run the java program, it is loaded first by the classloader. There are three built-in classloaders in Java.

Bootstrap ClassLoader: This is the first classloader which is the superclass of Extension classloader. It loads the rt.jar file which contains all class files of Java Standard Edition like java.lang package classes, java.net package classes, java.util package classes, java.io package classes, java.sql package classes, etc.

Extension ClassLoader: This is the child classloader of Bootstrap and parent classloader of System classloader. It loads the jar files located inside $JAVA_HOME/jre/lib/ext directory.

System/Application ClassLoader: This is the child classloader of Extension classloader. It loads the class files from the classpath. By default, the classpath is set to the current directory. You can change the classpath using “-cp” or “-classpath” switch. It is also known as Application classloader.

What are the various access specifiers in Java?

In Java, access specifiers are the keywords which are used to define the access scope of the method, class, or a variable. In Java, there are four access specifiers given below.

Public The classes, methods, or variables which are defined as public, can be accessed by any class or method.
Protected Protected can be accessed by the class of the same package, or by the sub-class of this class, or within the same class.
Default Default are accessible within the package only. By default, all the classes, methods, and variables are of default scope.
Private The private class, methods, or variables defined as private can be accessed within the class only.

What is the purpose of static methods and variables?

The methods or variables defined as static are shared among all the objects of the class. The static is the part of the class and not of the object. The static variables are stored in the class area, and we do not need to create the object to access such variables. Therefore, static is used in the case, where we need to define variables or methods which are common to all the objects of the class.

For example, In the class simulating the collection of the students in a college, the name of the college is the common attribute to all the students. Therefore, the college name will be defined as static.

What are the advantages of Packages in Java?

There are various advantages of defining packages in Java.

Packages avoid the name clashes.
The Package provides easier access control.
We can also have the hidden classes that are not visible outside and used by the package.
It is easier to locate the related classes.

What is object-oriented paradigm?

It is a programming paradigm based on objects having data and methods defined in the class to which it belongs. Object-oriented paradigm aims to incorporate the advantages of modularity and reusability. Objects are the instances of classes which interacts with one another to design applications and programs. There are the following features of the object-oriented paradigm.

Follows the bottom-up approach in program design.
Focus on data with methods to operate upon the object’s data
Includes the concept like Encapsulation and abstraction which hides the complexities from the user and show only functionality.
Implements the real-time approach like inheritance, abstraction, etc.
The examples of the object-oriented paradigm are C++, Simula, Smalltalk, Python, C#, etc.

What is an object?

The Object is the real-time entity having some state and behavior. In Java, Object is an instance of the class having the instance variables as the state of the object and the methods as the behavior of the object. The object of a class can be created by using the new keyword.

What is the difference between an object-oriented programming language and object-based programming language?

There are the following basic differences between the object-oriented language and object-based language.

Object-oriented languages follow all the concepts of OOPs whereas, the object-based language doesn’t follow all the concepts of OOPs like inheritance and polymorphism.
Object-oriented languages do not have the inbuilt objects whereas Object-based languages have the inbuilt objects, for example, JavaScript has window object.
Examples of object-oriented programming are Java, C#, Smalltalk, etc. whereas the examples of object-based languages are JavaScript, VBScript, etc.

What is the constructor?

The constructor can be defined as the special type of method that is used to initialize the state of an object. It is invoked when the class is instantiated, and the memory is allocated for the object. Every time, an object is created using the new keyword, the default constructor of the class is called. The name of the constructor must be similar to the class name. The constructor must not have an explicit return type.

How many types of constructors are used in Java?

Based on the parameters passed in the constructors, there are two types of constructors in Java.

Default Constructor: default constructor is the one which does not accept any value. The default constructor is mainly used to initialize the instance variable with the default values. It can also be used for performing some useful task on object creation. A default constructor is invoked implicitly by the compiler if there is no constructor defined in the class.

Parameterized Constructor: The parameterized constructor is the one which can initialize the instance variables with the given values. In other words, we can say that the constructors which can accept the arguments are called parameterized constructors.

What are the differences between the constructors and methods?

There are many differences between constructors and methods. They are given below.

Java Constructor	Java Method
A constructor is used to initialize the state of an object.	A method is used to expose the behavior of an object.
A constructor must not have a return type.	A method must have a return type.
The constructor is invoked implicitly.	The method is invoked explicitly.
The Java compiler provides a default constructor if you don’t have any constructor in a class.	The method is not provided by the compiler in any case.
The constructor name must be same as the class name.	The method name may or may not be same as class name.

What is the static variable?

The static variable is used to refer to the common property of all objects (that is not unique for each object), e.g., The company name of employees, college name of students, etc. Static variable gets memory only once in the class area at the time of class loading. Using a static variable makes your program more memory efficient (it saves memory). Static variable belongs to the class rather than the object.

What is the static method?

A static method belongs to the class rather than the object.

There is no need to create the object to call the static methods.

A static method can access and change the value of the static variable.

What are the restrictions that are applied to the Java static methods?

Two main restrictions are applied to the static methods.

The static method can not use non-static data member or call the non-static method directly.

this and super cannot be used in static context as they are non-static.

Why is the main method static?

Because the object is not required to call the static method. If we make the main method non-static, JVM will have to create its object first and then call main() method which will lead to the extra memory allocation.

***JAVA Interview Questions***

What is the difference between static (class) method and instance method?

static or class method	instance method
1)A method that is declared as static is known as the static method.	A method that is not declared as static is known as the instance method.
2)We don’t need to create the objects to call the static methods.	The object is required to call the instance methods.
3)Non-static (instance) members cannot be accessed in the static context (static method, static block, and static nested class) directly.	Static and non-static variables both can be accessed in instance methods.
4)For example: public static int cube(int n){ return nnn;}	For example: public void msg(){…}.

What is this keyword in java?

The this keyword is a reference variable that refers to the current object. There are the various uses of this keyword in Java. It can be used to refer to current class properties such as instance methods, variable, constructors, etc. It can also be passed as an argument into the methods or constructors. It can also be returned from the method as the current class instance.

What are the main uses of this keyword?

There are the following uses of this keyword.

this can be used to refer to the current class instance variable.
this can be used to invoke current class method (implicitly)
this() can be used to invoke the current class constructor.
this can be passed as an argument in the method call.
this can be passed as an argument in the constructor call.
this can be used to return the current class instance from the method.

What is the Inheritance?

Inheritance is a mechanism by which one object acquires all the properties and behavior of another object of another class. It is used for Code Reusability and Method Overriding. The idea behind inheritance in Java is that you can create new classes that are built upon existing classes. When you inherit from an existing class, you can reuse methods and fields of the parent class. Moreover, you can add new methods and fields in your current class also. Inheritance represents the IS-A relationship which is also known as a parent-child relationship.

There are five types of inheritance in Java.

Single-level inheritance
Multi-level inheritance
Multiple Inheritance
Hierarchical Inheritance
Hybrid Inheritance

Multiple inheritance is not supported in Java through class.

Why is Inheritance used in Java?

There are various advantages of using inheritance in Java that is given below.

Inheritance provides code reusability. The derived class does not need to redefine the method of base class unless it needs to provide the specific implementation of the method.
Runtime polymorphism cannot be achieved without using inheritance.
We can simulate the inheritance of classes with the real-time objects which makes OOPs more realistic.
Inheritance provides data hiding. The base class can hide some data from the derived class by making it private.
Method overriding cannot be achieved without inheritance. By method overriding, we can give a specific implementation of some basic method contained by the base class.

What is aggregation?

Aggregation can be defined as the relationship between two classes where the aggregate class contains a reference to the class it owns. Aggregation is best described as a has-a relationship. For example, The aggregate class Employee having various fields such as age, name, and salary also contains an object of Address class having various fields such as Address-Line 1, City, State, and pin-code. In other words, we can say that Employee (class) has an object of Address class. Consider the following example.

What is composition?

Holding the reference of a class within some other class is known as composition. When an object contains the other object, if the contained object cannot exist without the existence of container object, then it is called composition. In other words, we can say that composition is the particular case of aggregation which represents a stronger relationship between two objects. Example: A class contains students. A student cannot exist without a class. There exists composition between class and students.

What is the difference between aggregation and composition?

Aggregation represents the weak relationship whereas composition represents the strong relationship. For example, the bike has an indicator (aggregation), but the bike has an engine (composition).

Why does Java not support pointers?

The pointer is a variable that refers to the memory address. They are not used in Java because they are unsafe(unsecured) and complex to understand.

What are the main uses of the super keyword?

There are the following uses of super keyword.

super can be used to refer to the immediate parent class instance variable.
super can be used to invoke the immediate parent class method.
super() can be used to invoke immediate parent class constructor.

What are the differences between this and super keyword?

There are the following differences between this and super keyword.

The super keyword always points to the parent class contexts whereas this keyword always points to the current class context.
The super keyword is primarily used for initializing the base class variables within the derived class constructor whereas this keyword primarily used to differentiate between local and instance variables when passed in the class constructor.
The super and this must be the first statement inside constructor otherwise the compiler will throw an error.

What is object cloning?

The object cloning is used to create the exact copy of an object. The clone() method of the Object class is used to clone an object. The java.lang.Cloneable interface must be implemented by the class whose object clone we want to create. If we don’t implement Cloneable interface, clone() method generates CloneNotSupportedException.

protected Object clone() throws CloneNotSupportedException

What is method overloading?

Method overloading is the polymorphism technique which allows us to create multiple methods with the same name but different signature. We can achieve method overloading in two ways.

Changing the number of arguments
Changing the return type

Method overloading increases the readability of the program. Method overloading is performed to figure out the program quickly.

Can we overload the main() method?

Yes, we can have any number of main methods in a Java program by using method overloading.

What is method overriding:

If a subclass provides a specific implementation of a method that is already provided by its parent class, it is known as Method Overriding. It is used for runtime polymorphism and to implement the interface methods.

Rules for Method overriding:

The method must have the same name as in the parent class.
The method must have the same signature as in the parent class.
Two classes must have an IS-A relationship between them.

Difference between method Overloading and Overriding.

Method Overloading	Method Overriding
1) Method overloading increases the readability of the program.	Method overriding provides the specific implementation of the method that is already provided by its superclass.
2) Method overloading occurs within the class.	Method overriding occurs in two classes that have IS-A relationship between them.
3) In this case, the parameters must be different.	In this case, the parameters must be the same.

What is the final variable?

In Java, the final variable is used to restrict the user from updating it. If we initialize the final variable, we can’t change its value. In other words, we can say that the final variable once assigned to a value, can never be changed after that. The final variable which is not assigned to any value can only be assigned through the class constructor.

What is the difference between compile-time polymorphism and runtime polymorphism?

There are the following differences between compile-time polymorphism and runtime polymorphism.

SN	compile-time polymorphism	Runtime polymorphism
1	In compile-time polymorphism, call to a method is resolved at compile-time.	In runtime polymorphism, call to an overridden method is resolved at runtime.
2	It is also known as static binding, early binding, or overloading.	It is also known as dynamic binding, late binding, overriding, or dynamic method dispatch.
3	Overloading is a way to achieve compile-time polymorphism in which, we can define multiple methods or constructors with different signatures.	Overriding is a way to achieve runtime polymorphism in which, we can redefine some particular method or variable in the derived class. By using overriding, we can give some specific implementation to the base class properties in the derived class.
4	It provides fast execution because the type of an object is determined at compile-time.	It provides slower execution as compare to compile-time because the type of an object is determined at run-time.
5	Compile-time polymorphism provides less flexibility because all the things are resolved at compile-time.	Run-time polymorphism provides more flexibility because all the things are resolved at runtime.

What is the abstraction?

Abstraction is a process of hiding the implementation details and showing only functionality to the user. It displays just the essential things to the user and hides the internal information, for example, sending SMS where you type the text and send the message. You don’t know the internal processing about the message delivery. Abstraction enables you to focus on what the object does instead of how it does it. Abstraction lets you focus on what the object does instead of how it does it.

In Java, there are two ways to achieve the abstraction.

Abstract Class

Interface

What is the interface?

The interface is a blueprint for a class that has static constants and abstract methods. It can be used to achieve full abstraction and multiple inheritance. It is a mechanism to achieve abstraction. There can be only abstract methods in the Java interface, not method body. It is used to achieve abstraction and multiple inheritance in Java. In other words, you can say that interfaces can have abstract methods and variables. Java Interface also represents the IS-A relationship. It cannot be instantiated just like the abstract class. However, we need to implement it to define its methods. Since Java 8, we can have the default, static, and private methods in an interface.

What are the differences between abstract class and interface?

Abstract class	Interface
An abstract class can have a method body (non-abstract methods).	The interface has only abstract methods.
An abstract class can have instance variables.	An interface cannot have instance variables.
An abstract class can have the constructor.	The interface cannot have the constructor.
An abstract class can have static methods.	The interface cannot have static methods.
You can extend one abstract class.	You can implement multiple interfaces.
The abstract class can provide the implementation of the interface.	The Interface can’t provide the implementation of the abstract class.
The abstract keyword is used to declare an abstract class.	The interface keyword is used to declare an interface.
An abstract class can extend another Java class and implement multiple Java interfaces.	An interface can extend another Java interface only.
An abstract class can be extended using keyword extends	An interface class can be implemented using keyword implements
A Java abstract class can have class members like private, protected, etc.	Members of a Java interface are public by default.
Example: public abstract class Shape{ public abstract void draw(); }	Example: public interface Drawable{ void draw(); }

What are the advantages of Encapsulation in Java?

There are the following advantages of Encapsulation in Java:

By providing only the setter or getter method, you can make the class read-only or write-only. In other words, you can skip the getter or setter methods.

It provides you the control over the data. Suppose you want to set the value of id which should be greater than 100 only, you can write the logic inside the setter method. You can write the logic not to store the negative numbers in the setter methods.

It is a way to achieve data hiding in Java because other class will not be able to access the data through the private data members.

The encapsulate class is easy to test. So, it is better for unit testing.

The standard IDE’s are providing the facility to generate the getters and setters. So, it is easy and fast to create an encapsulated class in Java.

What are the advantages of defining packages in Java?

By defining packages, we can avoid the name conflicts between the same class names defined in different packages. Packages also enable the developer to organize the similar classes more effectively. For example, one can clearly understand that the classes present in java.io package are used to perform io related operations.

How to create packages in Java?

If you are using the programming IDEs like Eclipse, NetBeans, MyEclipse, etc. click on file->new->project and eclipse will ask you to enter the name of the package. It will create the project package containing various directories such as src, etc. If you are using an editor like notepad for java programming, use the following steps to create the package.

Define a package package_name. Create the class with the name class_name and save this file with your_class_name.java.

Now compile the file by running the following command on the terminal.

javac -d . your_class_name.java

The above command creates the package with the name package_name in the present working directory.

Now, run the class file by using the absolute class file name, like following.

java package_name.class_name

How can we access some class in another class in Java?

There are two ways to access a class in another class.

By using the fully qualified name: To access a class in a different package, either we must use the fully qualified name of that class, or we must import the package containing that class.

By using the relative path, We can use the path of the class that is related to the package that contains our class. It can be the same or subpackage.

How many types of exception can occur in a Java program?

There are mainly two types of exceptions: checked and unchecked. Here, an error is considered as the unchecked exception. According to Oracle, there are three types of exceptions:

Checked Exception: Checked exceptions are the one which are checked at compile-time. For example, SQLException, ClassNotFoundException, etc.

Unchecked Exception: Unchecked exceptions are the one which are handled at runtime because they can not be checked at compile-time. For example, ArithmaticException, NullPointerException, ArrayIndexOutOfBoundsException, etc.

Error: Error cause the program to exit since they are not recoverable. For Example, OutOfMemoryError, AssertionError, etc.

What is Exception Handling?

Exception Handling is a mechanism that is used to handle runtime errors. It is used primarily to handle checked exceptions. Exception handling maintains the normal flow of the program. There are mainly two types of exceptions: checked and unchecked. Here, the error is considered as the unchecked exception.

What is the difference between Checked Exception and Unchecked Exception?

1) Checked Exception:

The classes that extend Throwable class except RuntimeException and Error are known as checked exceptions, e.g., IOException, SQLException, etc. Checked exceptions are checked at compile-time.

2) Unchecked Exception:

The classes that extend RuntimeException are known as unchecked exceptions, e.g., ArithmeticException, NullPointerException, etc. Unchecked exceptions are not checked at compile-time.

What is finally block?

The “finally” block is used to execute the important code of the program. It is executed whether an exception is handled or not. In other words, we can say that finally block is the block which is always executed. Finally block follows try or catch block. If you don’t handle the exception, before terminating the program, JVM runs finally block, (if any). The finally block is mainly used to place the cleanup code such as closing a file or closing a connection. Here, we must know that for each try block there can be zero or more catch blocks, but only one finally block. The finally block will not be executed if program exits(either by calling System.exit() or by causing a fatal error that causes the process to abort).

***JAVA Interview Questions***

What is the difference between throw and throws?

throw keyword	throws keyword
1) The throw keyword is used to throw an exception explicitly.	The throws keyword is used to declare an exception.
2) The checked exceptions cannot be propagated with throw only.	The checked exception can be propagated with throws
3) The throw keyword is followed by an instance.	The throws keyword is followed by class.
4) The throw keyword is used within the method.	The throws keyword is used with the method signature.
5) You cannot throw multiple exceptions.	You can declare multiple exceptions, e.g., public void method()throws IOException, SQLException.

What is String Pool?

String pool is the space reserved in the heap memory that can be used to store the strings. The main advantage of using the String pool is whenever we create a string literal; the JVM checks the “string constant pool” first. If the string already exists in the pool, a reference to the pooled instance is returned. If the string doesn’t exist in the pool, a new string instance is created and placed in the pool. Therefore, it saves the memory by avoiding the duplicacy.

Why are the objects immutable in java?

Because Java uses the concept of the string literal. Suppose there are five reference variables, all refer to one object “sachin”. If one reference variable changes the value of the object, it will be affected by all the reference variables. That is why string objects are immutable in java.

What are the differences between String and StringBuffer?

The differences between the String and StringBuffer is given in the table below.

No.	String	StringBuffer
1)	The String class is immutable.	The StringBuffer class is mutable.
2)	The String is slow and consumes more memory when you concat too many strings because every time it creates a new instance.	The StringBuffer is fast and consumes less memory when you cancat strings.
3)	The String class overrides the equals() method of Object class. So you can compare the contents of two strings by equals() method.	The StringBuffer class doesn’t override the equals() method of Object class.

What are the differences between StringBuffer and StringBuilder?

The differences between the StringBuffer and StringBuilder is given below.

No.	StringBuffer	StringBuilder
1)	StringBuffer is synchronized, i.e., thread safe. It means two threads can’t call the methods of StringBuffer simultaneously.	StringBuilder is non-synchronized,i.e., not thread safe. It means two threads can call the methods of StringBuilder simultaneously.
2)	StringBuffer is less efficient than StringBuilder.	StringBuilder is more efficient than StringBuffer.

Why CharArray() is preferred over String to store the password?

String stays in the string pool until the garbage is collected. If we store the password into a string, it stays in the memory for a longer period, and anyone having the memory-dump can extract the password as clear text. On the other hand, Using CharArray allows us to set it to blank whenever we are done with the password. It avoids the security threat with the string by enabling us to control the memory.

What are the advantages of Java inner classes?

There are two types of advantages of Java inner classes.

Nested classes represent a special type of relationship that is it can access all the members (data members and methods) of the outer class including private.

Nested classes are used to develop a more readable and maintainable code because it logically groups classes and interfaces in one place only.

Code Optimization: It requires less code to write.

What are the disadvantages of using inner classes?

There are the following main disadvantages of using inner classes.

Inner classes increase the total number of classes used by the developer and therefore increases the workload of JVM since it has to perform some routine operations for those extra classes which result in slower performance.

IDEs provide less support to the inner classes as compare to the top level classes and therefore it annoys the developers while working with inner classes.

What is Garbage Collection?

Garbage collection is a process of reclaiming the unused runtime objects. It is performed for memory management. In other words, we can say that It is the process of removing unused objects from the memory to free up space and make this space available for Java Virtual Machine. Due to garbage collection java gives 0 as output to a variable whose value is not set, i.e., the variable has been defined but not initialized. For this purpose, we were using free() function in the C language and delete() in C++. In Java, it is performed automatically. So, java provides better memory management.

What is gc()?

The gc() method is used to invoke the garbage collector for cleanup processing. This method is found in System and Runtime classes. This function explicitly makes the Java Virtual Machine free up the space occupied by the unused objects so that it can be utilized or reused. Consider the following example for the better understanding of how the gc() method invoke the garbage collector.

How is garbage collection controlled?

Garbage collection is managed by JVM. It is performed when there is not enough space in the memory and memory is running low. We can externally call the System.gc() for the garbage collection. However, it depends upon the JVM whether to perform it or not.

How can an object be unreferenced?

There are many ways:

By nulling the reference

By assigning a reference to another

By anonymous object etc.

1) By nulling a reference:

Employee e=new Employee();

e=null;

2) By assigning a reference to another:

Employee e1=new Employee();

Employee e2=new Employee();

e1=e2;//now the first object referred by e1 is available for garbage collection

3) By anonymous object:

new Employee();

What is the purpose of the finalize() method?

The finalize() method is invoked just before the object is garbage collected. It is used to perform cleanup processing. The Garbage collector of JVM collects only those objects that are created by new keyword. So if you have created an object without new, you can use the finalize method to perform cleanup processing (destroying remaining objects). The cleanup processing is the process to free up all the resources, network which was previously used and no longer needed. It is essential to remember that it is not a reserved keyword, finalize method is present in the object class hence it is available in every class as object class is the superclass of every class in java. Here, we must note that neither finalization nor garbage collection is guaranteed.

What is the purpose of the Runtime class?

Java Runtime class is used to interact with a java runtime environment. Java Runtime class provides methods to execute a process, invoke GC, get total and free memory, etc. There is only one instance of java.lang.Runtime class is available for one java application. The Runtime.getRuntime() method returns the singleton instance of Runtime class.

What is the difference between final, finally and finalize?

No.	final	finally	finalize
1)	Final is used to apply restrictions on class, method, and variable. The final class can’t be inherited, final method can’t be overridden, and final variable value can’t be changed.	Finally is used to place important code, it will be executed whether an exception is handled or not.	Finalize is used to perform clean up processing just before an object is garbage collected.
2)	Final is a keyword.	Finally is a block.	Finalize is a method.

What is the purpose of the Runtime class?

What do you understand by an IO stream?

The stream is a sequence of data that flows from source to destination. It is composed of bytes. In Java, three streams are created for us automatically.

out: standard output stream
in: standard input stream
err: standard error stream

What is the difference between the Reader/Writer class hierarchy and the InputStream/OutputStream class hierarchy?

The Reader/Writer class hierarchy is character-oriented, and the InputStream/OutputStream class hierarchy is byte-oriented. The ByteStream classes are used to perform input-output of 8-bit bytes whereas the CharacterStream classes are used to perform the input/output for the 16-bit Unicode system. There are many classes in the ByteStream class hierarchy, but the most frequently used classes are FileInputStream and FileOutputStream. The most frequently used classes CharacterStream class hierarchy is FileReader and FileWriter.

What are the super most classes for all the streams?

All the stream classes can be divided into two types of classes that are ByteStream classes and CharacterStream Classes. The ByteStream classes are further divided into InputStream classes and OutputStream classes. CharacterStream classes are also divided into Reader classes and Writer classes. The SuperMost classes for all the InputStream classes is java.io.InputStream and for all the output stream classes is java.io.OutPutStream. Similarly, for all the reader classes, the super-most class is java.io.Reader, and for all the writer classes, it is java.io.Writer.

What are the FileInputStream and FileOutputStream?

Java FileOutputStream is an output stream used for writing data to a file. If you have some primitive values to write into a file, use FileOutputStream class. You can write byte-oriented as well as character-oriented data through the FileOutputStream class. However, for character-oriented data, it is preferred to use FileWriter than FileOutputStream.

Java FileInputStream class obtains input bytes from a file. It is used for reading byte-oriented data (streams of raw bytes) such as image data, audio, video, etc. You can also read character-stream data. However, for reading streams of characters, it is recommended to use FileReader class. Consider the following example for reading bytes from a file.

What is the purpose of using BufferedInputStream and BufferedOutputStream classes?

Java BufferedOutputStream class is used for buffering an output stream. It internally uses a buffer to store data. It adds more efficiency than to write data directly into a stream. So, it makes the performance fast. Whereas, Java BufferedInputStream class is used to read information from the stream. It internally uses the buffer mechanism to make the performance fast.

What are FilterStreams?

FilterStream classes are used to add additional functionalities to the other stream classes. FilterStream classes act like an interface which read the data from a stream, filters it, and pass the filtered data to the caller. The FilterStream classes provide extra functionalities like adding line numbers to the destination file, etc.

What is an I/O filter?

An I/O filter is an object that reads from one stream and writes to another, usually altering the data in some way as it is passed from one stream to another. Many Filter classes that allow a user to make a chain using multiple input streams. It generates a combined effect on several filters.

***JAVA Interview Questions***

In Java, How many ways you can take input from the console?

In Java, there are three ways by using which, we can take input from the console.

Using BufferedReader class: we can take input from the console by wrapping System.in into an InputStreamReader and passing it into the BufferedReader. It provides an efficient reading as the input gets buffered. Consider the following example.

Using Scanner class: The Java Scanner class breaks the input into tokens using a delimiter that is whitespace by default. It provides many methods to read and parse various primitive values. Java Scanner class is widely used to parse text for string and primitive types using a regular expression. Java Scanner class extends Object class and implements Iterator and Closeable interfaces.

Using Console class: The Java Console class is used to get input from the console. It provides methods to read texts and passwords. If you read the password using the Console class, it will not be displayed to the user. The java.io.Console class is attached to the system console internally.

What is the difference between Serializable and Externalizable interface?

No.	Serializable	Externalizable
1)	The Serializable interface does not have any method, i.e., it is a marker interface.	The Externalizable interface contains is not a marker interface, It contains two methods, i.e., writeExternal() and readExternal().
2)	It is used to “mark” Java classes so that objects of these classes may get the certain capability.	The Externalizable interface provides control of the serialization logic to the programmer.
3)	It is easy to implement but has the higher performance cost.	It is used to perform the serialization and often result in better performance.
4)	No class constructor is called in serialization.	We must call a public default constructor while using this interface.

What is serialization?

Serialization in Java is a mechanism of writing the state of an object into a byte stream. It is used primarily in Hibernate, RMI, JPA, EJB and JMS technologies. It is mainly used to travel object’s state on the network (which is known as marshaling). Serializable interface is used to perform serialization. It is helpful when you require to save the state of a program to storage such as the file. At a later point of time, the content of this file can be restored using deserialization. It is also required to implement RMI(Remote Method Invocation). With the help of RMI, it is possible to invoke the method of a Java object on one machine to another machine.

What is Deserialization?

Deserialization is the process of reconstructing the object from the serialized state. It is the reverse operation of serialization. An ObjectInputStream deserializes objects and primitive data written using an ObjectOutputStream.

What is the transient keyword?

If you define any data member as transient, it will not be serialized. By determining transient keyword, the value of variable need not persist when it is restored

What is Externalizable?

The Externalizable interface is used to write the state of an object into a byte stream in a compressed format. It is not a marker interface.

Give a brief description of Java socket programming?

Java Socket programming is used for communication between the applications running on different JRE. Java Socket programming can be connection-oriented or connectionless. Socket and ServerSocket classes are used for connection-oriented socket programming and DatagramSocket, and DatagramPacket classes are used for connectionless socket programming. The client in socket programming must know two information:

IP address of the server
port number

What is Socket?

A socket is simply an endpoint for communications between the machines. It provides the connection mechanism to connect the two computers using TCP. The Socket class can be used to create a socket.

What are the steps that are followed when two computers connect through TCP?

There are the following steps that are performed when two computers connect through TCP.

The ServerSocket object is instantiated by the server which denotes the port number to which, the connection will be made.

After instantiating the ServerSocket object, the server invokes accept() method of ServerSocket class which makes server wait until the client attempts to connect to the server on the given port.

Meanwhile, the server is waiting, a socket is created by the client by instantiating Socket class. The socket class constructor accepts the server port number and server name.

The Socket class constructor attempts to connect with the server on the specified name. If the connection is established, the client will have a socket object that can communicate with the server.

The accept() method invoked by the server returns a reference to the new socket on the server that is connected with the server.

What is the reflection?

Reflection is the process of examining or modifying the runtime behavior of a class at runtime. The java.lang.Class class provides various methods that can be used to get metadata, examine and change the runtime behavior of a class. The java.lang and java.lang.reflect packages provide classes for java reflection. It is used in:

IDE (Integrated Development Environment), e.g., Eclipse, MyEclipse, NetBeans.

Debugger

Test Tools, etc.

What is the purpose of using java.lang.Class class?

The java.lang.Class class performs mainly two tasks:

Provides methods to get the metadata of a class at runtime.

Provides methods to examine and change the runtime behavior of a class.

What are the ways to instantiate the Class class?

There are three ways to instantiate the Class class.

forName() method of Class class: The forName() method is used to load the class dynamically. It returns the instance of Class class. It should be used if you know the fully qualified name of the class. This cannot be used for “primitive types”.

getClass() method of Object class: It returns the instance of Class class. It should be used if you know the type. Moreover, it can be used with primitives.

the .class syntax: If a type is available, but there is no instance then it is possible to obtain a Class by appending “.class” to the name of the type. It can be used for “primitive data type” also.

What is the purpose of using javap?

The javap command disassembles a class file. The javap command displays information about the fields, constructors and methods present in a class file.

Syntax

javap fully_class_name

Can you access the private method from outside the class?

Yes, by changing the runtime behavior of a class if the class is not secured.

What are wrapper classes?

Wrapper classes are classes that allow primitive types to be accessed as objects. In other words, we can say that wrapper classes are built-in java classes which allow the conversion of objects to primitives and primitives to objects. The process of converting primitives to objects is called autoboxing, and the process of converting objects to primitives is called unboxing. There are eight wrapper classes present in java.lang package is given below.

Primitive Type	Wrapper class
boolean	Boolean
char	Character
byte	Byte
short	Short
int	Integer
long	Long
float	Float
double	Double

What are autoboxing and unboxing? When does it occur?

The autoboxing is the process of converting primitive data type to the corresponding wrapper class object, eg., int to Integer. The unboxing is the process of converting wrapper class object to primitive data type. For eg., integer to int. Unboxing and autoboxing occur automatically in Java. However, we can externally convert one into another by using the methods like valueOf() or xxxValue().

It can occur whenever a wrapper class object is expected, and primitive data type is provided or vice versa.

Adding primitive types into Collection like ArrayList in Java.

Creating an instance of parameterized classes ,e.g., ThreadLocal which expect Type.

Java automatically converts primitive to object whenever one is required and another is provided in the method calling.

When a primitive type is assigned to an object type.

What is object cloning?

The object cloning is a way to create an exact copy of an object. The clone() method of the Object class is used to clone an object. The java.lang.Cloneable interface must be implemented by the class whose object clone we want to create. If we don’t implement Cloneable interface, clone() method generates CloneNotSupportedException. The clone() method is defined in the Object class. The syntax of the clone() method is as follows:

protected Object clone() throws CloneNotSupportedException

What are the advantages and disadvantages of object cloning?

Advantage of Object Cloning:

You don’t need to write lengthy and repetitive codes. Just use an abstract class with a 4- or 5-line long clone() method.
It is the easiest and most efficient way of copying objects, especially if we are applying it to an already developed or an old project. Just define a parent class, implement Cloneable in it, provide the definition of the clone() method and the task will be done.
Clone() is the fastest way to copy the array.

Disadvantage of Object Cloning:

To use the Object.clone() method, we have to change many syntaxes to our code, like implementing a Cloneable interface, defining the clone() method and handling CloneNotSupportedException, and finally, calling Object.clone(), etc.
We have to implement the Cloneable interface while it does not have any methods in it. We have to use it to tell the JVM that we can perform a clone() on our object.
clone() is protected, so we have to provide our own clone() and indirectly call Object.clone() from it.
clone() does not invoke any constructor, so we do not have any control over object construction.
If you want to write a clone method in a child class, then all of its superclasses should define the clone() method in them or inherit it from another parent class. Otherwise, the super.clone() chain will fail.
clone() supports only shallow copying, but we will need to override it if we need deep cloning.

What is a native method?

A native method is a method that is implemented in a language other than Java. Natives methods are sometimes also referred to as foreign methods.

What is the purpose of the strictfp keyword?

Java strictfp keyword ensures that you will get the same result on every platform if you perform operations in the floating-point variable. The precision may differ from platform to platform that is why java programming language has provided the strictfp keyword so that you get the same result on every platform. So, now you have better control over the floating-point arithmetic.

What is the purpose of the System class?

The purpose of the System class is to provide access to system resources such as standard input and output. It cannot be instantiated. Facilities provided by System class are given below.

Standard input
Error output streams
Standard output
utility method to copy the portion of an array
utilities to load files and libraries
There are the three fields of Java System class, i.e., static printstream err, static inputstream in, and standard output stream.

What is a lightweight component?

Lightweight components are the one which does not go with the native call to obtain the graphical units. They share their parent component graphical units to render them. For example, Swing components, and JavaFX Components.

What is a heavyweight component?

The portable elements provided by the operating system are called heavyweight components. AWT is limited to the graphical classes provided by the operating system and therefore, It implements only the minimal subset of screen elements supported by all platforms. The Operating system dependent UI discovery tools are called heavyweight components.

What is an applet?

An applet is a small java program that runs inside the browser and generates dynamic content. It is embedded in the webpage and runs on the client side. It is secured and takes less response time. It can be executed by browsers running under many platforms, including Linux, Windows, Mac Os, etc. However, the plugins are required at the client browser to execute the applet. The following image shows the architecture of Applet.

when an applet is created, the following methods are invoked in order.

init()

start()

paint()

When an applet is destroyed, the following functions are invoked in order.

stop()

destroy()

What is a JavaBean?

JavaBean is a reusable software component written in the Java programming language, designed to be manipulated visually by a software development environment, like JBuilder or VisualAge for Java. t. A JavaBean encapsulates many objects into one object so that we can access this object from multiple places. Moreover, it provides the easy maintenance.

What is RMI?

The RMI (Remote Method Invocation) is an API that provides a mechanism to create the distributed application in java. The RMI allows an object to invoke methods on an object running in another JVM. The RMI provides remote communication between the applications using two objects stub and skeleton.

What is the purpose of stub and skeleton?

Stub:

The stub is an object, acts as a gateway for the client side. All the outgoing requests are routed through it. It resides at the client side and represents the remote object. When the caller invokes the method on the stub object, it does the following tasks:

It initiates a connection with remote Virtual Machine (JVM).

It writes and transmits (marshals) the parameters to the remote Virtual Machine (JVM).

It waits for the result.

It reads (unmarshals) the return value or exception.

It finally, returns the value to the caller.

Skeleton:

The skeleton is an object, acts as a gateway for the server side object. All the incoming requests are routed through it. When the skeleton receives the incoming request, it does the following tasks:

It reads the parameter for the remote method.

It invokes the method on the actual remote object.

It writes and transmits (marshals) the result to the caller.

What are the steps involved to write RMI based programs?

There are 6 steps which are performed to write RMI based programs.

Create the remote interface.
Provide the implementation of the remote interface.
Compile the implementation class and create the stub and skeleton objects using the rmic tool.
Start the registry service by the rmiregistry tool.
Create and start the remote application.
Create and start the client application.

What is the use of HTTP-tunneling in RMI?

HTTP tunneling can be defined as the method which doesn’t need any setup to work within the firewall environment. It handles the HTTP connections through the proxy servers. However, it does not allow outbound TCP connections.

What is JRMP?

JRMP (Java Remote Method Protocol) can be defined as the Java-specific, stream-based protocol which looks up and refers to the remote objects. It requires both client and server to use Java objects. It is wire level protocol which runs under RMI and over TCP/IP.

What is the difference between this() and super() in Java?

In Java, super() and this(), both are special keywords that are used to call the constructor.

this()	super()
1. this() represents the current instance of a class	1. super() represents the current instance of a parent/base class
2. Used to call the default constructor of the same class	2. Used to call the default constructor of the parent/base class
3. Used to access methods of the current class	3. Used to access methods of the base class
4. Used for pointing the current class instance	4. Used for pointing the superclass instance
5. Must be the first line of a block	5. Must be the first line of a block

What is Java String Pool?

Java String pool refers to a collection of Strings which are stored in heap memory. In this, whenever a new object is created, String pool first checks whether the object is already present in the pool or not. If it is present, then the same reference is returned to the variable else new object will be created in the String pool and the respective reference will be returned.

What is a classloader in Java?

The Java ClassLoader is a subset of JVM (Java Virtual Machine) that is responsible for loading the class files. Whenever a Java program is executed it is first loaded by the classloader. Java provides three built-in classloaders:

Bootstrap ClassLoader
Extension ClassLoader
System/Application ClassLoader

Why Java Strings are immutable in nature?

In Java, string objects are immutable in nature which simply means once the String object is created its state cannot be modified. Whenever you try to update the value of that object instead of updating the values of that particular object, Java creates a new string object. Java String objects are immutable as String objects are generally cached in the String pool. Since String literals are usually shared between multiple clients, action from one client might affect the rest. It enhances security, caching, synchronization, and performance of the application.

What is the difference between an array and an array list?

Array	ArrayList
Cannot contain values of different data types	Can contain values of different data types.
Size must be defined at the time of declaration	Size can be dynamically changed
Need to specify the index in order to add data	No need to specify the index
Arrays are not type parameterized	Arraylists are type
Arrays can contain primitive data types as well as objects	Arraylists can contain only objects, no primitive data types are allowed

What is a Map in Java?

In Java, Map is an interface of Util package which maps unique keys to values. The Map interface is not a subset of the main Collection interface and thus it behaves little different from the other collection types. Below are a few of the characteristics of Map interface:

Map doesn’t contain duplicate keys.
Each key can map at max one value.

***JAVA Interview Questions***

What is collection class in Java? List down its methods and interfaces.

In Java, the collection is a framework that acts as an architecture for storing and manipulating a group of objects. Using Collections you can perform various tasks like searching, sorting, insertion, manipulation, deletion, etc. Java collection framework includes the following:

Interfaces
Classes
Methods

What is Polymorphism?

Polymorphism is briefly described as “one interface, many implementations”. Polymorphism is a characteristic of being able to assign a different meaning or usage to something in different contexts – specifically, to allow an entity such as a variable, a function, or an object to have more than one form. There are two types of polymorphism:

Compile time polymorphism
Run time polymorphism

Compile time polymorphism is method overloading whereas Runtime time polymorphism is done using inheritance and interface.

What is runtime polymorphism or dynamic method dispatch?

In Java, runtime polymorphism or dynamic method dispatch is a process in which a call to an overridden method is resolved at runtime rather than at compile-time. In this process, an overridden method is called through the reference variable of a superclass.

What is abstraction in Java?

Abstraction refers to the quality of dealing with ideas rather than events. It basically deals with hiding the details and showing the essential things to the user. Thus you can say that abstraction in Java is the process of hiding the implementation details from the user and revealing only the functionality to them. Abstraction can be achieved in two ways:

Abstract Classes (0-100% of abstraction can be achieved)

Interfaces (100% of abstraction can be achieved)

What do you mean by an interface in Java?

An interface in Java is a blueprint of a class or you can say it is a collection of abstract methods and static constants. In an interface, each method is public and abstract but it does not contain any constructor. Thus, interface basically is a group of related methods with empty bodies. Example:

public interface Animal {

public void eat();

public void sleep();

public void run();

What is inheritance in Java?

Inheritance in Java is the concept where the properties of one class can be inherited by the other. It helps to reuse the code and establish a relationship between different classes. Inheritance is performed between two types of classes:

Parent class (Super or Base class)

Child class (Subclass or Derived class)

A class which inherits the properties is known as Child Class whereas a class whose properties are inherited is known as Parent class.

What is method overloading and method overriding?

Method Overloading:

In Method Overloading, Methods of the same class shares the same name but each method must have a different number of parameters or parameters having different types and order.

Method Overloading is to “add” or “extend” more to the method’s behavior.

It is a compile-time polymorphism.

The methods must have a different signature.

It may or may not need inheritance in Method Overloading.

Method Overriding:

In Method Overriding, the subclass has the same method with the same name and exactly the same number and type of parameters and same return type as a superclass.

Method Overriding is to “Change” existing behavior of the method.

It is a run time polymorphism.

The methods must have the same signature.

It always requires inheritance in Method Overriding.

What is multiple inheritance? Is it supported by Java?

MultipleInheritance -If a child class inherits the property from multiple classes is known as multiple inheritance. Java does not allow to extend multiple classes.

The problem with multiple inheritance is that if multiple parent classes have the same method name, then at runtime it becomes difficult for the compiler to decide which method to execute from the child class.

Therefore, Java doesn’t support multiple inheritance. The problem is commonly referred to as Diamond Problem.

What is encapsulation in Java?

Encapsulation is a mechanism where you bind your data(variables) and code(methods) together as a single unit. Here, the data is hidden from the outer world and can be accessed only via current class methods. This helps in protecting the data from any unnecessary modification. We can achieve encapsulation in Java by:

Declaring the variables of a class as private.

Providing public setter and getter methods to modify and view the values of the variables.

What is an association?

Association is a relationship where all object have their own lifecycle and there is no owner. Let’s take the example of Teacher and Student. Multiple students can associate with a single teacher and a single student can associate with multiple teachers but there is no ownership between the objects and both have their own lifecycle. These relationships can be one to one, one to many, many to one and many to many.

What is composition in Java?

Composition is again a specialized form of Aggregation and we can call this as a “death” relationship. It is a strong type of Aggregation. Child object does not have their lifecycle and if parent object deletes all child object will also be deleted. Let’s take again an example of a relationship between House and rooms. House can contain multiple rooms there is no independent life of room and any room can not belongs to two different houses if we delete the house room will automatically delete.

What is a servlet?

Java Servlet is server-side technologies to extend the capability of web servers by providing support for dynamic response and data persistence.

The javax.servlet and javax.servlet.http packages provide interfaces and classes for writing our own servlets.

All servlets must implement the javax.servlet.Servlet interface, which defines servlet lifecycle methods. When implementing a generic service, we can extend the GenericServlet class provided with the Java Servlet API. The HttpServlet class provides methods, such as doGet() and doPost(), for handling HTTP-specific services.

Most of the times, web applications are accessed using HTTP protocol and thats why we mostly extend HttpServlet class

What is Spring?

Wikipedia defines the Spring framework as “an application framework and inversion of control container for the Java platform. The framework’s core features can be used by any Java application, but there are extensions for building web applications on top of the Java EE platform.” Spring is essentially a lightweight, integrated framework that can be used for developing enterprise applications in java.

What is multithreading?

Multithreading is a process of executing multiple threads simultaneously. Multithreading is used to obtain the multitasking. It consumes less memory and gives the fast and efficient performance. Its main advantages are:

Threads share the same address space.
The thread is lightweight.
The cost of communication between the processes is low.

What is the thread?

A thread is a lightweight subprocess. It is a separate path of execution because each thread runs in a different stack frame. A process may contain multiple threads. Threads share the process resources, but still, they execute independently.

Differentiate between process and thread?

There are the following differences between the process and thread.

A Program in the execution is called the process whereas; A thread is a subset of the process
Processes are independent whereas threads are the subset of process.
Process have different address space in memory, while threads contain a shared address space.
Context switching is faster between the threads as compared to processes.
Inter-process communication is slower and expensive than inter-thread communication.
Any change in Parent process doesn’t affect the child process whereas changes in parent thread can affect the child thread.

What do you understand by inter-thread communication?

The process of communication between synchronized threads is termed as inter-thread communication.

Inter-thread communication is used to avoid thread polling in Java.

The thread is paused running in its critical section, and another thread is allowed to enter (or lock) in the same critical section to be executed.

It can be obtained by wait(), notify(), and notifyAll() methods.

What is the purpose of wait() method in Java?

The wait() method is provided by the Object class in Java. This method is used for inter-thread communication in Java. The java.lang.Object.wait() is used to pause the current thread, and wait until another thread does not call the notify() or notifyAll() method. Its syntax is given below.

public final void wait()

Why must wait() method be called from the synchronized block?

We must call the wait method otherwise it will throw java.lang.IllegalMonitorStateException exception. Moreover, we need wait() method for inter-thread communication with notify() and notifyAll(). Therefore It must be present in the synchronized block for the proper and correct communication.

What are the advantages of multithreading?

Multithreading programming has the following advantages:

Multithreading allows an application/program to be always reactive for input, even already running with some background tasks
Multithreading allows the faster execution of tasks, as threads execute independently.
Multithreading provides better utilization of cache memory as threads share the common memory resources.
Multithreading reduces the number of the required server as one server can execute multiple threads at a time.

What are the states in the lifecycle of a Thread?

A thread can have one of the following states during its lifetime:

New: In this state, a Thread class object is created using a new operator, but the thread is not alive. Thread doesn’t start until we call the start() method.

Runnable: In this state, the thread is ready to run after calling the start() method. However, the thread is not yet selected by the thread scheduler.

Running: In this state, the thread scheduler picks the thread from the ready state, and the thread is running.

Waiting/Blocked: In this state, a thread is not running but still alive, or it is waiting for the other thread to finish.

Dead/Terminated: A thread is in terminated or dead state when the run() method exits.

What is the difference between preemptive scheduling and time slicing?

Under preemptive scheduling, the highest priority task executes until it enters the waiting or dead states or a higher priority task comes into existence. Under time slicing, a task executes for a predefined slice of time and then reenters the pool of ready tasks. The scheduler then determines which task should execute next, based on priority and other factors.

What is context switching?

In Context switching the state of the process (or thread) is stored so that it can be restored and execution can be resumed from the same point later. Context switching enables the multiple processes to share the same CPU.

***JAVA Interview Questions***

Differentiate between the Thread class and Runnable interface for creating a Thread?

The Thread can be created by using two ways.

By extending the Thread class
By implementing the Thread class

However, the primary differences between both the ways are given below:

By extending the Thread class, we cannot extend any other class, as Java does not allow multiple inheritances while implementing the Runnable interface; we can also extend other base class(if required).

By extending the Thread class, each of thread creates the unique object and associates with it while implementing the Runnable interface; multiple threads share the same object

Thread class provides various inbuilt methods such as getPriority(), isAlive and many more while the Runnable interface provides a single method, i.e., run().

What does join() method?

The join() method waits for a thread to die. In other words, it causes the currently running threads to stop executing until the thread it joins with completes its task. Join method is overloaded in Thread class in the following ways.

public void join()throws InterruptedException

public void join(long milliseconds)throws InterruptedException

Describe the purpose and working of sleep() method.

The sleep() method in java is used to block a thread for a particular time, which means it pause the execution of a thread for a specific time. There are two methods of doing so.

Syntax:

public static void sleep(long milliseconds)throws InterruptedException

public static void sleep(long milliseconds, int nanos)throws InterruptedException

Working of sleep() method:

When we call the sleep() method, it pauses the execution of the current thread for the given time and gives priority to another thread(if available). Moreover, when the waiting time completed then again previous thread changes its state from waiting to runnable and comes in running state, and the whole process works so on till the execution doesn’t complete.

Can we call the run() method instead of start()?

Yes, calling run() method directly is valid, but it will not work as a thread instead it will work as a normal object. There will not be context-switching between the threads. When we call the start() method, it internally calls the run() method, which creates a new stack for a thread while directly calling the run() will not create a new stack.

What about the daemon threads?

The daemon threads are the low priority threads that provide the background support and services to the user threads. Daemon thread gets automatically terminated by the JVM if the program remains with the daemon thread only, and all other user threads are ended/died. There are two methods for daemon thread available in the Thread class:

public void setDaemon(boolean status): It used to mark the thread daemon thread or a user thread.

public boolean isDaemon(): It checks the thread is daemon or not.

What is shutdown hook?

The shutdown hook is a thread that is invoked implicitly before JVM shuts down. So we can use it to perform clean up the resource or save the state when JVM shuts down normally or abruptly. We can add shutdown hook by using the following method:

public void addShutdownHook(Thread hook){}

Runtime r=Runtime.getRuntime();

r.addShutdownHook(new MyThread());

Some important points about shutdown hooks are :

Shutdown hooks initialized but can only be started when JVM shutdown occurred.

Shutdown hooks are more reliable than the finalizer() because there are very fewer chances that shutdown hooks not run.

The shutdown hook can be stopped by calling the halt(int) method of Runtime class.

What is the synchronization?

Synchronization is the capability to control the access of multiple threads to any shared resource. It is used:

To prevent thread interference.

To prevent consistency problem.

When the multiple threads try to do the same task, there is a possibility of an erroneous result, hence to remove this issue, Java uses the process of synchronization which allows only one thread to be executed at a time. Synchronization can be achieved in three ways:

by the synchronized method

by synchronized block

by static synchronization

What is the purpose of the Synchronized block?

The Synchronized block can be used to perform synchronization on any specific resource of the method. Only one thread at a time can execute on a particular resource, and all other threads which attempt to enter the synchronized block are blocked.

Synchronized block is used to lock an object for any shared resource.

The scope of the synchronized block is limited to the block on which, it is applied. Its scope is smaller than a method.

What is static synchronization?

If you make any static method as synchronized, the lock will be on the class not on the object. If we use the synchronized keyword before a method so it will lock the object (one thread can access an object at a time) but if we use static synchronized so it will lock a class (one thread can access a class at a time).

What is the difference between notify() and notifyAll()?

The notify() is used to unblock one waiting thread whereas notifyAll() method is used to unblock all the threads in waiting state.

What is the deadlock?

Deadlock is a situation in which every thread is waiting for a resource which is held by some other waiting thread. In this situation, Neither of the thread executes nor it gets the chance to be executed. Instead, there exists a universal waiting state among all the threads. Deadlock is a very complicated situation which can break our code at runtime.

***JAVA Interview Questions***

How to detect a deadlock condition? How can it be avoided?

We can detect the deadlock condition by running the code on cmd and collecting the Thread Dump, and if any deadlock is present in the code, then a message will appear on cmd.

Ways to avoid the deadlock condition in Java:

Avoid Nested lock: Nested lock is the common reason for deadlock as deadlock occurs when we provide locks to various threads so we should give one lock to only one thread at some particular time.

Avoid unnecessary locks: we must avoid the locks which are not required.

Using thread join: Thread join helps to wait for a thread until another thread doesn’t finish its execution so we can avoid deadlock by maximum use of join method.

What is Thread Scheduler in java?

In Java, when we create the threads, they are supervised with the help of a Thread Scheduler, which is the part of JVM. Thread scheduler is only responsible for deciding which thread should be executed. Thread scheduler uses two mechanisms for scheduling the threads: Preemptive and Time Slicing.

Java thread scheduler also works for deciding the following for a thread:

It selects the priority of the thread.

It determines the waiting time for a thread

It checks the Nature of thread

How is the safety of a thread achieved?

If a method or class object can be used by multiple threads at a time without any race condition, then the class is thread-safe. Thread safety is used to make a program safe to use in multithreaded programming. It can be achieved by the following ways:

Synchronization
Using Volatile keyword
Using a lock based mechanism
Use of atomic wrapper classes

What is race-condition?

A Race condition is a problem which occurs in the multithreaded programming when various threads execute simultaneously accessing a shared resource at the same time. The proper use of synchronization can avoid the Race condition.

What is the volatile keyword in java?

Volatile keyword is used in multithreaded programming to achieve the thread safety, as a change in one volatile variable is visible to all other threads so one variable can be used by one thread at a time.

What do you understand by thread pool?

Java Thread pool represents a group of worker threads, which are waiting for the task to be allocated.

Threads in the thread pool are supervised by the service provider which pulls one thread from the pool and assign a job to it.

After completion of the given task, thread again came to the thread pool.

The size of the thread pool depends on the total number of threads kept at reserve for execution.

The advantages of the thread pool are :

Using a thread pool, performance can be enhanced.
Using a thread pool, better system stability can occur.

What is the difference between Synchronous programming and Asynchronous programming regarding a thread?

Synchronous programming: In Synchronous programming model, a thread is assigned to complete a task and hence thread started working on it, and it is only available for other tasks once it will end the assigned task.

Asynchronous Programming: In Asynchronous programming, one job can be completed by multiple threads and hence it provides maximum usability of the various threads.

What is the Collection framework in Java?

Collection Framework is a combination of classes and interface, which is used to store and manipulate the data in the form of objects. It provides various classes such as ArrayList, Vector, Stack, and HashSet, etc. and interfaces such as List, Queue, Set, etc. for this purpose.

What are the main differences between array and collection?

Array and Collection are somewhat similar regarding storing the references of objects and manipulating the data, but they differ in many ways. The main differences between the array and Collection are defined below:

Arrays are always of fixed size, i.e., a user can not increase or decrease the length of the array according to their requirement or at runtime, but In Collection, size can be changed dynamically as per need.

Arrays can only store homogeneous or similar type objects, but in Collection, heterogeneous objects can be stored.

Arrays cannot provide the ready-made methods for user requirements as sorting, searching, etc. but Collection includes readymade methods to use.

Explain various interfaces used in Collection framework?

Collection framework implements various interfaces, Collection interface and Map interface (java.util.Map) are the mainly used interfaces of Java Collection Framework. List of interfaces of Collection Framework is given below:

Collection interface: Collection (java.util.Collection) is the primary interface, and every collection must implement this interface.

Syntax:

public interface Collection<E>extends Iterable

Where <E> represents that this interface is of Generic type

List interface: List interface extends the Collection interface, and it is an ordered collection of objects. It contains duplicate elements. It also allows random access of elements.

Syntax:

public interface List<E> extends Collection<E>

Set interface: Set (java.util.Set) interface is a collection which cannot contain duplicate elements. It can only include inherited methods of Collection interface

Syntax:

public interface Set<E> extends Collection<E>

Queue interface: Queue (java.util.Queue) interface defines queue data structure, which stores the elements in the form FIFO (first in first out).

Syntax:

public interface Queue<E> extends Collection<E>

Dequeue interface: it is a double-ended-queue. It allows the insertion and removal of elements from both ends. It implants the properties of both Stack and queue so it can perform LIFO (Last in first out) stack and FIFO (first in first out) queue, operations.

Syntax:

public interface Dequeue<E> extends Queue<E>

Map interface: A Map (java.util.Map) represents a key, value pair storage of elements. Map interface does not implement the Collection interface. It can only contain a unique key but can have duplicate elements. There are two interfaces which implement Map in java that are Map interface and Sorted Map.
What is the difference between List and Set?

The List and Set both extend the collection interface. However, there are some differences between the both which are listed below.

The List can contain duplicate elements whereas Set includes unique items.
The List is an ordered collection which maintains the insertion order whereas Set is an unordered collection which does not preserve the insertion order.
The List interface contains a single legacy class which is Vector class whereas Set interface does not have any legacy class.
The List interface can allow n number of null values whereas Set interface only allows a single null value.

What is the difference between HashSet and TreeSet?

The HashSet and TreeSet, both classes, implement Set interface. The differences between the both are listed below.

HashSet maintains no order whereas TreeSet maintains ascending order.
HashSet impended by hash table whereas TreeSet implemented by a Tree structure.
HashSet performs faster than TreeSet.
HashSet is backed by HashMap whereas TreeSet is backed by TreeMap.

What is the difference between Set and Map?

The differences between the Set and Map are given below.

Set contains values only whereas Map contains key and values both.

Set contains unique values whereas Map can contain unique Keys with duplicate values.

Set holds a single number of null value whereas Map can include a single null key with n number of null values.

What is the difference between HashSet and HashMap?

The differences between the HashSet and HashMap are listed below.

HashSet contains only values whereas HashMap includes the entry (key, value). HashSet can be iterated, but HashMap needs to convert into Set to be iterated.

HashSet implements Set interface whereas HashMap implements the Map interface

HashSet cannot have any duplicate value whereas HashMap can contain duplicate values with unique keys.

HashSet contains the only single number of null value whereas HashMap can hold a single null key with n number of null values.

What is the difference between HashMap and TreeMap?

The differences between the HashMap and TreeMap are given below.

HashMap maintains no order, but TreeMap maintains ascending order.

HashMap is implemented by hash table whereas TreeMap is implemented by a Tree structure.

HashMap can be sorted by Key or value whereas TreeMap can be sorted by Key.

HashMap may contain a null key with multiple null values whereas TreeMap cannot hold a null key but can have multiple null values.

What is the difference between Collection and Collections?

The differences between the Collection and Collections are given below.

The Collection is an interface whereas Collections is a class.

The Collection interface provides the standard functionality of data structure to List, Set, and Queue. However, Collections class is to sort and synchronize the collection elements.

The Collection interface provides the methods that can be used for data structure whereas Collections class provides the static methods which can be used for various operation on a collection.

What does the hashCode() method?

The hashCode() method returns a hash code value (an integer number).

The hashCode() method returns the same integer number if two keys (by calling equals() method) are identical.

However, it is possible that two hash code numbers can have different or the same keys.

If two objects do not produce an equal result by using the equals() method, then the hashcode() method will provide the different integer result for both the objects.

Why we override equals() method?

The equals method is used to check whether two objects are the same or not. It needs to be overridden if we want to check the objects based on the property.

For example, Employee is a class that has 3 data members: id, name, and salary. However, we want to check the equality of employee object by the salary. Then, we need to override the equals() method.

What is the advantage of the generic collection?

There are three main advantages of using the generic collection.

If we use the generic class, we don’t need typecasting.
It is type-safe and checked at compile time.
Generic confirms the stability of the code by making it bug detectable at compile time.

What is JDBC?

JDBC is a Java API that is used to connect and execute the query to the database. JDBC API uses JDBC drivers to connect to the database. JDBC API can be used to access tabular data stored into any relational database.

What is JDBC Driver?

JDBC Driver is a software component that enables Java application to interact with the database. There are 4 types of JDBC drivers:

JDBC-ODBC bridge driver: The JDBC-ODBC bridge driver uses the ODBC driver to connect to the database. The JDBC-ODBC bridge driver converts JDBC method calls into the ODBC function calls. This is now discouraged because of the thin driver. It is easy to use and can be easily connected to any database.

Native-API driver (partially java driver): The Native API driver uses the client-side libraries of the database. The driver converts JDBC method calls into native calls of the database API. It is not written entirely in Java. Its performance is better than JDBC-ODBC bridge driver. However, the native driver must be installed on each client machine.

Network Protocol driver (fully java driver): The Network Protocol driver uses middleware (application server) that converts JDBC calls directly or indirectly into the vendor-specific database protocol. It is entirely written in Java. There is no requirement of the client-side library because of the application server that can perform many tasks like auditing, load balancing, logging, etc.

Thin driver (fully java driver): The thin driver converts JDBC calls directly into the vendor-specific database protocol. That is why it is known as the thin driver. It is entirely written in Java language. Its performance is better than all other drivers however these drivers depend upon the database.

What are the steps to connect to the database in java?

The following steps are used in database connectivity.

Registering the driver class:

The forName() method of the Class class is used to register the driver class. This method is used to load the driver class dynamically. Consider the following example to register OracleDriver class.

Class.forName(“oracle.jdbc.driver.OracleDriver”);

Creating connection:

The getConnection() method of DriverManager class is used to establish the connection with the database. The syntax of the getConnection() method is given below.

1) public static Connection getConnection(String url)throws SQLException

2) public static Connection getConnection(String url,String name,String password)

throws SQLException

Consider the following example to establish the connection with the Oracle database.

Connection con=DriverManager.getConnection(

“jdbc:oracle:thin:@localhost:1521:xe”,”system”,”password”);

Creating the statement:

The createStatement() method of Connection interface is used to create the Statement. The object of the Statement is responsible for executing queries with the database.

public Statement createStatement()throws SQLException

consider the following example to create the statement object

Statement stmt=con.createStatement();

Executing the queries:

The executeQuery() method of Statement interface is used to execute queries to the database. This method returns the object of ResultSet that can be used to get all the records of a table.

Syntax of executeQuery() method is given below.

public ResultSet executeQuery(String sql)throws SQLException

Example to execute the query

ResultSet rs=stmt.executeQuery(“select * from emp”);

while(rs.next()){

System.out.println(rs.getInt(1)+” “+rs.getString(2));

}

However, to perform the insert and update operations in the database, executeUpdate() method is used which returns the boolean value to indicate the successful completion of the operation.

Closing connection:

By closing connection, object statement and ResultSet will be closed automatically. The close() method of Connection interface is used to close the connection.

Syntax of close() method is given below.

public void close()throws SQLException

Consider the following example to close the connection.

con.close();

What are the JDBC API components?

The java.sql package contains following interfaces and classes for JDBC API.

Interfaces:

Connection: The Connection object is created by using getConnection() method of DriverManager class. DriverManager is the factory for connection.
Statement: The Statement object is created by using createStatement() method of Connection class. The Connection interface is the factory for Statement.
PreparedStatement: The PrepareStatement object is created by using prepareStatement() method of Connection class. It is used to execute the parameterized query.
ResultSet: The object of ResultSet maintains a cursor pointing to a row of a table. Initially, cursor points before the first row. The executeQuery() method of Statement interface returns the ResultSet object.
ResultSetMetaData: The object of ResultSetMetaData interface cotains the information about the data (table) such as numer of columns, column name, column type, etc. The getMetaData() method of ResultSet returns the object of ResultSetMetaData.
DatabaseMetaData: DatabaseMetaData interface provides methods to get metadata of a database such as the database product name, database product version, driver name, name of the total number of tables, the name of the total number of views, etc. The getMetaData() method of Connection interface returns the object of DatabaseMetaData.
CallableStatement: CallableStatement interface is used to call the stored procedures and functions. We can have business logic on the database through the use of stored procedures and functions that will make the performance better because these are precompiled. The prepareCall() method of Connection interface returns the instance of CallableStatement.

Classes:

DriverManager: The DriverManager class acts as an interface between the user and drivers. It keeps track of the drivers that are available and handles establishing a connection between a database and the appropriate driver. It contains several methods to keep the interaction between the user and drivers.
Blob: Blob stands for the binary large object. It represents a collection of binary data stored as a single entity in the database management system.
Clob: Clob stands for Character large object. It is a data type that is used by various database management systems to store character files. It is similar to Blob except for the difference that BLOB represent binary data such as images, audio and video files, etc. whereas Clob represents character stream data such as character files, etc. SQLException It is an Exception class which provides information on database access errors.
How can we set null value in JDBC PreparedStatement?

By using setNull() method of PreparedStatement interface, we can set the null value to an index. The syntax of the method is given below.

void setNull(int parameterIndex, int sqlType) throws SQLException

What are the benefits of PreparedStatement over Statement?

The benefits of using PreparedStatement over Statement interface is given below.

The PreparedStatement performs faster as compare to Statement because the Statement needs to be compiled everytime we run the code whereas the PreparedStatement compiled once and then execute only on runtime.
PreparedStatement can execute Parameterized query whereas Statement can only run static queries.
The query used in PreparedStatement is appeared to be similar every time. Therefore, the database can reuse the previous access plan whereas, Statement inline the parameters into the String, therefore, the query doesn’t appear to be same every time which prevents cache reusage.

What is the role of the JDBC DriverManager class?

The DriverManager class acts as an interface between user and drivers. It keeps track of the drivers that are available and handles establishing a connection between a database and the appropriate driver. The DriverManager class maintains a list of Driver classes that have registered themselves by calling the method DriverManager.registerDriver().

What are the functions of the JDBC Connection interface?

The Connection interface maintains a session with the database. It can be used for transaction management. It provides factory methods that return the instance of Statement, PreparedStatement, CallableStatement, and DatabaseMetaData.

What does the JDBC ResultSet interface?

The ResultSet object represents a row of a table. It can be used to change the cursor pointer and get the information from the database. By default, ResultSet object can move in the forward direction only and is not updatable. However, we can make this object to move the forward and backward direction by passing either TYPE_SCROLL_INSENSITIVE or TYPE_SCROLL_SENSITIVE in createStatement(int, int) method.

What does the JDBC ResultSetMetaData interface?

The ResultSetMetaData interface returns the information of table such as the total number of columns, column name, column type, etc.

Which interface is responsible for transaction management in JDBC?

The Connection interface provides methods for transaction management such as commit(), rollback() etc.

What is batch processing and how to perform batch processing in JDBC?

By using the batch processing technique in JDBC, we can execute multiple queries. It makes the performance fast. The java.sql.Statement and java.sql.PreparedStatement interfaces provide methods for batch processing. The batch processing in JDBC requires the following steps.

Load the driver class
Create Connection
Create Statement
Add query in the batch
Execute the Batch
Close Connection

What are CLOB and BLOB data types in JDBC?

BLOB: Blob can be defined as the variable-length, binary large object which is used to hold the group of Binary data such as voice, images, and mixed media. It can hold up to 2GB data on MySQL database and 128 GB on Oracle database. BLOB is supported by many databases such as MySQL, Oracle, and DB2 to store the binary data (images, video, audio, and mixed media).

CLOB: Clob can be defined as the variable-length, character-large object which is used to hold the character-based data such as files in many databases. It can hold up to 2 GB on MySQL database, and 128 GB on Oracle Database. A CLOB is considered as a character string.

What are the different types of lockings in JDBC?

A lock is a certain type of software mechanism by using which, we can restrict other users from using the data resource. There are four type of locks given in JDBC that are described below.

Row and Key Locks: These type of locks are used when we update the rows.

Page Locks: These type of locks are applied to a page. They are used in the case, where a transaction remains in the process and is being updated, deleting, or inserting some data in a row of the table. The database server locks the entire page that contains the row. The page lock can be applied once by the database server.

Table locks: Table locks are applied to the table. It can be applied in two ways, i.e., shared and exclusive. Shared lock lets the other transactions to read the table but not update it. However, The exclusive lock prevents others from reading and writing the table.

Database locks: The Database lock is used to prevent the read and update access from other transactions when the database is open.

What is the JDBC Rowset?

JDBC Rowset is the wrapper of ResultSet. It holds tabular data like ResultSet, but it is easy and flexible to use. The implementation classes of RowSet interface are as follows:

JdbcRowSet
CachedRowSet
WebRowSet
JoinRowSet
FilteredRowSet

What is the major difference between java.util.Date and java.sql.Date data type?

The major difference between java.util.Date and java.sql.Date is that, java.sql.Date represents date without time information whereas, java.util.Date represents both date and time information.

What does JDBC setMaxRows method do?

The setMaxRows(int i) method limits the number of rows the database can return by using the query. This can also be done within the query as we can use the limit cause in MySQL

What are the JDBC statements?

In JDBC, Statements are used to send SQL commands to the database and receive data from the database. There are various methods provided by JDBC statements such as execute(), executeUpdate(), executeQuery, etc. which helps you to interact with the database.

There is three type of JDBC statements given in the following table.

Statements	Explanation
Statement	Statement is the factory for resultset. It is used for general purpose access to the database. It executes a static SQL query at runtime.
PreparedStatement	The PreparedStatement is used when we need to provide input parameters to the query at runtime.
CallableStatement	CallableStatement is used when we need to access the database stored procedures. It can also accept runtime parameters.

What are the differences between Statement and PreparedStatement interface?

Statement	PreparedStatement
The Statement interface provides methods to execute queries with the database. The statement interface is a factory of ResultSet; i.e., it provides the factory method to get the object of ResultSet.	The PreparedStatement interface is a subinterface of Statement. It is used to execute the parameterized query.
In the case of Statement, the query is compiled each time we run the program.	In the case of PreparedStatement, the query is compiled only once.
The Statement is mainly used in the case when we need to run the static query at runtime.	PreparedStatement is used when we need to provide input parameters to the query at runtime.

What are the differences between execute, executeQuery, and executeUpdate?

execute	executeQuery	executeUpdate
The execute method can be used for any SQL statements(Select and Update both).	The executeQuery method can be used only with the select statement.	The executeUpdate method can be used to update/delete/insert operations in the database.
The execute method returns a boolean type value where true indicates that the ResultSet s returned which can later be extracted and false indicates that the integer or void value is returned.	The executeQuery() method returns a ResultSet object which contains the data retrieved by the select statement.	The executeUpdate() method returns an integer value representing the number of records affected where 0 indicates that query returns nothing.

What are the different types of ResultSet?

ResultSet is categorized by the direction of the reading head and sensitivity or insensitivity of the result provided by it. There are three general types of ResultSet.

Type	Description
ResultSet.TYPE_Forward_ONLY	The cursor can move in the forward direction only.
ResultSet.TYPE_SCROLL_INSENSITIVE	The cursor can move in both the direction (forward and backward). The ResultSet is not sensitive to the changes made by the others to the database.
ResultSet.TYPE_SCROLL_SENSITIVE	The cursor can move in both the direction. The ResultSet is sensitive to the changes made by the others to the database.

What are the differences between ResultSet and RowSet?

ResultSet	RowSet
ResultSet cannot be serialized as it maintains the connection with the database.	RowSet is disconnected from the database and can be serialized.
ResultSet object is not a JavaBean object	ResultSet Object is a JavaBean object.
ResultSet is returned by the executeQuery() method of Statement Interface.	Rowset Interface extends ResultSet Interface and returned by calling the RowSetProvider.newFactory().createJdbcRowSet() method.
ResultSet object is non-scrollable and non-updatable by default.	RowSet object is scrollable and updatable by default.

What are the differences between stored procedure and functions?

The differences between stored procedures and functions are given below:

Stored Procedure	Function
Is used to perform business logic.	Is used to perform the calculation.
Must not have the return type.	Must have the return type.
May return 0 or more values.	May return only one value.
The procedure supports input and output parameters.	The function supports only input parameter.
Exception handling using try/catch block can be used in stored procedures.	Exception handling using try/catch can’t be used in user-defined functions.

What is the difference between ArrayList and Vector?

No.	ArrayList	Vector
1)	ArrayList is not synchronized.	Vector is synchronized.
2)	ArrayList is not a legacy class.	Vector is a legacy class.
3)	ArrayList increases its size by 50% of the array size.	Vector increases its size by doubling the array size.
4)	ArrayList is not “thread-safe” as it is not synchronized.	Vector list is “thread-safe” as it?s every method is synchronized.

What is the difference between ArrayList and LinkedList?

No.	ArrayList	LinkedList
1)	ArrayList uses a dynamic array.	LinkedList uses a doubly linked list.
2)	ArrayList is not efficient for manipulation because too much is required.	LinkedList is efficient for manipulation.
3)	ArrayList is better to store and fetch data.	LinkedList is better to manipulate data.
4)	ArrayList provides random access.	LinkedList does not provide random access.
5)	ArrayList takes less memory overhead as it stores only object	LinkedList takes more memory overhead, as it stores the object as well as the address of that object.

What is the difference between Iterator and ListIterator?

Iterator traverses the elements in the forward direction only whereas ListIterator traverses the elements into forward and backward direction.

No.	Iterator	ListIterator
1)	The Iterator traverses the elements in the forward direction only.	ListIterator traverses the elements in backward and forward directions both.
2)	The Iterator can be used in List, Set, and Queue.	ListIterator can be used in List only.
3)	The Iterator can only perform remove operation while traversing the collection.	ListIterator can perform ?add,? ?remove,? and ?set? operation while traversing the collection.

What is the difference between Iterator and Enumeration?

No.	Iterator	Enumeration
1)	The Iterator can traverse legacy and non-legacy elements.	Enumeration can traverse only legacy elements.
2)	The Iterator is fail-fast.	Enumeration is not fail-fast.
3)	The Iterator is slower than Enumeration.	Enumeration is faster than Iterator.
4)	The Iterator can perform remove operation while traversing the collection.	The Enumeration can perform only traverse operation on the collection.

What is the difference between HashMap and Hashtable?

No.	HashMap	Hashtable
1)	HashMap is not synchronized.	Hashtable is synchronized.
2)	HashMap can contain one null key and multiple null values.	Hashtable cannot contain any null key or null value.
3)	HashMap is not ?thread-safe,? so it is useful for non-threaded applications.	Hashtable is thread-safe, and it can be shared between various threads.
4)	4) HashMap inherits the AbstractMap class	Hashtable inherits the Dictionary class.

What is the difference between Comparable and Comparator?

No.	Comparable	Comparator
1)	Comparable provides only one sort of sequence.	The Comparator provides multiple sorts of sequences.
2)	It provides one method named compareTo().	It provides one method named compare().
3)	It is found in java.lang package.	It is located in java.util package.
4)	If we implement the Comparable interface, The actual class is modified.	The actual class is not changed.

What is the difference between wait() and sleep() method?

wait()	sleep()
1) The wait() method is defined in Object class.	The sleep() method is defined in Thread class.
2) The wait() method releases the lock.	The sleep() method doesn’t release the lock.

What are the differences between processes and threads?

	Process	Thread
Definition	An executing instance of a program is called a process.	A thread is a subset of the process.
Communication	Processes must use inter-process communication to communicate with sibling processes.	Threads can directly communicate with other threads of its process.
Control	Processes can only exercise control over child processes.	Threads can exercise considerable control over threads of the same process.
Changes	Any change in the parent process does not affect child processes.	Any change in the main thread may affect the behavior of the other threads of the process.
Memory	Run in separate memory spaces.	Run in shared memory spaces.
Controlled by	Process is controlled by the operating system.	Threads are controlled by programmer in a program.
Dependence	Processes are independent.	Threads are dependent.

What is a finally block? Is there a case when finally will not execute?

Finally block is a block which always executes a set of statements. It is always associated with a try block regardless of any exception that occurs or not.
Yes, finally will not be executed if the program exits either by calling System.exit() or by causing a fatal error that causes the process to abort.

***JAVA Interview Questions*** ***JAVA Interview Questions*** ***JAVA Interview Questions***

What are the important methods of Java Exception Class?

Methods are defined in the base class Throwable. Some of the important methods of Java exception class are stated below.

String getMessage()– This method returns the message String about the exception. The message can be provided through its constructor.
public StackTraceElement[] getStackTrace() – This method returns an array containing each element on the stack trace. The element at index 0 represents the top of the call stack whereas the last element in the array represents the method at the bottom of the call stack.
Synchronized Throwable getCause()– This method returns the cause of the exception or null id as represented by a Throwable object.
String toString()– This method returns the information in String format. The returned String contains the name of Throwable class and localized message.
void printStackTrace()– This method prints the stack trace information to the standard error stream.
What is OutOfMemoryError in Java?

OutOfMemoryError is the subclass of java.lang.Error which generally occurs when our JVM runs out of memory.

What are the different types of garbage collectors in Java?

Garbage collection in Java a program which helps in implicit memory management. Since in Java, using the new keyword you can create objects dynamically, which once created will consume some memory. Once the job is done and there are no more references left to the object, Java using garbage collection destroys the object and relieves the memory occupied by it. Java provides four types of garbage collectors:

Serial Garbage Collector
Parallel Garbage Collector
CMS Garbage Collector
G1 Garbage Collector

Can we Overload or Override static methods in java?

Overriding: Overriding is related to run-time polymorphism. A subclass (or derived class) provides a specific implementation of a method in superclass (or base class) at runtime.

Overloading: Overloading is related to compile time (or static) polymorphism. This feature allows different methods to have same name, but different signatures, especially number of input parameters and type of input paramaters.

Can we overload static methods? The answer is ‘Yes’. We can have two ore more static methods with same name, but differences in input parameters

Can we Override static methods in java? We can declare static methods with same signature in subclass, but it is not considered overriding as there won’t be any run-time polymorphism. Hence the answer is ‘No’. Static methods cannot be overridden because method overriding only occurs in the context of dynamic (i.e. runtime) lookup of methods. Static methods (by their name) are looked up statically (i.e. at compile-time).

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