Java Interview Questions and Answers

java interview questions and answers

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Best Java Interview Questions and Answers

Java is one of the reliably used programming languages in all organizations. And, so you will find maximum job opportunities in the field of Java. Here, we have bought you some of the top 50 Java interview questions and answers created in consultation with expert interviews and certified Java experts. Prepare all these questions to excel in your Java career. We have also discussed with the aspirants who have attended Java interviews recently and so preparing these questions will elevate your confidence and will assist you in cracking the interviews.

So, if you are preparing to attend Core Java interviews, make use of these Java interview questions. We have compiled almost every concepts and topics in line with Java. These questions can be used by both professionals in the Java field who wish to extend their career roles and freshers who wish to start up their career in the field of Java. Don’t miss revising all these top 50 Java interview questions and answers before your Java interview. We wish you all success in your future career.

Java developed by James Gosling in the year 1991 is a platform-independent, object-oriented, robust and secure programming language, high-level multithreaded portable programming language that ensures maximum performance.

C++

Java

C++ is highly platform dependent

Java is truly platform independent

It is reliably utilized in the case of system programming

Java is reliably utilized in the case of application programming

C++ actually supports both operator overloading and Goto statement

Java does not support both operator overloading and Goto statement

It makes use of only compilers

Java makes use of both interpreters and compilers

Yes, absolutely Java is platform independent and this is because the byte codes of Java can operate on any platform that consists of any operating system like Linux, Windows, Mac, Sun Solaris and more. Thus, Java is totally platform independent.

A platform is nothing but a software or hardware environment in which a segment of software is performing and executing. It can be of both hardware-based and software-based while Java offers a high-end top performing software-based platform.

No, Java is not at all 100% object-oriented because Java also uses all the eight different primitive data types which include int, float, short, long, double, char, byte, Boolean which does not come under objects.

JDK

JRE

JVM

The full form of JDK is nothing but Java Development Kit

The full form of JRE is nothing but Java Runtime Environment

The full form of JVM is nothing but Java Virtual Machine

With the help of JDK, we can easily compile, pack or document all the required Java programs

JRE actually denotes a runtime environment where all the byte codes of Java run and operate

JVM is actually an abstract machine that offers a runtime environment where all the byte codes of Java can operate efficiently

It consists of both Development Kit and JRE

It is actually the process of implementing the most physically existing JVM

JVM undergoes three main notations which include implementation, runtime instance and specification

There are many types of memory areas allocated by Java while some are Heap, Stack, Class (Method) Area, Program Counter Register and Native Method Stack.

JIT compiler actually compiles all the identical segments of byte codes that ensure similar functionality at the same point of time in order to decrease the compilation time and to maximize the performance. The compiler actually translates the JVM (Java Virtual Machine) instruction sets into a specific set CPU instruction sets.

The most important essential and exceptional features of Java are as follows:

  • It is both simple and object-oriented
  • It is portable, architecture neutral and platform-independent
  • Java is highly robust and secure programming language
  • It ensures high performance
  • It is distributed, dynamic, multithreaded and interpreted

This habitat of Java comes from the byte code which acts as an intermediator between the both source and the machine code. Also, byte code is not platform dependent and so it can perform well in any system or computer.

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A Java Class is like a blueprint that holds all the data which includes variables (fields) and methods which can define the characteristics of the associated object.

Syntax of Java Class is as follows:

class ABC {

member variables // class body

methods}

A sub system of Java Virtual Machine that is used to effectively load all the class files is known as the classloader which initially loads every Java program.

A class that is nested within another specialized class is known as the inner class which has the access permission to operate classes nested within and the methods or variables that are found in the outer class.

A class that inherits from the super class or another class is defined as the sub class which can access all the protected methods, fields and public methods found in the super class.

The class that is required to convert Java primitives into the specified reference type or object is known as the wrapper class. All the primitive data types of Java consist of a particular data that has been completely dedicated to those data types. The wrapper class actually wraps all the primitive data types in the form of an object.

In Java, the singleton class consists of only one instance to which all the methods and variables belong to and this can be used in such criteria where there will be no need to restrict the number of objects belonging to a class.

Yes, in Java you can save a file in the form of .java which has to be compiled in the form of javac .java and can be executed with the help of java classname.

The keywords in Java that are used effectively to define the access scope of the variables, method or a class in known as the access specifiers.

There are four different types of access specifiers found in Java namely public, protected, default and private.

No, I will not face any error during the java program compilation because java does not consider the order of specifiers and we will get the resultant output without any hassles.

The static keyword like static method or static variable is used to share methods or variables among several objects belonging to a particular class apart from generating individual copies for each of the specific object.

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The most important benefits of packages in Java are as follows:

  • Name clashes are being avoided by packages in Java and it locates the related classes easily
  • Packages offer instant and effective access control
  • Packages can also hold some of the hidden class and protect it from displaying outside and used within the packages

The looping methodology in Java executes a particular statement or a block of specific statement as per programming continuously or repeatedly

There are three main looping used in Java namely – For loops, While loops and Do While loops.

The two essential keywords that are utilized in looping are break and continue statement, while the break statement is used in loop, the looping process will be instantly broken and if continue statement is used, the present iteration will be stopped and the next iteration will be continued or executed effectively.

An infinite loop in Java actually runs infinitely and continuously without any condition in order to break an infinite loop, we will have to add break logic along with the block of statements.

Syntax for infinite loop is as follows:

for (;;)

{

// statement to be executed

// Affix any specific break logic

}

An object holds several methods and classes that defines the state of the object and executes perfectly. There are various objects found in a Java program that instructs their role and activity within each other. This is actually a segment of core Java.

A particular programming paradigm in accordance with the objects that consists of methods and data that belongs to a specific class is known as an object-oriented paradigm, this brings out the benefits of reusability and modularity while objects are nothing but class instance that interacts with each other to develop reliable programs and applications.

The most useful features of object-oriented paradigm are as follows:

  • In undergoes the bottom-up approach to design a program effectively
  • In concentrates on the data that consists of necessary methods in order to operate effectively on the object’s data
  • It makes use of the encapsulation and abstraction techniques
  • Some of the object-oriented paradigm includes Simula, Python, C++, Smalltalk, C# and more

Object-oriented Programming Language

Object-based Programming Language

It undergoes OOPs concepts effectively

It does not follow any of the OOPs concept like polymorphism or inheritance

It does not consist of any in-built objects

It consists of several in-built objects

Some of the examples of object-oriented programming language include C#, Java, Smalltalk and more

Some of the examples of object-based programming language include VBScript, JavaScript and more

The ability of functions, variables and objects to take several different forms is known as polymorphism. In short it can be defined as “one interface and many implementations”.

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Polymorphism is of two types namely compile time polymorphism and run time polymorphism. A compile time polymorphism is known as method overloading while run time polymorphism can be achieved by using an interface or inheritance.

A method that can initialize the state of a particular object is known as the constructor. It comes into act only when the class is instantiated and the specific memory is allocated for the particular object. Whenever a new class is generated with a help of the new keyword, the constructor of that particular class will be called appropriately. The constructor same should resemble the class name and it should consist of an explicit return type.

In Java, there are two main constructors used and they are default constructor and parameterized constructor.

The constructor that is used to allocate default values to a particular object is known as the default constructor and the compiler of java implicitly creates a default constructor if the class consists of no constructor at all.

Float actually takes 4 bytes of memory while double takes 8 bytes of memory and float is defined to be a single precision floating point decimal number while double is defined to be a double precision decimal number.

StringBuffer

StringBuilder

It is synchronized and thread-safe

It is non-synchronized and not thread-safe

Heap is the storage area of StringBuffer and it can be modified instantly

Heap is the storage area of StringBuilder and it can be modified instantly

Slow performance

Speedy performance

Inheritance which comes under the object-oriented programming concept is used to inherit the derived class with available methods found in the base class.

Listed below are some of the useful techniques which can be used to avoid inheritance:

With the help of final keyword, private constructors and javadoc comment (“//”)

If all the methods are made final then there is no necessity to override that particular method

As we cannot utilize several methods in a single class, multiple inheritances are strictly prohibited or it is not used in Java.

In case a Java program consists of several methods with the same name but with different functionalities, then it can be referred to as the method overloading in Java.

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Overloading occurs when a Java program consists of several methods with the same name but with different functionalities while overriding occurs when two methods come with the same name and functionality but one method will be from the parent class and the other will be from the child class.

JSON is nothing but JavaScript Object Notation which is a substitute to XML and it is highly readable and lighter. It is an independent programming language and can be effectively used in server-server or client-server communication.

The main reasons why JSON should be preferred over XML are as follows:

  • JSON is highly readable and efficiently lighter than XML
  • It is independent, understandable and parse-able
  • XML supports only strings while JSON supports string, array, number or Boolean

Pointers actually makes the program complex and unsafe, as Java is to make the complete code simple, pointers cannot be used in Java.

Methods

Constructors

It defines the object’s behavior

It initializes the object’ state

It has a return type

It does not have a return type

Compiler does not provide any default method

In case the class does not have a constructor, then compiler provides a default constructor

The name of the method can be same or need not be same as the name of the class

The constructor name and the class name should always be similar

A non-access modifier or a special keyword type used in Java is known as the final keyword which includes final variable, final method and final class.

The collection of strings which are effectively stored in a Heap is known as the Java string pool. Once a new object is created in the Java program, the work of the string pool is to check the whether the similar types of objects are already available or not.  In case if similar object is present, then the string pool will return the same reference else it will generate respective reference and return the newly created one.

Static Method

Non-static Method

A static keyword is generally used before the name of the method

Here a static keyword is not necessary to be affixed before the name of the method

With a static keyword, we cannot access any of the non-static instances like methods or variables

Without the need to create any instance of the class, we can access both static variables and methods

The process of calling one constructor from the other in accordance with the current object is known as constructor chaining.

Constructor chaining is achieved by the two methods mentioned below:

  • Within the same class with the help of this()
  • From base class with the help of super()

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