Java Interview Questions

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By Vijay Singh Khatri

Java is one of the most extensively used programming languages. It is used in IT companies as a basic development language to design and develop software. Around the world, approximately 10 million people use Java, and there are 15 billion Java-supported devices. It is used for the development of software related to Big Data and several household devices like DTH boxes, house security, and smartphones.

Consequently, the demand for professionals that have expert knowledge of Java has increased exponentially over a short period of time. Therefore, the companies that hire Java professionals are always looking for expert candidates in this field. There are certain concepts and questions about which recruiters frequently ask during the interview session. In this article, we are going to cover all these questions to help you crack your interview successfully.

Frequently asked Java Interview Question

If you are preparing for a job interview that is hiring Java professionals, there are some questions you should know as they are frequently asked by the recruiters. Let us see some of the questions asked very frequently in Java interviews.

What are JVM, JRE, and JDK?

The Java Virtual Machine is often known as the JVM. The JVM works as an abstract virtual machine and is responsible for providing a run-time environment. Java byte codes are generally executed in the environment provided by the JVM.

There are three notations according to which JVM works, which are as follows:

  • Runtime Instance
  • Implementation
  • Specification

JRE is the abbreviation for Java Runtime Environment, which is a runtime environment that executes the Java Bytecodes. JRE is an implemented form of the Java Virtual Machine that exists in a physical state.

Java Development Kit is abbreviated as JDK. It is one of the most essential kits that is used to package, document, compile, and run Java programs.

While writing a Java program, what is the importance of the syntax: public static void main(String args[]).

The syntax: public static void main(String args[]) is used to denote the main method of the class. Additionally, this syntax is used to call the main method of the program which has the primary function of the program. Let us understand the function of each keyword specifically:

  • The public is the access modifier responsible for defining the access of the method. The term public means that the method can be accessed for any of the classes of the program.
  • Static is one of the keywords of Java. It is used to identify if the method is class-based or object-based. In the above syntax, the main method is made to be static. Therefore, it allows access without the creation of any instances of the given class. It is essential to specify the main method as static, otherwise, the compiler will give a runtime error, since the main method is not being invoked by the class directly.
  • Void is used to define the return type. The above syntax means that the above main method will not return a null value.
  • Main defines the main execution that the program is responsible for. The JVM searches for the main method to understand what is the starting point for the program.
  • String args [] are the parameters that are passed through to the main method.

Is the Java Platform Independent?

Yes, Java is regarded as a platform-independent programming language. This is due to the fact that every bytecode of Java can run on several systems and any of the operating systems.

Is Java 100% an object-oriented language?

  • No, Java is not completely object-oriented. There are eight primitive data types that process different values, such as int, byte, short, long, double, float, char, and Boolean. They accept different integer values and return them with the output.


What is a Wrapper Class?

Wrapper classes are a type of Java primitive class that is used as a reference object. The primitive data types have a class that is strictly dedicated to them. The term “wrapper” is given to them as they are responsible for wrapping data types of the class into objects.

Explain Constructors?

Constructors are a block of written codes that are defined to initialize the function of an object. The name of the constructor in the program must be the same as the class name. The constructor class is used to automatically call the object at the time of creation and it does not have a return type. Constructors are divided into two types: Default Constructors and Parameterized Constructors.

  • Default Constructor: A constructor is defined as default if there are no arguments constructor, which means that the programmer has not defined any constructor. It is used for the creation of objects and serves as an initializer for instance variables.
  • Parameterized Constructor: A constructor with the ability to initialize instance variables that have defined values is known as a parameterized constructor. Therefore, a constructor that has arguments is defined as parameterized.

What is an Array list? And How is it Different from a Vector?

An array list is a non-synchronous list that has high processing. The size of an array is doubled when any element is added to it. However, the array list has no definite increment size and is limited to only using iterators when they want to traverse another array list.

A Vector is different from an array list in several ways. It is synchronized and is comparatively slow as they are thread-safe. Also, unlike array lists, a vector has an increment size, and its size doubles by default. Additionally traversing, uses both Iterator and Enumeration.

Are equals() and == different in Java?

Equals () is a method in Java that is used to check equality between two variables and the logic of two different objects. Whereas “==” is known as the equality operator, which is used to compare two different primitives with each other or with other objects. “==” is a Boolean operator which is used in Java to compare values.

Explain the Difference between Stack Memory and Heap Memory?

Here are the differences between stack memory and heap memory:

  • Stack Memory can only be used for the execution of one thread. Whereas, heap memory is processed by every section of the program and application.
  • The heap memory provides global access for its objects, whereas the stack memory prevents the access of its objects from any other thread.
  • Stack Memory follows Last-In-First-Out to clear its memory. Whereas, in Heap memory, the generation of each memory is linked to every object.
  • Stack memory has the feature of lasting until the execution ends for the given thread. Whereas, heap memory has the lifetime of existing from start to end.
  • A newly created object is stored in the heap memory, whereas the stack memory has all the local variables.

Explain Packages. Also, what are the advantages of using Packages?

All the related classes and the interfaces are bundled together to form a collection known as Packages. It allows the developers to easily modularize several codes and then optimize them for reuse. Additionally, the syntax that is being used in the packages can also be imported to other classes and can be reused accordingly.

The advantages of packages are:

  • It allows programmers to avoid any kind of clashes between names.
  • Programmers have better control of the syntax.
  • It helps in the creation of a hierarchical structure so that the classes could be easily located.

Explain Object Oriented Programming?

OOPs, or Object-Oriented Programming is a programming method in which programs are organized into a structure. The program is more oriented towards the object and holds no reference to functions or their logic. OOPs, are used by programmers that need to create complex and lengthy programs, which have to be constantly updated and have logic that has no part in it. Java is an object-oriented programming language.

Explain the different concepts of Object-Oriented Programming language.

There are several concepts of OOPs that have made it very popular. Let us understand some of the concepts that are associated with them.

  • Inheritance: When one of the classes acquires the property of another class, it is known as inheritance.
  • Encapsulation: It is known as the wrapping up of data together with the syntax in order to work as a single unit.
  • Abstraction: It is the process of hiding the details of implementations from programmers and only showing the functionality.
  • Polymorphism: When an object, variable, or function has the ability to develop multiple forms of itself, it is known as polymorphism.

Explain the difference between an Instance and a Local Variable?

The scope of a local variable is limited to the local method and is generally used inside blocks, methods, or constructors, allowing the program to use the variable only inside the local method. The benefit of defining a local variable is that none of the other methods can interpret the variable.

Whereas, instance variables are defined within the main method so that they can be used in the entire class. These variables are said to be declared before declaring the method. Additionally, every object that is declared in the class has a separate copy of its variables. Therefore, when used in a particular method, they do not tend to alter the value of any other variable.

Explain the difference between a Method and a Constructor.

A method and a constructor have the same function. But, let us see how they are different:

  • A method represents the behavior of the current object. Whereas, a constructor generally initializes the state in which the object is.
  • A method has a return type, whereas a constructor does not have a return type.
  • A method must be invoked explicitly, whereas constructors are generally called inside.
  • In a method, the compiler does not provide a default method. Whereas, in a constructor, the compiler does provide a default constructor.
  • The name of the constructor is always the same as that of the class, whereas there is no such case in a method.

Explain everything about the keyword Final?

The keyword Final works as a non-access modifier and is considered special in Java. There are different contexts in which the keyword can be used.

  • Final variable

Once the value of a variable assigned as final is declared, it cannot be altered again. A value can only be assigned to them if there are no values initially assigned to them.

  • Final method

A method that has been declared final cannot be overridden by any derived class.

  • Final class

A class that has been declared as final cannot have any more subclasses. However, it can extend any other subsequent class within the program.

Final Words

Java is a very broad field of programming. It provides one of the best programming languages which is very easy to learn and implement. It works together with several other systems and does not need a runtime environment. It is a high-level programming language that has special and powerful tools that can help in the development of any application.

It is easy to use syntax which is integrated with several other features of Java, as packages make programming easier, and help in calling different pre-recorded functions easily. Many companies use Java as their programming language to develop several products and software. Therefore, they are always in need of programmers and developers that have knowledge of Java. Therefore, if you are actively looking for a job in any of the IT companies, you must have better knowledge of Java and all of its concepts.

We have tried to cover as many possible Java interview questions that are very frequently asked. Therefore, if you are looking to get a job in an IT company that is looking for a Java programmer, keep the above-discussed questions in mind and make an impression on the interviewer.

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