Top 50 Java Interview Questions and Answers

Java Interview Questions and Answers: An Overview

Java is a high-level versatile programming language. It has a wide range of applications. Almost all the big companies like Amazon and Google hire Java Developers. Though you have learned Java or you are a Certified Java Developer you need to be aware of Java Technical Interview Questions to crack the technical round of interview. Therefore the following article provides you with a comprehensive guide on Java Interview Questions and Answers to help you prepare for interviews.

Basic Java Interview Questions for Freshers

1. Differentiate between Java and C++.

The following differences are there:

• C++ is only a compiled language, whereas Java is a compiled and interpreted language.
• Java programs are machine-independent whereas a C++ program can run only on the machine on which it is compiled.
• C++ allows users to use pointers in the program. Whereas it's not the case with Java. Java internally uses pointers.
• C++ supports the concept of multiple inheritances while Java doesn't support this. It can be achieved by using interfaces in Java.

2. Is Java a complete object-oriented programming language?

• Everything in Java is under the classes. And we can access that by creating the objects. This makes it an object-oriented language.
• But it has the support of primitive data types like int, float, char, boolean, double, etc. This is not a feature of object-oriented languages.
• Therefore, Java is not a pure object-oriented programming language because the primitive data types it supports don't directly belong to the Integer classes.

3. What is JVM?

JVM stands for Java Virtual Machine. It is a Java interpreter. It is responsible for loading, verifying, and executing the bytecode created in Java. Its implementation is known as JRE. Although it is platform dependent which means the software of JVM is different for different Operating Systems it plays a vital role in making Java platform Independent.

4. What do you mean by JIT?

JIT stands for Just-in-Time compiler and is a part of JRE(Java Runtime Environment). It is used for better performance of the Java applications during run-time. The step-by-step working process of JIT is mentioned below:

• Source code is compiled with Javac compiler to form bytecode
• Bytecode is further passed on to JVM. The .class files are loaded at run time by JVM and with the help of an interpreter, these are converted to machine-understandable code.
• JIT is a part of JVM. When the JIT compiler is enabled, the JVM analyzes the method calls in the .class files and compiles them to get more efficient and native machine code at run time. It also ensures that the prioritized method calls are optimized.
• The JVM then executes the optimized code directly instead of interpreting the code again. This increases the performance and speed of the execution.

5. What gives Java its 'write once and run anywhere' nature?

It is the bytecode. Java compiler converts the Java programs into the .class file known as the 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 machine.

6. What is a ClassLoader?

Java Classloader is the program of JRE (Java Runtime Environment). The task of ClassLoader is to dynamically load the required Java classes and interfaces to the JVM during the execution of the bytecode or created .class file. Because of classloaders, the Java run time system does not need to know about files and file systems.

7. Explain public static void main(String args[]) in Java.

• public: the public is the access modifier responsible for mentioning who can access the element or the method and what is the limit. It is responsible for making the main function globally available. It is made public so that JVM can invoke it from outside the class as it is not present in the current class.
• static: It is a keyword with which you can use the element without initiating the class to avoid the unnecessary allocation of the memory.
• void: void is a keyword used to specify that a method doesn’t return anything. We use void with the main function because it doesn’t return anything.
• main: main helps JVM to identify that the declared function is the main function.
• String args[]: It stores Java command-line arguments and is an array of type java.lang.String class.

8. Can the main method be overloaded?

Yes, the main method can be overloaded. We can create as many overloaded main methods as we want. However, JVM has a predefined calling method that JVM will only call the main method with the definition of

public static void main(string[] args)

Example

We'll see the demonstration in the Java Compiler

class Main {
 public static void main(String[] args) {
 System.out.println("This is the main method");
 }

 public static void main(int[] args) {
 System.out.println("Overloaded integer array main method");
 }

 public static void main(char[] args) {
 System.out.println("Overloaded character array main method");
 }

 public static void main(double[] args) {
 System.out.println("Overloaded double array main method");
 }

 public static void main(float args) {
 System.out.println("Overloaded float main method");
 }
}

Output

This is the main method

9. What are the different data types in Java?

The data types in Java are categorized into two categories as mentioned below:

1. Primitive Data Type
2. Non-primitive data Type or Object Data type

1. Primitive Data Type: Primitive data are single values with no special capabilities. There are 8 primitive data types in Java:
   1. boolean: stores value true or false
   2. byte: stores an 8-bit signed two’s complement integer
   3. char: stores a single 16-bit Unicode character
   4. short: stores a 16-bit signed two’s complement integer
   5. int: stores a 32-bit signed two’s complement integer
   6. long: stores a 64-bit two’s complement integer
   7. float: stores a single-precision 32-bit IEEE 754 floating-point
   8. double: stores a double-precision 64-bit IEEE 754 floating-point
2. Non-Primitive Data Type: They are reference data types that contain a memory address of the variable’s values because it is not able to directly store the values in the memory. Types of Non-Primitive data types are:
   1. String
   2. Array
   3. Class
   4. Object
   5. Interface

10. How many types of operators are there in Java?

There are multiple types of operators in Java. They are as follows:

1. Arithmetic operators in java
2. Relational operators in java
3. Logical operators
4. Assignment operators
5. Unary operators
6. Bitwise operators

11. Differentiate between equals() method and equality operator == in Java

12. What are the main concepts of OOPs in Java?

The main OOPs concepts in Java are:

• Inheritance
• Polymorphism
• Abstraction
• Encapsulation

13. What is a Class Variable?

Class variables also known as static variable are declared with the static keyword in a class, but outside a method, constructor or a block. There would only be one copy of each class variable per class, regardless of how many objects are created from it.

class Example {
 public static int ctr = 0;

 public Example() {
 ctr++;
 }

 public static void main(String[] args) {
 Example obj1 = new Example();
 Example obj2 = new Example();
 Example obj3 = new Example();

 System.out.println("Number of objects created are " + Example.ctr);
 }
}

Output

Number of objects created are 3

Read more: Variables in Java

14. How is an infinite loop declared in Java?

We will see how to make all three loops in Java an infinite one.

1. for Loop

   
   for (;;)
   {
    //body of the for loop... 
   }
   

2. while loop

   
   while(true) 
   { 
    // body of the loop...
   } 
   

3. do...while loop

   
   do 
   { 
    // body of the loop... 
   }while(true);

15. What if I write static public void instead of public static void?

The Java program will run successfully if you write a static public void main instead of the conventional public static void main. Java is flexible regarding the order of access modifiers, and both declarations are valid for the main method.

16. What are the key differences between Java constructors and Java methods?

17. When a byte datatype is used?

The byte data type can be useful for saving memory in large arrays, where memory savings matters. They can also be used in place of int where their limits help to clarify your code.

18. What is method overloading?

Method overloading in Java works for two or more methods or functions stored in the same class with the same name

1. By Changing the number of arguments
2. By Changing the data type of arguments

Read More: compile time polymorphism in Java

19. What are the various access specifiers in Java?

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

20. What is a dot operator in Java?

Dot operator in Java is a syntactic element. It is used for three purposes:

1. It separates a variable and method from a reference variable.
2. It is also used to access classes and sub-packages from a package.
3. It is also used to access the member of a package or a class.

Example

System.out.println("This is the main method");
import java.io.*; 

Intermediate Java Interview Questions and Answers

21. In how many ways can you take input from the console?

We can do this in four ways in Java Online Compiler:

1. Using Command line argument

   
   class ScholarHat {
    public static void main(String[] args) {
    // Check if the length of the args array is greater than 0
    if (args.length > 0) {
    System.out.println("The command line arguments are:");
    // Iterating the args array and printing the command line arguments
    for (String value : args)
    System.out.println(value);
    } else
    System.out.println("No command line arguments found.");
    }
   }
   

   Run Code >>
   • Compile the code in cmd using the below command

     javac ScholarHat.java
     

   • Run the program with command line arguments

     java ScholarHat WelcometoScholarHat
     

   Output

   The command line arguments are:
   WelcometoScholarHat
   

2. Using Buffered Reader Class

   
   import java.io.*;
   
   class ScholarHat {
    public static void main(String[] args) throws IOException {
    // Enter data using BufferedReader
    BufferedReader reader = new BufferedReader(new InputStreamReader(System.in));
   
    System.out.println("Please enter some text:");
   
    // Reading data using readLine
    String input = reader.readLine();
    
    // Printing the read line
    System.out.println("You entered: " + input);
    }
   }
   

   Run Code >>

   Output

   Please enter some text:
   Hello, ScholarHat
   You entered: Hello, ScholarHat
   

3. Using Console Class

   
   public class ScholarHat { 
    public static void main(String[] args) 
    { 
    // Using Console to input data from the user 
    String x = System.console().readLine(); 
    System.out.println("You entered string " + x); 
    } 
   }
   

   Run Code >>

   Output

   You entered string Hello, ScholarHat!
   

4. Using Scanner Class

   
   import java.util.Scanner;
   
   public class ScholarHat {
    public static void main(String[] args) {
    Scanner in = new Scanner(System.in);
    System.out.print("Enter a string: ");
    String str = in.nextLine();
    System.out.println("You entered string " + str);
    }
   }
   

   Run Code >>

   Output

   Enter a string:
   Hello, ScholarHat
   You entered string Hello, ScholarHat
   

22. What do we get in the JDK file?

JDK (Java Development Kit) is the package that contains various tools, Compilers, Java Runtime Environment, etc.

1. JRE: Java Runtime Environment contains a library of Java classes + JVM. JAVA Classes contain some predefined methods that help Java programs to use that feature, build, and execute. We know that there is a system class in Java that contains the print-stream method, and with the help of this, we can print something on the console.
2. JVM: Java Virtual Machine is a part of JRE that executes the Java program at the end. It is part of JRE, but it is software that converts bytecode into machine-executable code to execute on hardware.

23. What is the difference between the throw and throws keywords in Java?

• The throw keyword in Java is used to manually throw the exception to the calling method.
• The throws keyword in Java is used in the function definition to inform the calling method that this method throws the exception. So if you are calling, then you have to handle the exception.

class Main {
 public static int divideByZero(int a, int b) throws ArithmeticException {
 if (a == 0 || b == 0)
 throw new ArithmeticException("Division by zero is not allowed");
 return a / b;
 }

 public static void main(String[] args) {
 try {
 int result = divideByZero(10, 0);
 System.out.println("Result: " + result);
 } catch (ArithmeticException e) {
 System.out.println("Exception caught: " + e.getMessage());
 }
 }
}

In the above Java code, the divideByZero() method checks for division by zero and throws an ArithmeticException if either a or b is zero. In the main method, we catch the exception and print a message indicating that division by zero is not allowed.

Output

Enter a string:
Exception caught: Division by zero is not allowed

24. What is the Is-A relationship in OOPs in Java?

In Java, inheritance is an is-a relationship. We can use inheritance only if there is an is-a relationship between two classes. For example,

In the above figure:

• A car is a Vehicle
• A bus is a Vehicle
• A truck is a Vehicle
• A bike is a Vehicle

Here, the Car class can inherit from the Vehicle class, the bus class can inherit from the Vehicle class, and so on.

Read More: Inheritance in Java

25. What do you understand by the term Vector in Java?

The Vector class is an implementation of the List interface with which you can create resizable arrays similar to the ArrayList class.

Syntax

Vector vector = new Vector<>();

• Vector can be imported using Java.util.Vector.
• Elements of the Vector can be accessed using index numbers.
• Vectors are synchronized in nature i.e. they only use a single thread ( only one process is performed at a particular time ).
• The vector contains many methods that are not part of the collections framework.

Read More: Java Arrays

26. What do you understand by constructor overloading in Java?

Constructor overloading in Java happens in Java when more than one constructor is declared inside a class but with different parameters. If an object in a class is created by using a new keyword, it generates a constructor in that class.

Example of Constructor Overloading in Java Playground

public class ScholarHat {
    // function for adding two integers
    void add(int a, int b) {
        int sum = a + b;
        System.out.println("Addition of two integers: " + sum);
    }

    // function for concatenating two strings
    void add(String s1, String s2) {
        String con_str = s1 + s2;
        System.out.println("Concatenated strings: " + con_str);
    }

    public static void main(String args[]) {
        ScholarHat obj = new ScholarHat();
        
        // addition of two numbers
        obj.add(10, 10);
        
        // concatenation of two strings
        obj.add("Operator ", "overloading ");
    }
}

Output

Addition of two integer :20
Concatenated strings :Operator overloading

27. Is it necessary that each try block must be followed by a catch block?

No, a catch block doesn't need to be present after a try block. A try block should be followed either by a catch block or by the finally block. If there are chances of more exceptions to occur, they should be declared using the throws clause of the method.

Read more: Exception handling in Java

28. What is the difference between a program and a process?

• A program can be defined as lines of code written to accomplish a particular task. Whereas the process can be defined as the programs which are under execution.
• A program doesn't execute directly by the CPU. First, the resources are allocated to the program and when it is ready for execution then it is a process.

29. Differentiate between an object-oriented programming language and an object-based programming language.

30. Write a Java program to create and throw custom exceptions.

class ScholarHat {
 public static void main(String[] args) throws CustomException {
 // Throwing the custom exception by passing the message
 throw new CustomException("This is an example of custom exception");
 }
}

// Creating Custom Exception Class
class CustomException extends Exception {
 // Defining Constructor to throw exception message
 public CustomException(String message) {
 super(message);
 }
}

31. What happens if there are multiple main methods inside one class in Java?

The program will not compile. The Java Compiler will give a warning that the method has been already defined inside the class.

32. How is the creation of a string using new() different from that of a literal?

1. When a String is created using the new() keyword, a new String object is created in memory every time, even though the content of the String is the same as an existing String. It means that two String objects with the same content will be stored in different memory locations, and comparing them using the == operator will return false, even if their contents are the same.

   Example

   
   public class StringComparison {
       public static void main(String[] args) {
           String str1 = new String("Hello ScholarHat");
           String str2 = new String("Hello ScholarHat");
   
           // Comparing references of str1 and str2
           System.out.println(str1 == str2);
       }
   }
   

   Try it Yourself >>

   Output

   false
   

2. When a String is created using a literal, the JVM checks if a String with the same content already exists in the String pool. If a String with the same content already exists in the String pool, a reference to that String object is returned instead of creating a new String object. This means that two String literals with the same content will reference the same String object in the memory, and comparing them using the == operator will return true.

   Example

   
   public class StringComparison {
       public static void main(String[] args) {
           String str1 = new String("Hello ScholarHat");
           String str2 = new String("Hello ScholarHat");
   
           // Comparing references of str1 and str2
           System.out.println(str1 == str2);
       }
   }
   

   Try it Yourself >>

   Output

   true
   

For more information, click on Strings in Java

33. Is it possible to call a constructor of a class inside another constructor?

Yes, this is possible and this can be termed as constructor chaining and can be achieved using this().

Read More: constructor chaining in Java

34. Are int array[] and int[] array different or the same?

In Java, both int array[] and int[] array are valid syntaxes for declaring an array of integers.

int arr[] //C-Style syntax to declare an array.

int[] arr //Java-Style syntax to declare an array.

35. What are the different ways to create objects in Java?

1. Using new keyword
2. Using new instance
3. Using clone() method
4. Using deserialization
5. Using the newInstance() method of the constructor class

To get detailed insights on all these methods refer to Classes and Objects in Java OOPs

Advanced Java Interview Questions and Answers

36. What are the differences between abstract class and interface?

37. Explain System.out.println().

System.out.println() prints the message on the console.

• System - It is a class present in java.lang package.
• out is the static variable of type PrintStream class present in the System class
• .println() is the method present in the PrintStream class.

38. Can this keyword be used to refer to static members?

Example in Java Online Editor

public class ScholarHat {
 static int i = 50;

 public ScholarHat() {
 System.out.println(this.i);
 }

 public static void main(String[] args) {
 ScholarHat t = new ScholarHat();
 }
}

Output

50

39. Explain super in Java.

class ScholarHat {
 ScholarHat() {
 System.out.println("ScholarHat class is created");
 }
}

class Student extends ScholarHat {
 Student() {
 System.out.println("Student class is created");
 }
}

public class Test {
 public static void main(String[] args) {
 Student s = new Student();
 }
}
 

ScholarHat class is created
Student class is created

interface Animal {
    void sound();
}

class Dog implements Animal {
    public void sound() {
        System.out.println("Woof!");
    }
}

public class Main {
    public static void main(String[] args) {
        // Creating a Dog object
        Dog dog = new Dog();
        
        // Casting Dog object to Animal interface reference
        Animal animal = (Animal) dog;
        
        // Calling the sound() method through the Animal interface reference
        animal.sound(); 
    }
}

Woof

int[][] arr = new int[][] {
 {1, 2, 3}, 
 {4, 5}, 
 {6, 7, 8, 9}
};
 

import java.util.ArrayList;
import java.util.List;

public class Main {
    public static void main(String[] args) {
        // Create a list
        List list = new ArrayList<>();

        // Add elements to the list
        list.add("Apple");
        list.add("Banana");
        list.add("Orange");

        // Print the list
        System.out.println("List: " + list);
    }
}

List: [Apple, Banana, Orange]