Whether you're preparing for positions, looking at a particular employment switch, or looking for your next independent undertaking, it is fundamental to dominate Java. Due to its adaptability, platform independence, and robust features, Java is one of the most popular programming languages worldwide.

We've compiled the top 30 Java interview questions with answers in this comprehensive survey to give you the knowledge and self-assurance you need to succeed in future interviews. We should plunge and level up our Java abilities for better professional outcomes. Let’s Go!

Basic JAVA Interview Questions & Answers

1. What is JAVA?

JAVA is an object-oriented multiplatform programming language. It is a versatile programming language that focuses on data and the methods that operate it. It is prominently used to develop web, mobile or desktop applications. Many common uses of JAVA are:

a.  Game Development

Java is especially popular for Android game development with frameworks like LibGDX, easing complexity and making it a preferred choice for diverse game types across multiple platforms.

b.  Cloud Computing

JAVA’s WORA (Write Once, Run Anywhere) ideology is suitable for cloud computing. It allows seamless program execution across multiple platforms, which is why cloud providers prefer it.  

c.  Artificial Intelligence

Java's platform independence, robust security, and scalability make it ideally suited for AI development, efficiently handling complex AI algorithms.

d. IOT

JAVA is ideal for creating sensors and hardware devices with absolute security and reliability.

2. Is JAVA an independent platform?

Yes, JAVA is a platform-independent language, which implies that the JAVA codes can be executed in any environment. It doesn’t mean it can be run everywhere, but its code can be executed in any operating system. 

3. What is the difference between JVM, JDK & JRE?

4.  List the top features of JAVA Programming Language.

JAVA is one of the most commonly used programming languages, with myriad valuable features that make it better than others.

• It is easy to understand JAVA’s programming language and syntax.

• It is platform-independent and, thus, can run programs anywhere in software or hardware and still get the same result.

• It is a robust language with amazing features like exception handling and garbage collection.

• It is an OOP language that focuses on data supporting the concepts of class and four key features in OOP.

• It is faster than most traditional programming languages.

• It supports dealing with multiple tasks at once.

• It does not rely on any architecture.

• It is a secure language.

  
  

5.  What is a JAVA Virtual Machine?

The Java Virtual Machine, abbreviated as JVM, is an interpreter in Java. It is responsible for actions related to bytecode in Java, like loading, verifying, and executing. However, the JVM software is different for different Operating Systems.

6.  What is JIT Compiler?

The JIT (Just-In-Time) Compiler is an element of the JAVA Runtime Environment that boosts the productivity of JAVA programs by interpreting bytecode into native machine code during execution. This implies that instead of decoding JAVA bytecode each time an application runs, the JIT compiler compiles it once into machine code before directly executing it for better performance. This approach helps JAVA applications to run faster the more they are executed since the JIT compiler may optimise machine code based on runtime data.

7.  Explain these terms:

These terms are the four pillars of the OOP language, a computer programming model that uses data and objects rather than logic or function.  

a. Inheritance

In JAVA, classes can repurpose or adapt the properties of other classes, encouraging the reusability of codes. This inheritance of code is known as inheritance.

b. Encapsulation

As the name suggests, it encapsulates the data and internal state of objects from direct access by the program.

c. Polymorphism

The feature allows for the performance of a single function in several ways. Different types of objects can pass through the same interference through polymorphism

d. Abstraction

Java allows you to create abstraction by utilising abstract classes and interfaces, which helps isolate the impact of code modifications.

8.  What are the exceptions in JAVA? How do you handle them?

Exception handling in JAVA can be done using more than one method. One is try-catch-finally, which manages all types of exceptions. Another is the try-with-resources method, which makes cleaning easier for resources.

Try-catch-finally example:

public class Main {

public static void main(String[] args) {

     try {

         int[] numbers = {1, 2, 3};

            System.out.println(numbers[3]); // This will throw an ArrayIndexOutOfBoundsException

     } catch (ArrayIndexOutOfBoundsException e) {

            System.out.println("An exception occurred: " + e.getMessage());

     } finally {

            System.out.println("The 'try-catch' is finished.");

     }

}

}

Try-with-resources example

import java.io.*;

public class Main {

public static void main(String[] args) {

     try (BufferedReader br = new BufferedReader(new FileReader("test.txt"))) {

         String line;

         while ((line = br.readLine()) != null) {

                System.out.println(line);

         }

     } catch (IOException e) {

            System.out.println("An exception occurred: " + e.getMessage());

     }

}

}

9.  What is the use of the ‘main’ method in JAVA?

The 'main' method in Java is where any independent Java application begins to execute. It is necessary as it acts as the entry point for the Java Runtime Environment before you start operating your program. When you launch a Java program, this method is called first, and it controls the program's following steps, such as setting up objects and calling other methods required by the application.

10.  What is a Classloader?

A ClassLoader in Java is a component of the Java Runtime Environment that actively uploads Java classes into the Java Virtual Machine. It reads binary data from a developed Java class and converts it to an executable version.

Experienced JAVA Interview Questions & Answers

With an experience of 2 years

1.  State the difference between a ‘Linked list’ and an ‘array list’.

2.  What is the major difference between StringBuffer and StringBuilder in JAVA?

i.  StringBuffer:

• StringBuffer is a Java class that implements many of the characteristics of strings.

• It indicates fixed-length, unaltered text arrays. StringBuffer is thread-secure; therefore, it can be used in an environment with multiple threads.

• Characters and substrings can be inserted in the middle of a StringBuffer or extended at the end.

• It develops automatically to accommodate additions and frequently reallocates more characters than necessary.

Example:

public class GFG {

public static void main(String args[]) {

     StringBuffer str = new StringBuffer("Hello");

     str.append(" World!");

     System.out.println(str); // Output: Hello World!

}

}

 ii. StringBuilder

• StringBuilder is identical to StringBuffer but indicates an elastic string of characters.

• Since a String object in Java generates an unchangeable sequence of characters, StringBuilder offers a substitute by permitting modifications.

• StringBuilder is not thread-secure, unlike StringBuffer.

• It is more efficient than StringBuffer because it eliminates the synchronisation overhead.

Example:

public class GFG {

public static void main(String args[]) {

     StringBuilder str = new StringBuilder("Hello");

     str.append(" World!");

     System.out.println(str); // Output: Hello World!

}

}

3.  Describe the JAVA thread lifecycle.

Certainly! Here’s a more comprehensive breakdown of the Java thread lifecycle:

 i.  New State:

• The thread was created but is still not functioning.

• The thread scheduler awaits scheduling.

ii.  Runnable State:

• Actively running or preparing to run.

• The scheduler gives CPU time.

• Yielding enables other threads to operate.

 iii. Blocked State:

• This happens when a thread attempts to take over the lock controlled by another thread.

• Wait for the lock to be unlocked.

• When the lock becomes available, the code restores to accessible mode.

iv. Waiting State:

• Methods such as wait() and join() were utilized for entering the data.

• Waits for alert or set time.

• Returns to runnable upon notification or termination.

v. Timed Waiting State:

• Similar to waiting, but with a break.

• Threads wait for a specific period.

• Returns to the accessible state after timeout or notice.

vi. Terminated State:

• The thread completes its operation normally.

• Terminates owing to an unresolved exception.

• Cannot restart.

With an experience of 3 years

1. How does the ‘optional’ class prevent ‘NullPointerExceptions’ in JAVA?

The 'Optional' class in Java is an envelope object that may or may not have a non-null value. Using 'Optional' explicitly requires callers to manage the possibility of a value's absence, avoiding 'NullPointerExceptions'. Instead of returning 'null' for functions that might not consistently be able to retrieve a value, an 'Optional' object is returned. This requires the caller to check for the current value before implementing it, using methods such as 'isPresent()' and 'orElse()', dramatically minimising the likelihood of receiving a 'NullPointerException'.

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2. Difference between HashMap and HashSet in JAVA?

Both HashSet and HashMap are data structures with unique purposes and some key differences. A Java developer needs to be aware of these game-changing differences.

3. Why can one not override the static method in JAVA?

In Java, static methods are associated with the class rather than any of its instances. As a result, you cannot override a static method's functionality depending on a class instance. On the other hand, a static method can be rewritten in a subclass, allowing the subclass to have its version of the technique; however, this is referred to as hiding rather than overriding.

With an experience of 5 years

1.  List the difference between the Volatile & Transient Variable in JAVA.

In JAVA, these two variables have specific purpose and uses:

Volatile Variable:

This keyword assures that the value of a variable is constantly read from and returned to the main memory without being cached by any threads. This is implemented in multiple threads to ensure that changes performed in one thread are promptly seen in the other threads.

Transient Variable:

This term indicates that a value should not be sequenced. Transitory variables are disregarded when a component is sequenced; therefore, their values are not preserved.

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2. Describe false sharing in terms of multithreading.

False sharing in multithreading happens when threads unnecessarily interact by writing to adjacent data on the same storage cached line. This can slow down the software since changes in one thread can affect the others, resulting in repeated unnecessary data reloads.

3. What is the core difference among these keywords in Java

a. Final

b. Finally

c. Finalise         

Often confused among the purposes, these three keywords play a significant role in JAVA programming. Let’s discuss their essential purposes:

a. Final keyword:

Useful for declaring constants, avoiding method overriding, and class inheriting.

b. Finally keyword:

It is a part of exception handling and specifies a section of code that is always performed following a try/catch block, irrespective of whether an exception was raised.

c. Finalize keyword:

An approach used in garbage collection to specify precise actions that should occur when an object is going to be retrieved by the person collecting it.

For Freelancers

1. How do you stay updated with the latest JAVA libraries as a freelancer?

When asked such a question, mentioning strong learning strategies and involvement that helped you in your freelancing journey is essential. You can go with something like:

As a Java freelancer, I often read Java blogs such as Oracle and Baeldung and participate in Java forums or groups on Stack Overflow, GitHub, and Reddit. I also visit JavaOne and other development conferences, and I have recently taken online courses through companies such as Redrob. Redrob keeps me updated with its enlightening content and provides mock interview questions, skill assessment tests, coding quizzes, and so on. This maintains my abilities and allows me to provide clients with the most current and effective solutions.

2. How do you test the JAVA application before delivering?

When facing such a question, present a detailed and structured strategy that helps you. Here’s how you could structure your answer:

Before releasing a Java application, I set up a clear testing plan. I start by unit testing every element with JUnit. Then, I tested the integration to ensure all components were executed well together. System evaluation confirms that the application functions effectively in a near-production setting. I additionally conduct user acceptance tests for clients to ensure the application satisfies their requirements. Finally, after making any changes, I run regression tests to ensure that they do not adversely affect existing functionality.

3. What tools do you preferably use for JAVA development?

You can frame your answer with something like:

I primarily use IntelliJ IDEA for Java development because of its excellent coding aid and integration with numerous frameworks and version control systems. Maven and Gradle are my go-to tools for build and dependency management. Git is my preferred version control technology, which I frequently manage through GitLab or GitHub. I also use JUnit for unit testing to assure code reliability and Jenkins for ongoing integration and delivery. Together, these tools improve my development workflow, project management and greater-quality code.

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FAQs

·  What key concepts are to remember when preparing for a JAVA interview?

These key ideas incorporate OOP standards, exemption handling, multithreading, assortments structure, JDBC, Servlets, JSP, Spring system, and Rest.

·  What are the effective ways to handle exceptions in JAVA applications?

Use try-catch blocks, particular exception classes, finally blocks, and advanced methods like try-with-resources for cleanup.

·  Where can I get more practice material related to JAVA interview questions?

For more helpful practice content, you can visit Redrob. Redrob is India’s best online job interview preparation platform. And it doesn’t end here; it also dives deeper into the specific role-based assessment tests.