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30+ Java Interview Questions And Answers

Java is a widely-used programming language known for its portability across platforms, which makes it the go-to choice for developing cross-platform applications. It's implemented in billions of devices globally, from data centers and gaming consoles to smartphones. Below is a collection of essential Java interview questions designed to reveal a candidate's understanding and expertise in Java, starting with foundational knowledge for beginners and moving on to complex scenarios for advanced Java developers.


Most asked Java interview questions



What is Java and how is it used in programming?

Java is a high-level, class-based, object-oriented programming language that is designed to have as few implementation dependencies as possible. It's used for building platform-independent applications, which means once written, can run on any operating system without modification.


What are the main principles of Object-Oriented Programming?

The main principles include Encapsulation, which is the concept of bundling the data and methods that work on the data within one unit. Inheritance, a mechanism that allows one object to acquire all the properties and behaviors of a parent object. Polymorphism, which allows actions to act differently based on the object that the action is being performed on. Lastly, Abstraction, which means using simple things to represent complexity.


Can Java run on any platform?

Yes, Java can run on any platform that has a compatible Java Virtual Machine (JVM). This is because Java is platform-independent at the source level, which is achieved through compilation into bytecode that can be executed by the JVM.


What is the difference between JDK, JRE, and JVM?

The JDK (Java Development Kit) is a software development environment used for developing Java applications. The JRE (Java Runtime Environment) contains libraries, the JVM, and other components to run applications written in Java. The JVM (Java Virtual Machine) is a runtime instance that interprets the compiled Java bytecode and enables execution.


What is a 'class' in Java?

A 'class' in Java is a blueprint from which individual objects are created. It contains fields (also known as variables) and methods to define the behavior.


What is Garbage Collection in Java?

Garbage Collection is Java's process of automatically freeing the memory by deleting objects that are no longer reachable in your code, which helps in managing memory efficiently.


Can you explain method overloading in Java?

Method overloading in Java occurs when you have multiple methods in the same class with the same name but different parameters (type, number, or both). It allows methods to handle different types of input while maintaining a consistent naming convention.


What is an 'interface' in Java?

An interface in Java is a reference type, similar to a class, that can contain only constants, method signatures, default methods, static methods, and nested types. It is a way to achieve abstraction and multiple inheritances in Java.


Why are Strings in Java immutable?

Strings in Java are immutable to enhance security, simplify string concatenation operations, and make it possible for the JVM to optimize String handling, such as by storing them in a common pool for reusability.


What are the access modifiers in Java?

The access modifiers in Java dictate how classes, methods, and variables can be accessed. They include public (accessible from anywhere), protected (accessible within the package and subclasses), default (no keyword, accessible within the package), and private (accessible within the class only).


What is a Constructor in Java?

A constructor in Java is a block of code that initializes a newly created object. It must have the same name as the class and cannot return a value.


Explain the 'final' keyword in Java.

The 'final' keyword in Java is used to apply restrictions: it can make a variable unchangeable (constant), a method no longer overrideable, or a class non-extendable.


Describe the Java Collections Framework.

The Java Collections Framework is a set of classes and interfaces that implement commonly reusable collection data structures like lists, sets, queues, and maps.


What is a 'package' and how is it used in Java?

A 'package' in Java is a namespace that organizes sets of related classes and interfaces. It is used to avoid name conflicts and to control access to classes and interfaces.


What does the following code snippet do?

public void changeValue(final int x) {
    x = x + 5;

This code attempts to change the value of a method parameter marked as final. This will result in a compilation error because final parameters cannot be modified.

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Explain the concept of Reflection in Java and where it's used.

Reflection in Java is a feature that allows examination and modification of the runtime behavior of applications. It's used in frameworks for serialization, deserialization, and testing, as it can manipulate the internal properties of classes, interfaces, and objects.


Discuss how memory leaks occur in Java.

Memory leaks in Java happen when objects are no longer needed but remain unreachable by the Garbage Collector due to lingering object references, often in collections like maps or lists.


Describe the Java Memory Model.

The Java Memory Model specifies how threads interact with memory in Java. It defines the visibility guarantee for shared variables when they are accessed from multiple threads, ensuring that interactions do not cause unexpected results.


What is bytecode in Java?

Bytecode in Java is a highly optimized set of instructions designed to be executed by the Java Virtual Machine (JVM). It's the result of compiling Java source code, enabling platform independence.


Differentiate between Checked and Unchecked Exceptions in Java.

Checked exceptions must be either caught or declared in the method signature, forcing error handling. Unchecked exceptions, like RuntimeException, do not have this requirement and can be caught optionally.


What are generics in Java?

Generics enable types (classes and interfaces) to be parameters when defining classes, interfaces, and methods. They provide stronger type checks at compile time and eliminate casting in code.


Explain the 'volatile' keyword in Java.

The 'volatile' keyword in Java is used to mark a variable as being stored in main memory. It ensures that reads and writes to the variable are done directly from and to main memory, making their changes visible to other threads immediately.


Discuss the role of the 'transient' keyword in Java.

The 'transient' keyword marks fields not to be serialized. It's used when you don't want the state of an object to be part of the serialized form.


What does the 'synchronized' keyword signify?

The 'synchronized' keyword controls access to a block of code or method by only allowing one thread at a time to execute it, ensuring thread safety during concurrent execution.


How do you ensure that a Java class is immutable?

To ensure a Java class is immutable, make all fields private and final, do not provide setters, and create deep copies of mutable objects when needed.


Describe the types of references in Java and how garbage collection relates to them.

Java has strong, soft, weak, and phantom references that determine the eligibility of an object for garbage collection based on reachability. Strong references prevent GC, while soft and weak references allow it. Phantom references are enqueued after an object is marked for GC.


What is the difference between fail-fast and fail-safe iterators in Java?

Fail-fast iterators immediately throw a ConcurrentModificationException if a collection is modified during iteration. Fail-safe iterators use a copy of the collection to avoid this exception.


Explain dependency injection in Java.

Dependency injection in Java is a design pattern where an object's dependencies are provided externally, rather than the object creating them. It's a key aspect of the inversion of control principle used in Java frameworks like Spring.


What is the 'double-checked locking' pattern in Java and why is it used?

Double-checked locking is a design pattern that reduces the overhead of acquiring a lock by testing the locking criterion without actually acquiring the lock. It's used to improve performance while ensuring that the object is lazily initialized in a thread-safe manner.


What happens in the JVM when the following Java code is executed?

String str1 = "Hello";
String str2 = new String("Hello");
String str3 = str2.intern();

When this code is executed, 'str1' references a String from the String pool. 'str2' creates a new String object. 'str3' also references the String from the String pool due to the intern() method, making 'str1' and 'str3' point to the same object.


Java Interview Tips

Understanding the Question

  • Take a moment to fully digest the interview question. If anything is unclear, don't hesitate to ask the interviewer for clarification. Making sure you understand the question can prevent mistakes in your answer and shows that you prioritize accuracy and understanding over speed.

Methodical Problem Solving

  • When faced with a tough technical question, approach it methodically. Break the problem down into smaller, more manageable parts, and walk your interviewer through your problem-solving process. Communicating your thought process can be just as important as arriving at the correct answer.

Be Honest About What You Know

  • If there is a question that you don't know the answer to, it's usually better to be honest about it rather than trying to bluff. You can then demonstrate your ability to reason or how you would find the answer. This honest approach can build the interviewer's trust and showcase your learning mindset.

Use Examples to Demonstrate Understanding

  • If you can, use examples to illustrate your response, whether it's from previous work experiences or hypothetical scenarios. This helps demonstrate your understanding of the concept and your ability to apply it practically.

Stay Calm and Stay Positive

  • No matter how challenging the question may seem, staying calm and maintaining a positive attitude is crucial. Panicking can cloud your judgement and hinder your ability to answer effectively. Take deep breaths and remember that it's okay to take a moment to collect your thoughts before responding.


How much does it cost to hire a Java developer?

Costs typically start at $45/hour, adjusted based on experience and project complexity. Read more: How much does it cost to hire a Java developer?

Where can I hire a Java developer?

You can connect with leading Java developers through specialized tech staffing agencies like FireHire, renowned for our vast talent network. Read more: Where can I hire a Java developer?

How do I hire a Java programmer?

Connect with FireHire to discuss your requirements, and within just 5 days, we'll introduce you to your ideal Java programmer. Read more: How do I hire a Java programmer?

Why using FireHire for hiring Java developers is the best choice?

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