20 Java Interview Questions with Sample Answers

What Are Java Interview Questions?

Technical and behavioral questions designed to assess a candidate's knowledge of Java programming are known as Java interview questions. These questions can focus on:

• Core Java concepts such as OOP, exception handling, multithreading, collections, etc.
• Frameworks and tools like Spring, Hibernate, Maven, and JUnit
• Real-world problem-solving using Java
• Best practices, performance tuning, and design patterns
• Code readability, scalability, and maintainability

Hiring teams use these core Java questions for interview rounds to filter candidates who have strong logic, solid coding practices, and hands-on experience in Java development.

For other related roles, explore Qandle’s HR interview questions from our HR toolkit library.

Java Interview Questions with Answers

Here are 20 essential Java interview questions along with detailed sample answers that will help you assess candidates at various levels of experience from fresher to experienced professionals.

1. What are the key features of Java?

Answer: Java is a platform-independent, object-oriented, high-level programming language. Its key features include:

• Write Once, Run Anywhere (WORA)
• Automatic memory management with Garbage Collection
• Multithreading support
• Robust exception handling
• Secure runtime environment

2. What makes JDK, JRE, and JVM different from each other?

Answer:

• JDK (Java Development Kit): A software development environment with tools and libraries to develop Java applications.
• JRE (Java Runtime Environment): Provides libraries and JVM to run Java applications.
• JVM (Java Virtual Machine): This abstract machine that executes Java bytecode guarantees platform independence.

3. In Java, what distinguishes == from .equals()?

Answer:

• == compares object references (memory location).
• .equals() compares object values.

For instance, two strings with identical content can return true when using .equals() but false while using ==.

4. What is inheritance in Java?

Answer: Using the extends keyword, inheritance enables a class to inherit the attributes and behaviors of another class. It promotes code reusability and establishes an 'is-a' relationship between parent and child classes.

5. What is polymorphism in Java?

Answer: Polymorphism allows objects to behave differently based on context. There are two types:

• Compile-time (method overloading)
• Runtime (method overriding)

This enhances flexibility and scalability in object-oriented programming.

6. What is an interface in Java?

Answer: An interface defines a contract of methods that a class must implement. Unlike abstract classes, interfaces cannot have instance variables or constructors and support multiple inheritance.

7. What is the difference between an abstract class and an interface?

Answer:

• Abstract class: Can use both concrete and abstract methods.
• Interface: Contains only abstract methods (until Java 8) and is fully abstract. Does not support constructors or state.

Use interfaces to define capabilities and abstract classes to define shared behavior.

8. What are access modifiers in Java?

Answer: Java provides four access modifiers:

• public: Accessible everywhere
• protected: Accessible within the package and subclasses
• default: Accessible within the package
• private: Accessible only within the class

These control visibility and encapsulation.

9. What is exception handling in Java?

Answer: Exception handling is a mechanism to handle runtime errors using try, catch, finally, and throw. It ensures the program continues running even when unexpected errors occur.

10. What distinguishes checked exceptions from unchecked exceptions?

Answer:

• Checked exceptions: Must be handled at compile time (IOException, for example).
• Unchecked exceptions: Occur at runtime and are not mandatory to catch (e.g., NullPointerException).

11. Explain the concept of multithreading in Java.

Answer: Multithreading allows concurrent execution of two or more threads, helping improve performance. Java offers the Thread class and Runnable interface.

12. What is the Java Collection Framework?

Answer: It is a set of classes and interfaces to handle groups of objects. Common interfaces include List, Set, Map, and implementations include ArrayList, HashSet, HashMap.

13. How is HashMap different from Hashtable?

Answer:

• HashMap permits one null key and several null values; it is not synchronized.
• Hashtable is synchronized, does not allow null keys or values.

HashMap is preferred for non-threaded environments.

14. What is garbage collection in Java?

Answer: Garbage collection is the process of automatically releasing memory that is occupied by unavailable objects. Java uses automatic garbage collectors like G1, CMS, etc., to manage heap space.

15. What is the final keyword in Java?

Answer: final can be used with:

• Variables (to make them constants)
• Methods (to prevent overriding)
• Classes (to prevent inheritance)

16. What is the purpose of the static keyword?

Answer: static is used to declare class-level members. Static variables and methods belong to the class rather than instances, and are accessed using the class name.

17. What are constructors in Java?

Answer: One unique way to initialize objects is with a constructor. It lacks a return type and shares the same name as the class. Java supports constructor overloading.

18. What is the difference between pass by value and pass by reference?

Answer: Java is strictly pass-by-value. When you pass an object, the reference (memory address) is passed by value. So changes to object fields persist, but reassigning the object doesn't affect the original.

19. What is method overloading and overriding?

Answer:

• Overloading: Using different parameter types with the same method name in the same class.
• Overriding: A subclass provides a specific implementation for a method already defined in the superclass.

20. Why is Java considered platform-independent?

Answer: Bytecode, which can run on any platform with a Java Virtual Machine (JVM), is created when Java code is compiled. Java's 'write once, run anywhere' capability makes it platform-independent.

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