Top 50 Java Interview Questions and Answers for Freshers

Java remains one of the most popular programming languages, widely used in enterprise applications, Android development, and web services. For freshers entering the software industry, cracking a Java interview often revolves around understanding core Java concepts, object-oriented programming, exception handling, data structures, and coding questions.This knowledgebase compiles the Top 50 Java Interview Questions and Answers tailored specifically for beginners.

1. Core Java Fundamentals

Q1. What is Java?

Java is a high-level, class-based, object-oriented programming language designed to have as few implementation dependencies as possible. It is platform-independent and widely used for web, mobile, and enterprise applications.

Q2. Why is Java platform-independent?

Java programs run on the Java Virtual Machine (JVM), which abstracts underlying hardware. Java code is compiled into bytecode, which the JVM interprets, enabling the same program to run on any platform.

Q3. What is JVM, JRE, and JDK?

JVM (Java Virtual Machine): Runs Java bytecode.

JRE (Java Runtime Environment): Includes JVM and libraries to run Java applications.

JDK (Java Development Kit): Includes JRE + tools to develop, compile, and debug Java programs.

Q4. Explain the main features of Java.

Platform independence

Object-oriented

Simple syntax

Security

Portability

Multithreading support

Automatic memory management (Garbage Collection)

Q5. What is the difference between JDK and JRE?

JDK: For development (includes compilers, debuggers).

JRE: For running Java programs (includes JVM + libraries).

2. Object-Oriented Programming (OOP) Concepts

Q6. What is Object-Oriented Programming?

OOP is a paradigm based on objects representing data and methods. Java supports Encapsulation, Inheritance, Polymorphism, and Abstraction.

Q7. What are the four pillars of OOP?

Encapsulation

Inheritance

Polymorphism

Abstraction

Q8. What is a Class and an Object?

Class: A blueprint/template for creating objects.

Object: An instance of a class containing data and behavior.

Q9. What is Inheritance?

Inheritance allows one class (subclass) to inherit fields and methods from another class (superclass), promoting code reuse.

Q10. What is Polymorphism?

Ability of a method to take multiple forms:

Compile-time: Method overloading

Runtime: Method overriding

Q11. What is Encapsulation?

Restricting access to variables/methods using access modifiers, providing abstraction and security.

Q12. What is an Abstract Class?

A class that cannot be instantiated and may contain abstract methods that subclasses must override.

Q13. What is an Interface?

A contract containing abstract methods that implementing classes must define. Interfaces support multiple inheritance.

3. Java Basics and Syntax

Q14. What are data types in Java?

Primitive types: int, float, double, boolean, char, etc.

Reference types: Objects, Arrays, Classes.

Q15. What is the difference between == and equals()?

==: Compares object references.

equals(): Compares object content.

Q16. What is a Constructor?

A special method used to initialize objects when created.

Q17. What are the types of constructors?

Default constructor (no parameters).

Parameterized constructor.

Q18. What is Method Overloading?

Defining multiple methods with the same name but different parameter lists.

Q19. What is Method Overriding?

Redefining a superclass method in a subclass with the same signature.

Q20. What is the static keyword?

Defines class-level variables/methods shared across all instances.

Q21. Difference between String, StringBuilder, and StringBuffer?

String: Immutable.

StringBuilder: Mutable, faster, not thread-safe.

StringBuffer: Mutable, synchronized (thread-safe).

Q22. What are access modifiers in Java?

public → accessible everywhere

private → accessible within class only

protected → accessible within package + subclasses

default (package-private) → accessible within package only

4. Exception Handling and Multithreading

Q23. What is Exception Handling?

Mechanism to handle runtime errors using try, catch, finally blocks.

Q24. Difference between checked and unchecked exceptions?

Checked: Checked at compile-time (e.g., IOException).

Unchecked: Occur at runtime (e.g., NullPointerException).

Q25. What is finally block?

A block that always executes after try/catch, typically used for cleanup.

Q26. What are threads in Java?

Lightweight units of execution allowing concurrent processing.

Q27. How to create a thread in Java?

Extending Thread class.

Implementing Runnable interface.

Q28. What is synchronization?

Mechanism to control access to shared resources among threads, preventing race conditions.

Q29. What is deadlock?

A situation where two or more threads wait indefinitely for resources held by each other.

5. Collections and Data Structures

Q30. What are Java Collections?

A framework (List, Set, Map, etc.) for storing/manipulating groups of objects.

Q31. Difference between ArrayList and LinkedList?

ArrayList: Dynamic array, faster for indexing.

LinkedList: Doubly linked list, faster for insertions/deletions

Q32. What is HashMap?

A key-value collection allowing fast retrieval using hashing.

Q33. Difference between HashMap and Hashtable?

HashMap: Not synchronized, faster.

Hashtable: Synchronized, slower.

Q34. Difference between Set and List?

Set: Unique elements, no duplicates.

List: Allows duplicates, maintains insertion order.

Q35. Difference between Comparable and Comparator?

Comparable: Defines default natural ordering (compareTo).

Comparator: Defines custom ordering (compare).

6. Practical Coding and Scenario Questions

Q36. How to reverse a String in Java?

Using StringBuilder.reverse()

Manual character swapping

Q37. Write a program to check if a number is prime.

(Implementation expected in interview).

Q38. Write a program to find factorial of a number.

Q39. How to find duplicate elements in an array?

Using HashSet or HashMap.

Q40. Explain bubble sort algorithm and implement it in Java.

Q41. Explain binary search algorithm and its implementation.

Q42. How to implement a Stack or Queue in Java?

Using Stack/Queue classes or LinkedList.

Q43. Difference between shallow copy and deep copy?

Shallow copy: Copies references.

Deep copy: Copies actual object values.

7. Advanced Basics for Freshers

Q44. Difference between final, finally, and finalize?

final: Keyword for constants, methods, or classes.

finally: Block for cleanup code in exception handling.

finalize(): Method called by garbage collector before object destruction.

Q45. What are wrapper classes in Java?

Object representation of primitive types (e.g., Integer, Double).

Q46. What is autoboxing and unboxing?

Autoboxing: Converting primitive to wrapper class.

Unboxing: Converting wrapper class to primitive.

Q47. Explain garbage collection in Java.

Automatic memory management removing unused objects.

Q48. What is the Java Memory Model?

Heap: Stores objects.

Stack: Stores local variables and method calls.

Q49. What is an anonymous inner class?

A class without a name defined inside another class, used for quick implementations.

Q50. What is the use of the volatile keyword?

Ensures changes to a variable are visible across threads immediately.