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Top 50 Core Java Interview Questions and Answers

Anandita Doda
Java Developer Questions and Answers
Java Developer Questions and Answers

Java is one of the most widely used and enduring programming languages in the world. From enterprise-grade applications to Android development and backend systems, Java forms the backbone of countless software solutions. Its strong object-oriented foundation, cross-platform capabilities, and robust standard libraries have made it a favourite among Core Java developers and employers alike.

Contents
Target AudienceKey Topics to Revise Before a Java InterviewSection 1: Java Basics & OOPs (Questions 1–10)Section 2: Data Types, Operators & Flow Control (11–20)Section 3: Exception Handling (21–30)Section 4: Collections Framework & Java 8 Features (31–40)Section 5: Multithreading, Memory, and Advanced Topics (41–50)Conclusion

For anyone seeking a career in software development—whether as a fresher, experienced engineer, or backend developer—Core Java interview questions are often the first line of assessment. These questions test not just your theoretical understanding, but also your ability to think logically and apply concepts to real-world programming problems.

In this blog, we bring you a curated list of the Top 50 Core Java Interview Questions and Answers, organised by topic. Each answer is written in simple language, with enough depth to help you revise effectively and build confidence before your interview.

Target Audience

This blog is designed for a wide range of readers preparing for Core Java interviews. Whether you are just starting out or already have experience, this guide will help you sharpen your understanding and boost your confidence.

It is especially useful for:

  • Fresh Graduates and Entry-Level Programmers
    Those preparing for campus placements or their first Java developer job.
  • Software Developers with 1–3 Years of Experience
    Professionals preparing for technical interviews to move into better roles or switch companies.
  • Backend and Full-Stack Developers
    Candidates needing to revise Java fundamentals as part of their broader tech stack.
  • Career Switchers and Bootcamp Graduates
    People transitioning into tech roles who want a focused and practical Java interview resource.
  • Technical Recruiters and Hiring Managers
    Those looking for reliable Core Java questions to include in interview rounds.

Whether you are applying to a product company, a service-based firm, or a startup, this blog gives you the structured practice and understanding needed to handle Java interviews confidently.

Key Topics to Revise Before a Java Interview

Before diving into interview questions, it is important to refresh your understanding of the core areas of Java. These topics form the backbone of most technical discussions and assessments during interviews for Java roles:

1. Object-Oriented Programming (OOPs) Concepts

Understand the four pillars:

  • Inheritance
  • Encapsulation
  • Abstraction
  • Polymorphism
    Know how Java implements them through classes, interfaces, access modifiers, and method overriding/overloading.

2. Java Basics

Brush up on:

  • Data types (primitive vs reference)
  • Operators
  • Control statements (if-else, switch, loops)

3. Exception Handling

Understand:

  • try-catch-finally blocks
  • Checked vs unchecked exceptions
  • Custom exceptions
  • throw vs throws

4. Collections Framework

Know the differences and use cases of:

  • List, Set, Map
  • ArrayList vs LinkedList
  • HashMap vs TreeMap
    Also, be familiar with iterators, generics, and Concurrent collections.

5. Multithreading and Concurrency

Understand:

  • Thread life cycle
  • Runnable vs Thread
  • synchronized, wait(), notify(), ExecutorService
  • Basics of concurrent API (Atomic, ReentrantLock)

6. Java Memory Management

Cover:

  • Stack and heap memory
  • Garbage Collection
  • Strong vs weak references
  • Memory leaks

7. Interfaces and Abstract Classes

Understand the differences, use cases, and how they support abstraction and multiple inheritance in Java.

8. Java 8 Features

Focus on:

  • Lambda expressions
  • Streams API
  • Functional interfaces
  • Default and static methods in interfaces

9. Important Java Classes and Methods

Know commonly used methods from:

  • String, Object, Arrays, Collections, Math, System

10. Java Keywords and Modifiers

Review important keywords:

  • final, static, transient, volatile, synchronized, this, super, instanceof

Mastering these areas will ensure you are well-prepared to tackle both conceptual and practical questions during the interview.

Section 1: Java Basics & OOPs (Questions 1–10)

1. What is Java?
Answer: Java is a high-level, object-oriented, platform-independent programming language developed by Sun Microsystems (now Oracle). It uses the WORA principle—Write Once, Run Anywhere—thanks to the Java Virtual Machine (JVM).

2. What are the main features of Java?
Answer:

  • Object-Oriented
  • Platform Independent
  • Secure
  • Robust
  • Multithreaded
  • High performance with Just-In-Time (JIT) compiler

3. What is the difference between JDK, JRE, and JVM?
Answer:

  • JDK (Java Development Kit): Full environment for Java development (JRE + compilers + tools).
  • JRE (Java Runtime Environment): Runtime for running Java apps (JVM + libraries).
  • JVM (Java Virtual Machine): Engine that runs bytecode on different machines.

4. What is an object in Java?
Answer: An object is an instance of a class. It contains state (fields/attributes) and behavior (methods/functions).

5. What is a class in Java?
Answer: A class is a blueprint for creating objects. It defines variables and methods that the objects will have.

6. What are the four pillars of Object-Oriented Programming in Java?
Answer:

  • Encapsulation: Binding data and methods together.
  • Abstraction: Hiding internal implementation.
  • Inheritance: One class acquires features of another.
  • Polymorphism: Many forms—method overloading and overriding.

7. What is the difference between method overloading and method overriding?
Answer:

  • Overloading: Same method name with different parameters (compile-time).
  • Overriding: Subclass redefines a method from the parent class (run-time).

8. What is the difference between a constructor and a method?
Answer:

  • Constructor initializes an object and has no return type.
  • Method performs actions and has a return type.

9. What is the purpose of the this keyword?
Answer: this refers to the current object instance. It is used to resolve naming conflicts between instance variables and parameters.

10. What is encapsulation in Java?
Answer: Encapsulation means keeping data (fields) private and providing public getter/setter methods. It helps protect the object’s state and enforce rules.

Section 2: Data Types, Operators & Flow Control (11–20)

11. What are the different data types in Java?
Answer: Java has two main types:

  • Primitive: byte, short, int, long, float, double, char, boolean
  • Non-primitive: Strings, Arrays, Classes, Interfaces

12. What is the default value of int and boolean in Java?
Answer:

  • int: 0
  • boolean: false

13. What is the difference between == and .equals() in Java?
Answer:

  • == checks if two references point to the same object.
  • .equals() checks if two objects have the same content (can be overridden).

14. What are the basic operators in Java?
Answer:

  • Arithmetic: +, –, *, /, %
  • Relational: ==, !=, >, <, >=, <=
  • Logical: &&, ||, !
  • Bitwise: &, |, ^, ~, <<, >>

15. What is the use of the instanceof operator?
Answer: It checks whether an object is an instance of a particular class or subclass.

16. What are the different control statements in Java?
Answer:

  • Conditional: if, else, switch
  • Looping: for, while, do-while
  • Jump: break, continue, return

17. What is the difference between break and continue?
Answer:

  • break exits the loop completely.
  • continue skips the current iteration and continues with the next one.

18. What is a ternary operator in Java?
Answer:
A shortcut for if-else:
condition ? value_if_true : value_if_false

19. What is type casting?
Answer: Converting one data type into another.

  • Implicit (widening): int to long
  • Explicit (narrowing): double to int

20. Can a switch statement work with Strings in Java?
Answer: Yes, from Java 7 onwards, switch supports String values.

Section 3: Exception Handling (21–30)

21. What is an exception in Java?
Answer:
An exception is an unwanted event that disrupts the normal flow of a program. It is an object that represents an error or unexpected condition during runtime.

22. What is the difference between checked and unchecked exceptions?
Answer:

  • Checked exceptions are checked at compile-time (e.g., IOException, SQLException).
  • Unchecked exceptions are checked at runtime (e.g., NullPointerException, ArrayIndexOutOfBoundsException).

23. What are the key keywords used in exception handling?
Answer:

  • try, catch, finally, throw, throws

24. Can you write code that uses try-catch?
Answer:

try {
    int result = 10 / 0;
} catch (ArithmeticException e) {
    System.out.println("Cannot divide by zero.");
}

25. What is the purpose of the finally block?
Answer:
The finally block always executes after try and catch, regardless of whether an exception occurred. It is typically used for resource cleanup.

26. Can you catch multiple exceptions in one catch block?
Answer: Yes, using the pipe (|) symbol:

catch (IOException | SQLException e) {
    // handle both
}

27. What is the difference between throw and throws?
Answer:

  • throw is used to explicitly throw an exception.
  • throws declares exceptions a method can throw.

28. Can a finally block be executed even if there is a return statement in try?
Answer: Yes. The finally block will execute even if there is a return inside the try or catch.

29. What is a custom exception in Java?
Answer: A user-defined class that extends Exception or RuntimeException to create specific error types.

30. What happens if an exception is not caught?
Answer: The JVM handles it, prints a stack trace, and terminates the program abnormally.

Section 4: Collections Framework & Java 8 Features (31–40)

31. What is the Java Collections Framework?
Answer:
It is a set of classes and interfaces in java.util for storing and manipulating groups of objects. Key interfaces include List, Set, Queue, and Map.

32. What is the difference between ArrayList and LinkedList?
Answer:

  • ArrayList is faster for accessing elements.
  • LinkedList is faster for adding/removing elements from the middle or ends.

33. What is the difference between HashSet and TreeSet?
Answer:

  • HashSet is unordered and faster.
  • TreeSet is sorted in natural order and slower due to tree-based structure.

34. What is the difference between HashMap and Hashtable?
Answer:

  • HashMap is not synchronized and allows one null key.
  • Hashtable is synchronized and does not allow any null key or value.

35. What is the difference between List and Set?
Answer:

  • List allows duplicates and maintains insertion order.
  • Set does not allow duplicates and may or may not preserve order.

36. What are lambda expressions in Java?
Answer:
Introduced in Java 8, lambda expressions allow writing anonymous functions in a concise way:

(x, y) -> x + y;

37. What is a functional interface?
Answer:
An interface with a single abstract method. Common ones include Runnable, Callable, Comparator, and custom ones annotated with @FunctionalInterface.

38. What is the Stream API?
Answer:
A Java 8 feature that lets you process data in a functional style:

list.stream().filter(x -> x > 10).collect(Collectors.toList());

39. What is the difference between map() and flatMap() in streams?
Answer:

  • map() transforms each element.
  • flatMap() flattens nested structures like lists of lists.

40. What are method references in Java 8?
Answer: A shorthand for calling a method using ::. Example:

list.forEach(System.out::println);

Section 5: Multithreading, Memory, and Advanced Topics (41–50)

41. What is multithreading in Java?
Answer:
Multithreading is a feature that allows concurrent execution of two or more threads (lightweight processes) to maximize CPU utilization and improve performance.

42. How do you create a thread in Java?
Answer:
Two ways:

  1. Extend the Thread class and override run()
  2. Implement the Runnable interface and pass it to a Thread object

43. What is the difference between process and thread?
Answer:

  • A process is a complete, independent program.
  • A thread is a smaller unit within a process, sharing memory and resources.

44. What is the synchronized keyword used for?
Answer:
It is used to lock a method or block of code so that only one thread can access it at a time, preventing race conditions.

45. What is the difference between wait() and sleep()?
Answer:

  • wait() pauses the thread and releases the lock.
  • sleep() pauses the thread but does not release the lock.

46. What is garbage collection in Java?
Answer:
Garbage Collection (GC) is the automatic process of removing unused or unreachable objects from memory to free up space.

47. What are strong, weak, and soft references?
Answer:

  • Strong: Default; prevents GC
  • Soft: Collected when memory is low
  • Weak: Collected in the next GC cycle

48. What is the purpose of the volatile keyword?
Answer:
It tells the JVM that a variable may be changed by multiple threads and should always read the value from main memory, not the thread’s cache.

49. What is the difference between final, finally, and finalize()?
Answer:

  • final: Prevents changes (variable, method, or class)
  • finally: Block that always executes after try-catch
  • finalize(): Method called by GC before object removal (deprecated)

50. What is the Java Memory Model (JMM)?
Answer:
The JMM defines how threads interact through memory. It ensures consistency and visibility of shared variables when accessed by multiple threads.

Conclusion

Mastering Core Java is a crucial step for any aspiring software developer or Java professional. The language forms the foundation of many enterprise-level applications, mobile apps, and backend systems across industries. This blog has covered the top 50 Core Java interview questions and answers, helping you strengthen your understanding across key areas such as OOPs, exception handling, collections, multithreading, and Java 8 features.

Whether you are a beginner preparing for your first role or a seasoned developer looking to refresh your knowledge, consistent revision, hands-on practice, and conceptual clarity are the keys to success. Keep coding, explore real-world problems, and walk into your Java interviews with confidence and clarity.

Boost your chances and get ready to learn and prepare with the Core Java Developer Free Practice Test. Start practicing now!

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