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Top 25 Java Interview Questions and Answers for Software Engineers

Java continues to dominate enterprise, backend, and Android development, making it one of the most in-demand skills for software engineers. Whether you’re preparing for FAANG interviews or mid-sized product companies, these 25 most frequently asked Java interview questions will help you master concepts, coding challenges, and JVM internals.

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  1. What are the main features of Java?

Java is built on the principle of WORA (Write Once, Run Anywhere).

Key Features:

  • Object-Oriented → Uses classes and objects.
  • Platform Independent → Bytecode runs on JVM across OS.
  • Robust & Secure → Memory management, exception handling.
  • Multithreaded → Handles multiple threads at once.
  • Portable → Runs on any device with JVM.

Diagram: Java Features Overview

[ Java Program ]

       ↓ Compile

[ Bytecode ] → [ JVM ] → [ Any OS ]

 

  1. Difference between JDK, JRE, and JVM

Component

Full Form

Role

JDK

Java Development Kit

Tools for development (javac, debugger).

JRE

Java Runtime Environment

Libraries + JVM for execution.

JVM

Java Virtual Machine

Converts bytecode → machine code.

Diagram:

JDK = JRE + Development Tools

JRE = JVM + Libraries

JVM = Execution Engine

 

  1. Explain OOP concepts in Java.
  • Encapsulation → Wrapping data/methods.
  • Inheritance → Extending functionality.
  • Polymorphism → One interface, many forms.
  • Abstraction → Hiding implementation.

3. Explain OOP concepts in Java

Code Example:

				
					abstract class Animal { abstract void sound(); }
class Dog extends Animal {
  void sound() { System.out.println("Bark"); }
}
    1. What is the difference between == and .equals()?

    Operator

    Compares

    Example

    ==

    References (memory location)

    “abc” == “abc” → true

    .equals()

    Content

    “abc”.equals(new String(“abc”)) → true

     

    1. String vs StringBuilder vs StringBuffer

    Class

    Mutable

    Thread-Safe

    Performance

    String

    No

    Yes

    Slower

    StringBuilder

    Yes

    No

    Faster

    StringBuffer

    Yes

    Yes

    Slower (due to synchronization)

    StringBuilder sb = new StringBuilder(“Java”);

    sb.append(” Rocks”);

    System.out.println(sb);

     

    1. Explain Java Memory Model (Heap vs Stack)

    Diagram:

    Stack Memory       Heap Memory

    —————    ——————-

    – Local vars       – Objects

    – Method calls     – Class instances

    – Thread-specific  – Shared across threads

     

    1. What is Garbage Collection?

    Java automatically reclaims unused memory.

    • Generational GC: Young, Old, Permanent generations.

       

    • Algorithms: Mark-and-Sweep, Copying, Compacting.

       

7. What is Garbage Collection 
				
					public class GCExample {
  public static void main(String[] args) {
    String s = new String("Hello");
    s = null; // eligible for GC
    System.gc();
  }
}
  1. Difference between final, finally, and finalize()

Term

Meaning

final

Keyword → constants, no inheritance, no override.

finally

Block in exception handling → always runs.

finalize()

Cleanup before GC (deprecated).

  1. Checked vs Unchecked Exceptions
  • Checked → Compile-time (IOException, SQLException).
  • Unchecked → Runtime (NullPointerException).

try {

  FileReader fr = new FileReader(“test.txt”); // checked

} catch (IOException e) { e.printStackTrace(); }

 

  1. HashMap vs Hashtable

Feature

HashMap

Hashtable

Thread-safe

No

Yes

Nulls

1 null key allowed

Not allowed

Performance

Faster

Slower

 

  1. Explain ConcurrentHashMap.
  • Thread-safe.
  • Divides map into segments → reduces contention.
  • Allows concurrent reads/writes.

ConcurrentHashMap<Integer, String> map = new ConcurrentHashMap<>();

map.put(1, “Java”);

 

  1. synchronized vs volatile

Keyword

Ensures

Use Case

synchronized

Mutual exclusion

Thread-safe methods

volatile

Visibility of changes

Flags, simple variables

 

  1. What are Streams in Java 8?

List<String> names = Arrays.asList(“Tom”,”Jerry”,”Mike”);

names.stream()

     .filter(s -> s.startsWith(“M”))

     .forEach(System.out::println);

 

  1. JVM, JIT, and ClassLoader
  • JVM → Executes bytecode.

  • JIT → Compiles hot code to native machine code.

  • ClassLoader → Dynamically loads classes.

Diagram:

Source → Bytecode → ClassLoader → JVM → Native Code

 

  1. Explain Escape Analysis.

JVM optimization: decides if object can be on stack instead of heap → reduces GC load.

15. Explain Escape Analysis

16. Implement Singleton in Java.

				
					public class Singleton {
  private static volatile Singleton instance;
  private Singleton() {}
  public static Singleton getInstance() {
    if (instance == null) {
      synchronized (Singleton.class) {
        if (instance == null) instance = new Singleton();
      }
    }
    return instance;
  }
}

17. Reverse a String

				
					String str = "Interview";
String reversed = new StringBuilder(str).reverse().toString();

18. Prime Number Program

				
					boolean isPrime(int n) {
  if (n <= 1) return false;
  for (int i = 2; i <= Math.sqrt(n); i++) {
    if (n % i == 0) return false;
  }
  return true;
}

19. Producer-Consumer Problem

				
					BlockingQueue<Integer> q = new ArrayBlockingQueue<>(5);

// Producer
new Thread(() -> {
  try { q.put(1); } catch(Exception e){}
}).start();

// Consumer
new Thread(() -> {
  try { System.out.println(q.take()); } catch(Exception e){}
}).start();
  1. Detect Deadlock in Java
  • Use ThreadMXBean from ManagementFactory.
  • Analyze thread dumps.

ThreadMXBean bean = ManagementFactory.getThreadMXBean();

long[] ids = bean.findDeadlockedThreads();

20. Detect Deadlock in Java

21. LRU Cache with LinkedHashMap

				
					class LRU<K,V> extends LinkedHashMap<K,V> {
  private int capacity;
  LRU(int cap){ super(cap,0.75f,true); this.capacity=cap; }
  protected boolean removeEldestEntry(Map.Entry<K,V> eldest){
    return size() > capacity;
  }
}
  1. How does Java support microservices?
  • Frameworks → Spring Boot, Micronaut, Quarkus.
  • Communication → REST, gRPC, Kafka.
  1. Securing Java Web Applications
  • Use Spring Security.
  • Avoid SQL injection with PreparedStatement.
  • Enable HTTPS and JWT authentication.
  1. New Features in Java 17
  • Sealed Classes.
  • Pattern Matching for Switch.
  • Strong Encapsulation.

sealed interface Shape permits Circle, Square {}

final class Circle implements Shape {}

  1. How to Optimize Java Performance?
  • Use efficient data structures.
  • Minimize object creation.
  • Use profilers (VisualVM, JConsole).
  • Apply caching.
  • Tune JVM parameters.

 

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