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☕ Core Java Interview Questions 2026

Core Java Interview Questions, OOPs, Collections, Multi-threading, and JVM Architecture.

All (20) 🟢 Easy 🟡 Medium 🔴 Hard
1
What is the difference between JDK, JRE, and JVM?
Easy ▼
JDK (Java Development Kit): Complete toolkit for developers. Includes compiler (javac), JRE, debugger, and development tools.

JRE (Java Runtime Environment): Required to RUN Java programs. Includes JVM + core libraries. No compiler.

JVM (Java Virtual Machine): Executes bytecode. Platform-specific but makes Java platform-independent.

Relation: JDK ⊃ JRE ⊃ JVM
2
What are the 4 pillars of OOP in Java?
Easy ▼
1. Encapsulation: Wrapping data + methods into a class. Use private fields + public getters/setters.

2. Inheritance: Child class acquires properties of parent class using extends keyword.

3. Polymorphism: Same method behaves differently. Two types:
  • Compile-time (Method Overloading)
  • Runtime (Method Overriding)


4. Abstraction: Hiding implementation details. Achieved via abstract classes and interfaces.
3
What is the difference between == and .equals() in Java?
Easy ▼
== operator: Compares object references (memory address), not content.

.equals(): Compares actual content/values.

Example:
String a = new String("hello");
String b = new String("hello");
a == b // false (different objects)
a.equals(b) // true (same content)

Note: For String literals, == may work due to String Pool, but always use .equals() for safe comparison.
4
What is the difference between ArrayList and LinkedList?
Easy ▼
ArrayList:
  • Dynamic array internally
  • Fast random access O(1)
  • Slow insert/delete in middle O(n)
  • Less memory overhead


LinkedList:
  • Doubly linked list internally
  • Slow random access O(n)
  • Fast insert/delete O(1)
  • More memory (stores prev/next pointers)


Use ArrayList when you read frequently. Use LinkedList when you insert/delete frequently.
5
What is method overloading vs method overriding?
Easy ▼
Method Overloading (Compile-time Polymorphism):
  • Same method name, different parameters
  • Happens in same class
  • Resolved at compile time

int add(int a, int b)
double add(double a, double b)

Method Overriding (Runtime Polymorphism):
  • Same method signature in parent + child class
  • Child class provides specific implementation
  • Uses @Override annotation
  • Resolved at runtime
6
What is the difference between abstract class and interface?
Medium ▼
Abstract Class:
  • Can have both abstract and concrete methods
  • Can have instance variables
  • Supports constructors
  • Single inheritance only
  • Use when classes share code


Interface:
  • All methods abstract by default (Java 7)
  • Java 8+ allows default/static methods
  • Only public static final variables
  • Multiple interface implementation allowed
  • Use to define a contract/behavior
7
Explain the concept of Java Collections Framework.
Medium ▼
Java Collections Framework provides a unified architecture to store and manipulate groups of objects.

Key Interfaces:
  • List — ordered, allows duplicates (ArrayList, LinkedList)
  • Set — no duplicates (HashSet, TreeSet, LinkedHashSet)
  • Map — key-value pairs (HashMap, TreeMap, LinkedHashMap)
  • Queue — FIFO structure (PriorityQueue, LinkedList)


Root interface: Collection (except Map)
8
What is multithreading? How do you create a thread in Java?
Medium ▼
Multithreading: Executing multiple threads simultaneously to maximize CPU usage.

Two ways to create a thread:

1. Extend Thread class:
class MyThread extends Thread {
public void run() { System.out.println("Thread running"); }
}
new MyThread().start();

2. Implement Runnable interface (Preferred):
class MyTask implements Runnable {
public void run() { System.out.println("Running"); }
}
new Thread(new MyTask()).start();
9
What is the difference between HashMap and Hashtable?
Medium ▼
HashMap:
  • Not synchronized (not thread-safe)
  • Allows one null key and multiple null values
  • Faster performance
  • Part of Java 1.2 Collections


Hashtable:
  • Synchronized (thread-safe)
  • Does not allow any null key or value
  • Slower due to synchronization
  • Legacy class (Java 1.0)


Tip: Use ConcurrentHashMap instead of Hashtable for thread-safe operations.
10
What is the Java Memory Model? Explain Heap and Stack.
Medium ▼
Stack Memory:
  • Stores method calls, local variables, references
  • LIFO order
  • Thread-specific (each thread has its own stack)
  • Automatically freed when method returns
  • StackOverflowError when full


Heap Memory:
  • Stores all objects and class instances
  • Shared across threads
  • Managed by Garbage Collector
  • OutOfMemoryError when full


String Pool is a special area in heap for string literals.
11
What are Java 8 Streams? Give an example.
Medium ▼
Java 8 Streams provide a declarative way to process collections of data.

Key Operations:
  • Intermediate: filter(), map(), sorted(), distinct()
  • Terminal: collect(), forEach(), count(), reduce()


Example — filter even numbers:
List nums = Arrays.asList(1,2,3,4,5,6);
List evens = nums.stream()
.filter(n -> n % 2 == 0)
.collect(Collectors.toList());
// Output: [2, 4, 6]
12
What is a Lambda Expression in Java 8?
Easy ▼
A lambda expression is a short block of code that takes parameters and returns a value. It implements a functional interface.

Syntax: (parameters) -> expression

Without Lambda:
Runnable r = new Runnable() {
public void run() { System.out.println("Hello"); }
};

With Lambda:
Runnable r = () -> System.out.println("Hello");

Common use: With Collections.sort(), Stream API, event handlers.
13
What is the difference between final, finally, and finalize?
Easy ▼
final: Keyword
  • Variable: value cannot change (constant)
  • Method: cannot be overridden
  • Class: cannot be extended


finally: Block in try-catch that always executes (for cleanup).
try { } catch(e) { } finally { // always runs }

finalize(): Method called by GC before object is garbage collected. Deprecated in Java 9+.
14
What is Exception Handling in Java? What is checked vs unchecked exception?
Easy ▼
Exception Handling keywords: try, catch, finally, throw, throws

Checked Exceptions:
  • Checked at compile time
  • Must be handled or declared
  • Examples: IOException, SQLException, FileNotFoundException


Unchecked Exceptions (RuntimeException):
  • Checked at runtime
  • No mandatory handling
  • Examples: NullPointerException, ArrayIndexOutOfBoundsException, ClassCastException
15
What is Garbage Collection in Java?
Medium ▼
Garbage Collection (GC) is the process of automatically reclaiming heap memory occupied by objects that are no longer referenced.

How it works:
  • JVM tracks object references
  • Objects with no references become eligible for GC
  • GC runs in background (daemon thread)


GC Algorithms: Serial GC, Parallel GC, G1 GC (default Java 9+), ZGC

You cannot force GC, but can request: System.gc() (not guaranteed)
16
What is the Singleton Design Pattern? Implement it in Java.
Hard ▼
Singleton ensures only ONE instance of a class exists throughout the application.

Thread-safe implementation:
public class Singleton {
private static volatile Singleton instance;

private Singleton() { } // private constructor

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

Use cases: Database connection, Logger, Configuration manager
17
What is the difference between String, StringBuilder, and StringBuffer?
Easy ▼
String:
  • Immutable — any change creates new object
  • Thread-safe (immutable)
  • Stored in String Pool


StringBuilder:
  • Mutable — in-place modifications
  • NOT thread-safe
  • Faster than StringBuffer
  • Use for single-thread string manipulation


StringBuffer:
  • Mutable — in-place modifications
  • Thread-safe (synchronized)
  • Slower than StringBuilder
  • Use in multi-threaded environment
18
What is Java Reflection API?
Hard ▼
Reflection allows a program to inspect and modify its own structure and behavior at runtime.

Capabilities:
  • Get class name, methods, fields, constructors
  • Invoke methods dynamically
  • Access private members
  • Create objects without knowing the class at compile time


Example:
Class cls = Class.forName("java.lang.String");
Method[] methods = cls.getMethods();
for(Method m : methods) System.out.println(m.getName());

Used in: Frameworks (Spring, Hibernate), IDEs, Testing tools
19
What are generics in Java and why are they used?
Medium ▼
Generics allow you to write type-safe code that works with different data types without casting.

Benefits:
  • Type safety at compile time
  • Eliminates ClassCastException
  • Code reusability


Generic Class:
class Box {
T value;
Box(T v) { this.value = v; }
T get() { return value; }
}
Box intBox = new Box<>(42);
Box strBox = new Box<>("Hello");

Wildcards: ? extends T (upper bound), ? super T (lower bound)
20
What is the difference between Comparable and Comparator in Java?
Hard ▼
Comparable (java.lang):
  • Interface with compareTo() method
  • Class itself defines its natural ordering
  • Modifies the actual class

class Student implements Comparable {
public int compareTo(Student s) { return this.age - s.age; }
}

Comparator (java.util):
  • Interface with compare() method
  • External class defines custom ordering
  • Multiple sort orders possible without modifying class

Collections.sort(list, (a,b) -> a.name.compareTo(b.name));