Top 50 Java Interview Questions for Freshers 2026

Last Updated: March 2026 | Level: Freshers & 0-2 Years Experience | Read Time: ~20 min

Master these 50 Java interview questions before your next technical round. Curated from real interview experiences at top MNCs, startups, and product companies. Java is the primary language tested at Infosys, TCS, and Wipro — pair this guide with our Infosys Placement Papers 2026 and TCS Placement Papers 2026 for end-to-end preparation.

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Table of Contents

1. Core Java (Q1–Q15)
2. OOP Concepts (Q16–Q25)
3. Collections Framework (Q26–Q35)
4. Multithreading (Q36–Q43)
5. Java 8+ Features (Q44–Q50)

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Core Java

Q1. What is the difference between JDK, JRE, and JVM? Easy

Memory tip: JDK ⊃ JRE ⊃ JVM

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Q2. What is the difference between == and .equals() in Java? Easy

String a = new String("hello");
String b = new String("hello");

System.out.println(a == b);       // false (different objects)
System.out.println(a.equals(b));  // true (same content)

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Q3. What is autoboxing and unboxing in Java? Easy

int num = 42;
Integer boxed = num;       // autoboxing
int unboxed = boxed;       // unboxing

List<Integer> list = new ArrayList<>();
list.add(10);              // autoboxing int → Integer

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Q4. What is the final keyword in Java? Easy

final int MAX = 100;          // constant variable
// MAX = 200;                 // compile error

final class Immutable {}     // cannot be subclassed
// class Sub extends Immutable {} // compile error

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Q5. Explain the difference between String, StringBuilder, and StringBuffer. Medium

// String (immutable)
String s = "Hello";
s = s + " World";  // new object created

// StringBuilder (mutable, not thread-safe)
StringBuilder sb = new StringBuilder("Hello");
sb.append(" World");  // modifies same object

// StringBuffer (mutable, thread-safe)
StringBuffer sbuf = new StringBuffer("Hello");
sbuf.append(" World");

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Q6. What is the String Pool in Java? Medium

String a = "hello";   // stored in pool
String b = "hello";   // returns existing pool reference
String c = new String("hello");  // forces new heap object

System.out.println(a == b);  // true (same pool reference)
System.out.println(a == c);  // false (different object)
System.out.println(a == c.intern());  // true (intern() adds to pool)

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Q7. What are access modifiers in Java? Easy

Modifier	Class	Package	Subclass	World
private	✅	❌	❌	❌
default	✅	✅	❌	❌
protected	✅	✅	✅	❌
public	✅	✅	✅	✅

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Q8. What is a static method and when would you use it? Easy

class MathUtils {
    static int square(int n) {
        return n * n;
    }
}

// Call without creating object
int result = MathUtils.square(5);  // 25

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Q9. What is exception handling in Java? Explain try-catch-finally. Medium

try {
    int result = 10 / 0;  // throws ArithmeticException
} catch (ArithmeticException e) {
    System.out.println("Cannot divide by zero: " + e.getMessage());
} finally {
    System.out.println("This always runs");
}

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Q10. What is the difference between checked and unchecked exceptions? Medium

// Checked exception — must handle
try {
    FileReader fr = new FileReader("file.txt");  // throws IOException
} catch (IOException e) { }

// Unchecked exception — optional handling
String s = null;
s.length();  // throws NullPointerException at runtime

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Q11. What is the this keyword in Java? Easy

class Person {
    String name;
    
    Person(String name) {
        this.name = name;  // 'this.name' = field, 'name' = parameter
    }
}

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Q12. What is type casting in Java? Medium

// Widening (automatic)
int i = 100;
long l = i;       // automatic widening

// Narrowing (explicit)
double d = 9.99;
int n = (int) d;  // explicit cast, n = 9 (decimal truncated)

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Q13. What is a Java interface? How is it different from an abstract class? Medium

interface Drawable {
    void draw();                          // abstract
    default void show() {                 // default method (Java 8+)
        System.out.println("Showing...");
    }
}

abstract class Shape {
    int color;                            // state allowed
    abstract double area();               // abstract method
    void describe() { }                   // concrete method
}

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Q14. What is garbage collection in Java? Medium

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Q15. What is the difference between Array and ArrayList? Easy

// Array — fixed size
int[] arr = new int[5];
arr[0] = 10;

// ArrayList — dynamic size
ArrayList<Integer> list = new ArrayList<>();
list.add(10);
list.add(20);
list.remove(0);  // removes element at index 0

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OOP Concepts

Q16. What are the four pillars of OOP? Easy

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Q17. What is the difference between method overloading and method overriding? Medium

// Overloading (same class, different params)
class Calculator {
    int add(int a, int b) { return a + b; }
    double add(double a, double b) { return a + b; }
}

// Overriding (subclass reimplements parent method)
class Animal { void sound() { System.out.println("..."); } }
class Dog extends Animal {
    @Override
    void sound() { System.out.println("Woof!"); }
}

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Q18. What is a constructor? What is constructor chaining? Easy

class Employee {
    String name;
    int age;
    
    Employee(String name) {
        this(name, 25);  // chains to next constructor
    }
    
    Employee(String name, int age) {
        this.name = name;
        this.age = age;
    }
}

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Q19. What is the super keyword? Easy

class Vehicle {
    String brand = "Generic";
    void start() { System.out.println("Vehicle starting"); }
}

class Car extends Vehicle {
    String brand = "Toyota";
    
    void display() {
        System.out.println(brand);        // Toyota (child)
        System.out.println(super.brand);  // Generic (parent)
        super.start();                    // calls parent method
    }
}

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Q20. What is an abstract class? When should you use it? Medium

abstract class Shape {
    String color;
    
    Shape(String color) { this.color = color; }
    
    abstract double area();  // must be implemented by subclasses
    
    void displayColor() {    // shared concrete method
        System.out.println("Color: " + color);
    }
}

class Circle extends Shape {
    double radius;
    Circle(String color, double r) { super(color); this.radius = r; }
    
    @Override
    double area() { return Math.PI * radius * radius; }
}

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Q21. What is polymorphism? Give a real-world example. Medium

class Payment {
    void process() { System.out.println("Processing payment..."); }
}
class CreditCard extends Payment {
    @Override void process() { System.out.println("Processing credit card"); }
}
class UPI extends Payment {
    @Override void process() { System.out.println("Processing UPI"); }
}

Payment p = new CreditCard();  // parent ref, child object
p.process();  // "Processing credit card" — runtime dispatch

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Q22. What is encapsulation? Why is it important? Easy

class BankAccount {
    private double balance;  // hidden
    
    public double getBalance() { return balance; }  // controlled read
    
    public void deposit(double amount) {
        if (amount > 0) balance += amount;          // controlled write
    }
}

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Q23. Can an interface extend another interface? Medium

interface Readable { void read(); }
interface Writable { void write(); }
interface ReadWrite extends Readable, Writable { }  // multiple extension

class File implements ReadWrite {
    public void read() { System.out.println("Reading..."); }
    public void write() { System.out.println("Writing..."); }
}

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Q24. What is the difference between aggregation and composition? Hard

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Q25. What is the Liskov Substitution Principle? Hard

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Collections Framework

Q26. What is the Java Collections Framework? Easy

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Q27. What is the difference between ArrayList and LinkedList? Medium

ArrayList<String> arrayList = new ArrayList<>();  // backed by array
LinkedList<String> linkedList = new LinkedList<>(); // backed by linked nodes
linkedList.addFirst("first");  // O(1) — LinkedList advantage

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Q28. What is the difference between HashMap and HashSet? Easy

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Q29. What is the difference between HashMap and TreeMap? Medium

Map<String, Integer> hashMap = new HashMap<>();    // unordered
Map<String, Integer> treeMap = new TreeMap<>();    // sorted by key

treeMap.put("banana", 2);
treeMap.put("apple", 1);
treeMap.put("cherry", 3);
// Iterates: apple, banana, cherry (sorted)

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Q30. How does HashMap work internally? Hard

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Q31. What is the difference between fail-fast and fail-safe iterators? Hard

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Q32. What is LinkedHashMap? Medium

Map<String, Integer> lhm = new LinkedHashMap<>();
lhm.put("c", 3); lhm.put("a", 1); lhm.put("b", 2);
// Iterates in insertion order: c, a, b

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Q33. What is the difference between Iterator and ListIterator? Medium

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Q34. What is Collections.synchronizedList()? Medium

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Q35. What is the difference between Comparable and Comparator? Medium

// Comparable — class implements it
class Student implements Comparable<Student> {
    int age;
    public int compareTo(Student other) { return this.age - other.age; }
}

// Comparator — external sorting logic
Comparator<Student> byName = (s1, s2) -> s1.name.compareTo(s2.name);
students.sort(byName);

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Multithreading

Q36. What is a thread? How do you create one in Java? Easy

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

// Method 2: Implement Runnable (preferred)
Thread t = new Thread(() -> System.out.println("Runnable running"));
t.start();

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Q37. What is the difference between start() and run() in threads? Easy

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Q38. What is synchronization in Java? Medium

class Counter {
    private int count = 0;
    
    synchronized void increment() {  // only one thread at a time
        count++;
    }
}

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Q39. What is a deadlock? How can you prevent it? Hard

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Q40. What is the volatile keyword? Hard

class SharedFlag {
    volatile boolean running = true;  // always read from main memory
    
    void stop() { running = false; }
    void doWork() {
        while (running) { /* work */ }
    }
}

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Q41. What is ExecutorService? Medium

ExecutorService executor = Executors.newFixedThreadPool(4);

Future<Integer> future = executor.submit(() -> {
    return 42;  // task returning a value
});

int result = future.get();  // blocks until result ready
executor.shutdown();

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Q42. What is the difference between wait() and sleep()? Medium

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Q43. What is a ThreadLocal variable? Hard

ThreadLocal<Integer> threadId = ThreadLocal.withInitial(() -> 0);

// Each thread gets its own copy
Runnable task = () -> {
    threadId.set((int)(Math.random() * 100));
    System.out.println("Thread ID: " + threadId.get());
};

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Java 8+ Features

Q44. What are lambda expressions? Easy

// Before Java 8 (anonymous class)
Runnable r1 = new Runnable() {
    public void run() { System.out.println("Running"); }
};

// Java 8 lambda
Runnable r2 = () -> System.out.println("Running");

// With Collections
List<String> names = Arrays.asList("Alice", "Bob", "Charlie");
names.forEach(name -> System.out.println(name));

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Q45. What is the Stream API? Medium

List<Integer> numbers = Arrays.asList(1, 2, 3, 4, 5, 6, 7, 8, 9, 10);

int sumOfEvenSquares = numbers.stream()
    .filter(n -> n % 2 == 0)       // keep evens: 2,4,6,8,10
    .map(n -> n * n)                // square: 4,16,36,64,100
    .reduce(0, Integer::sum);       // sum: 220

System.out.println(sumOfEvenSquares);  // 220

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Q46. What is a functional interface? Medium

@FunctionalInterface
interface Transformer<T> {
    T transform(T input);
}

Transformer<String> upper = s -> s.toUpperCase();
System.out.println(upper.transform("hello"));  // HELLO

// Built-in functional interfaces
Predicate<Integer> isEven = n -> n % 2 == 0;
Function<String, Integer> strLen = String::length;

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Q47. What is Optional in Java 8? Medium

Optional<String> name = Optional.of("Alice");
Optional<String> empty = Optional.empty();

System.out.println(name.orElse("Unknown"));   // Alice
System.out.println(empty.orElse("Unknown"));  // Unknown

// Method chaining
Optional<String> result = Optional.of("  hello  ")
    .map(String::trim)
    .filter(s -> !s.isEmpty());

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Q48. What are default methods in interfaces? Medium

interface Greetable {
    default void greet() {
        System.out.println("Hello!");
    }
    
    default void greetFormal(String name) {
        System.out.println("Good day, " + name);
    }
}

class Person implements Greetable {
    // greet() is inherited — no need to override
}

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Q49. What is method reference in Java 8? Medium

List<String> names = Arrays.asList("Alice", "Bob", "Charlie");

// Lambda
names.forEach(name -> System.out.println(name));

// Method reference (equivalent)
names.forEach(System.out::println);

// Static method reference
List<String> upperNames = names.stream()
    .map(String::toUpperCase)  // instance method ref
    .collect(Collectors.toList());

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Q50. What is the difference between map() and flatMap() in streams? Hard

// map() — one to one
List<String> words = Arrays.asList("Hello World", "Java 8");
List<String[]> mapped = words.stream()
    .map(s -> s.split(" "))    // Stream<String[]>
    .collect(Collectors.toList());

// flatMap() — one to many, then flatten
List<String> flatMapped = words.stream()
    .flatMap(s -> Arrays.stream(s.split(" ")))  // Stream<String>
    .collect(Collectors.toList());
// Result: ["Hello", "World", "Java", "8"]

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Quick Revision Tips

Topic	Key Points
JVM vs JDK vs JRE	JDK > JRE > JVM
String Pool	Literals cached, new String() bypasses pool
HashMap	Array of buckets, hashCode + equals, O(1) avg
Thread Safety	synchronized, volatile, atomic, concurrent collections
Java 8	Lambdas, Streams, Optional, Default methods, Method refs

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