OOP Interview Questions

What are the four pillars of OOP?

Pillar	Meaning	Java feature
Encapsulation	hide data, expose behaviour	`private` fields + public methods
Inheritance	reuse and extend a parent type	`extends`, `implements`
Polymorphism	one interface, many implementations	overloading, overriding
Abstraction	show what, hide how	abstract classes, interfaces

What is the difference between method overloading and overriding?

	Overloading	Overriding
Where	same class	subclass
Signature	same name, different parameters	same name and parameters
Resolved at	compile time (static polymorphism)	runtime (dynamic dispatch)
Return type	can differ	same or covariant (subtype)
static / private methods	can be overloaded	cannot be overridden (static ones are hidden)

Show runtime polymorphism with an example.

Example

The reference type is Payment, but the method of the actual object runs.

import java.util.List;

public class Main {
    public static void main(String[] args) {
        List<Payment> payments = List.of(new UpiPayment(), new CardPayment());
        for (Payment p : payments) {
            p.pay(500);
        }
    }
}

abstract class Payment {
    abstract void pay(int amount);
}

class UpiPayment extends Payment {
    void pay(int amount) { System.out.println("Paid Rs " + amount + " via UPI"); }
}

class CardPayment extends Payment {
    void pay(int amount) { System.out.println("Paid Rs " + amount + " via Card"); }
}

Output

Paid Rs 500 via UPI
Paid Rs 500 via Card

What is the difference between an abstract class and an interface?

	Abstract class	Interface
Inheritance	a class extends only one	a class implements many
State	can have instance fields	only `public static final` constants
Constructors	yes	no
Methods	abstract and concrete, any access	abstract, `default`, `static`, `private` (Java 9+)
Use when	related classes share state/code ("is-a")	unrelated classes share a capability ("can-do")

Why does Java not support multiple inheritance of classes?

To avoid the diamond problem: if two parents define the same method, the child would not know which one to inherit. Java allows multiple inheritance of types through interfaces. If two interfaces give conflicting default methods, the class must override it and can pick one with A.super.method().

What is constructor chaining? Are constructors inherited?

Constructors are not inherited. Every constructor first calls a parent constructor (implicitly super()), so object creation runs from the top of the hierarchy down.

Example

Parent constructors run first.

public class Main {
    public static void main(String[] args) {
        new C();
    }
}

class A { A() { System.out.println("A constructor"); } }
class B extends A { B() { System.out.println("B constructor"); } }
class C extends B { C() { System.out.println("C constructor"); } }

Output

A constructor
B constructor
C constructor

What are access modifiers in Java?

Modifier	Class	Package	Subclass (other package)	Everywhere
private	yes	no	no	no
default (none)	yes	yes	no	no
protected	yes	yes	yes	no
public	yes	yes	yes	yes