☕Java Programming
OOP in Java
Classes, objects, constructors, inheritance, method overriding, and polymorphism in Java.
BeginnerIntermediateAdvanced
What You Will Learn in OOP in Java
Object-Oriented Programming in Java organises code as objects (instances of classes) that encapsulate data and behaviour, supporting inheritance, polymorphism, and abstraction.
- Four pillars: Encapsulation, Inheritance, Polymorphism, Abstraction.
- Class = blueprint; Object = instance with actual memory allocation.
- Constructor: special method with same name as class, no return type, initialises objects.
- Inheritance: `extends` keyword; Java supports single class inheritance, multiple interface inheritance.
- Polymorphism: compile-time (method overloading) and runtime (method overriding).
- `super` refers to parent class; `this` refers to current instance.
Syntax
class Parent {
void display() { System.out.println("Parent"); }
}
class Child extends Parent {
@Override
void display() { System.out.println("Child"); } // runtime polymorphism
void display(String msg) { System.out.println(msg); } // overloading
}Complete Code Example
class Animal {
String name;
Animal(String name) { this.name = name; }
void sound() { System.out.println(name + " makes a sound"); }
}
class Dog extends Animal {
Dog(String name) { super(name); }
@Override
void sound() { System.out.println(name + " barks"); }
}
public class Main {
public static void main(String[] args) {
Animal a = new Dog("Rex"); // upcasting
a.sound(); // Output: Rex barks (runtime polymorphism)
}
}Example
A `Dog` class extending `Animal` and overriding `sound()` demonstrates runtime polymorphism.
Expected Exam Questions — OOP in Java
Q1.What is encapsulation? How is it achieved in Java?
Answer: Encapsulation wraps data (fields) and methods together in a class, hiding internal state from outside. It is achieved by declaring fields as `private` and providing `public` getter/setter methods.
Q2.What is the difference between method overloading and overriding?
Answer: Overloading: same method name, different parameters, same class — resolved at compile time. Overriding: child class redefines parent's method with same signature — resolved at runtime via dynamic dispatch.
Q3.What is an abstract class? How does it differ from an interface?
Answer: Abstract class: can have both abstract (no body) and concrete methods, can have constructors, supports single inheritance. Interface: all methods are abstract by default (Java 7), supports multiple inheritance. Use abstract class for IS-A with shared code; interface for CAN-DO contracts.
Q4.What is the difference between `IS-A` and `HAS-A` relationships?
Answer: IS-A (inheritance): `Dog extends Animal` — Dog IS-A Animal. HAS-A (composition/aggregation): `Car has Engine` — Car HAS-A Engine object as a field. Favour composition over inheritance for looser coupling.
Q5.Can a constructor be inherited in Java?
Answer: No. Constructors are not inherited. However, `super()` in a child class constructor calls the parent constructor. If not written explicitly, Java implicitly calls `super()` (no-arg parent constructor).
🔘 MCQ Practice — OOP in Java
MCQ 1.Which OOP concept is demonstrated when the same method behaves differently in different objects?
A. Encapsulation
B. Abstraction
C. Polymorphism ✓
D. Inheritance
✓ Correct Answer: Polymorphism
MCQ 2.A class cannot extend more than one class in Java. This is a limitation of:
A. Multiple Inheritance ✓
B. Abstraction
C. Polymorphism
D. Encapsulation
✓ Correct Answer: Multiple Inheritance
MCQ 3.Which keyword is used to call a parent class constructor?
A. this()
B. parent()
C. super() ✓
D. base()
✓ Correct Answer: super()
Download OOP in Java PDF Notes
Get the complete OOP in Java notes as a PDF — free for enrolled students, or browse our public study materials library.