Unit 4 · The Four Pillars in Depth · 10 min

Inheritance

Inheritance lets a new class reuse the fields and methods of an existing class using the keyword `extends`.

Why this matters

It removes duplicate code. Common features go in the parent class; only the differences go in the child.

Real-life analogy

You inherit features from your parents and add your own. A Car is a Vehicle with extra car-specific things.

Syntax

class Child extends Parent { }

Example

Java

class Person {
    String name;

    void introduce() {
        System.out.println("I am " + name);
    }
}

class Student extends Person {
    int rollNumber;

    void showRoll() {
        System.out.println(name + " has roll number " + rollNumber);
    }
}

public class Main {
    public static void main(String[] args) {
        Student s = new Student();
        s.name = "Ananya Rao";     // inherited from Person
        s.rollNumber = 12;
        s.introduce();           // inherited method
        s.showRoll();
    }
}

Output

I am Ananya Rao
Ananya Rao has roll number 12

Step by step

1`class Student extends Person` makes Student the child (subclass) of Person.
2Student automatically gets the field `name` and the method `introduce()`.
3Student adds its own field rollNumber and method showRoll().
4This is an IS-A relationship: a Student IS-A Person.

Key points

extends creates a parent-child relationship
Types: single, multilevel, hierarchical
Java does NOT support multiple inheritance of classes (use interfaces instead)
private members are not directly accessible in the child

Common mistakes

Trying `class A extends B, C` — not allowed in Java
Expecting to access a private parent field directly in the child
Using inheritance where composition (HAS-A) is more suitable

Try it yourself

Create Vehicle and Car classes using inheritance.
Build a three-level chain: Person → Employee → Manager.

Open the practice notebook