Inheritance Basics · Inheritance in Java | Java

IS-A Relationship

Inheritance lets a class (the subclass or child) reuse fields and methods of another class (the superclass or parent) using extends. It models an IS-A relationship: a Car IS-A Vehicle.

Vehicle            (brand, start())
        /      \
      Car      Bike         (Car adds doors, Bike adds hasGear)

Java supports single inheritance for classes: a class extends only one class.
Multilevel is fine: ElectricCar extends Car extends Vehicle.
Private members are not accessible in the child (they still exist inside the object).
Constructors are not inherited.

First Example

Example

public class Main {
    public static void main(String[] args) {
        Car car = new Car();
        car.brand = "Tata";
        car.doors = 4;
        car.start();        // inherited from Vehicle
        car.openBoot();     // defined in Car
        System.out.println(car.brand + " car with " + car.doors + " doors");
    }
}

class Vehicle {
    String brand;

    void start() {
        System.out.println(brand + " vehicle starting");
    }
}

class Car extends Vehicle {
    int doors;

    void openBoot() {
        System.out.println("Boot opened");
    }
}

Output

Tata vehicle starting
Boot opened
Tata car with 4 doors

Types of Inheritance

Type	Example	Allowed for classes?
Single	`B extends A`	yes
Multilevel	`C extends B`, `B extends A`	yes
Hierarchical	`B extends A`, `C extends A`	yes
Multiple	`C extends A, B`	no (use interfaces)

Note: Multiple inheritance of classes is not allowed because of the "diamond problem": if both parents had the same method, Java could not know which one to use.