Inheritance

IS-A with extends

• class Dog extends Animal means Dog **is an** Animal. Dog inherits visible fields and methods.
• Java allows **one** superclass. Multiple inheritance of *classes* is forbidden (interfaces provide multiple *types*).
• Use inheritance for genuine specialization, not just to reuse a utility method — prefer composition (HAS-A) then.

class Animal {
    String name;

    Animal(String name) {
        this.name = name;
    }

    void speak() {
        System.out.println(name + " makes a sound");
    }
}

class Dog extends Animal {
    Dog(String name) {
        super(name); // first statement
    }

    @Override
    void speak() {
        System.out.println(name + " barks");
    }
}

public class InheritDemo {
    public static void main(String[] args) {
        Animal a = new Dog("Rex");
        a.speak();
    }
}

super and constructors

• If you do not write super(...), the compiler inserts super(). That fails if the parent has no no-arg constructor.
• super.method() calls the superclass implementation (useful inside an override).
• Private members are not inherited in the usable sense — the subclass cannot access them.

Object and types of inheritance

Fragile base class and final

• Overridable methods called from a superclass constructor see a not-yet-initialized subclass — a classic bug. Do not call overridable methods from constructors.
• final class cannot be extended. final method cannot be overridden. sealed classes (Java 17) list permitted subclasses.
• protected members are visible to subclasses (and the same package).
• Inheritance copies the API contract. If you only wanted reuse, a private field of that type is simpler (composition).

Multilevel constructors

class Vehicle {
    Vehicle() {
        System.out.println("Vehicle");
    }
}

class Car extends Vehicle {
    Car() {
        System.out.println("Car");
    }
}

class EvCar extends Car {
    EvCar() {
        System.out.println("EvCar");
    }

    public static void main(String[] args) {
        new EvCar();
    }
}