Inheritance vs Composition in java

They are absolutely different. Inheritance is an "is-a" relationship. Composition is a "has-a".


In the composition approach, the subclass becomes the "front-end class," and the superclass becomes the "back-end class." With inheritance, a subclass automatically inherits an implemenation of any non-private superclass method that it doesn't override. With composition, by contrast, the front-end class must explicitly invoke a corresponding method in the back-end class from its own implementation of the method. This explicit call is sometimes called "forwarding" or "delegating" the method invocation to the back-end object.

The composition approach to code reuse provides stronger encapsulation than inheritance, because a change to a back-end class needn't break any code that relies only on the front-end class. 

Inheritance Example:

class Fruit {


    // Return int number of pieces of peel that
    // resulted from the peeling activity.
    public int peel() {


        System.out.println("Peeling is appealing.");
        return 1;
    }
}


class Apple extends Fruit {
}


class Example1 {


    public static void main(String[] args) {


        Apple apple = new Apple();
        int pieces = apple.peel();
    }
}

if we want to change return type to peel() then it will effect to main method also so solution is composition as below:

Composition Example:

class Fruit {


    // Return int number of pieces of peel that
    // resulted from the peeling activity.
    public int peel() {


        System.out.println("Peeling is appealing.");
        return 1;
    }
}


class Apple {


    private Fruit fruit = new Fruit();


    public int peel() {
        return fruit.peel();
    }
}


class Example2 {


    public static void main(String[] args) {


        Apple apple = new Apple();
        int pieces = apple.peel();
    }
}



ref: https://www.javaworld.com/article/2076814/core-java/inheritance-versus-composition--which-one-should-you-choose-.html