OOP Java: Better to extend a class or create a new instance of the subclass? [duplicate]

Question

I'm having trouble with the concepts of object oriented programming. Is it better to extend a class or create a new object within a class? Under which circumstances would you extend a subclass versus creating a new instance of that subclass within the calling class? The example is in Java but I imagine the concepts will work in other OOP languages.

I appreciate your insights

class Mini {
// I want to use the members of this class in another class
    int myInt;

    public Mini(int myInt){
        this.myInt = myInt;
    }

    public int myMethod(){
        return this.myInt;
    }
}

// should I create a new instance of Mini here?
class Maxi {
    Mini myMini = new Mini(5);

    public Maxi(){
        int miniInt = myMini.myMethod();
        System.out.print(miniInt );
    }
}

// or should I have Maxi extend Mini?
class Maxi extends Mini {
    public Maxi(int myInt){
        System.out.print(this.myInt);
    }
}

Answer 1

You extend a class when it has an is-a relationship. For example, a Cat is an Animal. A Cat is not a CatFood but it might use CatFood.

In your case, I'm not sure what MiniMaxi are, but it doesn't sound like Maxi is a Mini, rather it uses one.

In general, try to favor composition (using objects) over inheritance, especially in single inheritance languages like java.

Answer 2

The best answer to that question is another question: does class B use class A or does class B build on class A?