Java Interface-Implementation Pair [closed]

Answer 1

I'd also use static methods to declare default functionality for stateless functionality.

For stateful functionality I'd prefer composition instead of inheritance. With composition and using delegates/adapters you can combine default functionality from many sources.

e.g.

public interface StuffDoer{
    void doStuff();
    void doOtherStuff();
}

public class MyStuffDoer implements StuffDoer{
    private final StuffDoer mixin;
    public  MyStuffDoer(StuffDoer mixin){
        this.mixin = mixin;
    }
    public void doStuff(){
        mixin.doStuff();
    }
    public void doOtherStuff(){
        mixin.doOtherStuff();
    }

}

public class MyStuffDoer2 implements StuffDoer{
    private final StuffDoer mixin1, mixin2;
    public  MyStuffDoer(StuffDoer mixin1, StuffDoer mixin2){
        this.mixin1 = mixin1;
        this.mixin2 = mixin2;
    }
    public void doStuff(){
        mixin1.doStuff();
    }
    public void doOtherStuff(){
        mixin2.doOtherStuff();
    }

}

For simple cases inheritance is ok as well, but it is not really very flexible.

Implementing multiple interfaces is also a case where this approach scales better.

public interface A{
    void doStuff();
}

public interface B{
    void doOtherStuff();
}

public class MyStuffDoer implements A, B{
    private final A mixin1;
    private final B mixin2;
    public  MyStuffDoer(A mixin1, B mixin2){
        this.mixin1 = mixin1;
        this.mixin2 = mixin2;
    }
    public void doStuff(){
        mixin1.doStuff();
    }
    public void doOtherStuff(){
        mixin2.doOtherStuff();
    }
}

You can't do this with abstract classes. I have used this composition approach on a few projects, and it worked quite fine.