Question

I am having a hard time understanding why the error below happens. If #1 is ok, why is #2 not?

public interface IFoobar<DATA extends IFoobar> {
    void bigFun();
}

class FoobarImpl<DATA extends IFoobar> implements IFoobar<DATA> {
    public void bigFun() {
        DATA d = null;
        IFoobar<DATA> node = d;    //#1 ok
        d = node;                  //#2 error
    }
}

Answer 1

Because DATA is a type of IFoobar, but not the other way around. It’s no different than:

String d = null;
Object o = d;                //#1 ok
d = o;                       //#2 error