类型与泛型不匹配

这是一个界面:

public interface Foo extends Comparable>  {
   ...
}

并且有一些类实现此接口:

public class Bar extends Something implements Foo {
    public Vector> giveBar() {
        ...
    }
}

public class Boo extends SomethingElse implements Foo {
    public Vector> giveBoo() {
        ...
    }
}

现在我想在矢量中保留一堆Foos(可能真的是Foos或Boos).

Bar bar = new Bar();
Boo boo = new Boo();
Vector> vector;
if (...) 
   vector = bar.giveBar();
else
   vector = boo.giveBoo();

我明白了:

Type mismatch: cannot convert from Vector> to Vector>

同样适用于:

Vector vector;
if (...) 
   vector = giveBar();
else
   vector = giveBoo();

是Bar和Boo的超类是否能解决这个问题的唯一解决方案？

所有代码归结为:

Vector vector = new Vector();

Vector> vector = new Vector>();
vector.add(new Vector());