Question

I asked a similiar question 10 minutes ago, but pasted the wrong code snippet. I’m really sorry about that.

I’m currently facing an issue with base and subclasses.

While having a single object as parameter (method single) the compiler doesn’t complain.

But if it comes to lists the compiler forces me to declare the list as <? extends Base>

After that I’m no longer allowed to add objects of the base type to that list.

The error message: “The method list(List<Generics.Base>) in the type Generics.C is not applicable for the arguments (List<Generics.Sub>)”

public class Generics {

    class Base {    }

    class Sub extends Base{     }

    interface I {
        public void list( List<Base> list );
        public void single( Base list );
    }

    class C implements I {
        public void list( List<Base> b) {       }
        public void single( Base p) {       }
    }

    void test() {
        C c = new C();
        c.single( new Sub() );
        List<Sub> b = new ArrayList<Sub>(); 
        c.list( b );                    // error message as above
    }

    public static void main( String[] args) {
        Generics g = new Generics();
        g.test();
    }
}

Is there any other way but declaring the list-methods argument as type <? extends Base> ?

Answer 1

Below are the 2 ways to do it….

public void list(List<? extends Base> list){

}

Or

public <T extends Base> void list(List<T> list){

}