Java泛型类中的超类

Question

This is my first time here, but I do my best to describe my problem:

I have an abstract class A . I also have abstract classes B and C extending A . Then I have classes extending B or C .

Now I have a generic class only accepting classes extending A . ( class XYZ<ABC extends A>… )

Now I basically want to create a method that works with the generic class, but only with instances that use a class extending B .

I tried something like this:

public void method(XYZ<B> t){}

This sadly didn't work. The IDE wanted me to have either the type of t or the other way around.

Now the questions:
Why does this not work? It seems that it wouldn't cause any problems, because a subclass has to provide all methods from the superclass.
Is there a way around this except of making a method for every single subclass of B or changing the type of the object i want to use?

example code:

public class main {

public static void main(String[] args) {
    ArrayList<Abc> foo = new ArrayList<>();
    xyz(foo);
}
private void xyz(ArrayList<B> abs){}


private static abstract class  A{}
private static abstract class  B extends A{}
private static abstract class  C extends A{}
private static class Abc extends B{}
}

Answer 1

Try it like this:

public class Main {

    public static void main(String[] args) {
        ArrayList<Abc> foo = new ArrayList<>();
        xyz(foo);
    }

    private static <T extends B> void xyz(ArrayList<T> abs){
        System.out.print("running xyz");
    }


    private static abstract class  A{}
    private static abstract class  B extends A{}
    private static abstract class  C extends A{}
    private static class Abc extends B{}
}

Seems to be what you are aiming for. Take a look here for more information.