Java generic parameter as exact subclass?

Question

Assuming we have a method like this:

public void foo(Class<? extends ClassBar> x) {
    ...
}

By modifying the generic expression;

< ? extends ClassBar >

Is it possible to ensure that ClassBar.class can't be passed in but anything extends ClassBar directly or indirectly be passed in WITHOUT throwing an exception on the runtime?

Answer 1

If you have only a bunch of classes extending ClassBar you can follow these two approaches.

---

Solution 1:

have all subclasses of ClassBar extend a custom interface (except for ClassBar itself), and change the method signature to:

public <T extends ClassBar & MyInterface> void foo(Class<T> x) {
    ...
}

Solution 2:

use something similar to this @AndyTurner's trick and provide instances only for specific types.

E.g:

class ClassBar {}

class ClassBarA extends ClassBar{}
class ClassBarB extends ClassBar{}

Your class containing foo:

class Foo<T extends ClassBar> {
    private Foo() {} // private constructor

    public static <T extends ClassBarA> Foo<T> instance(T c) {
        return new Foo<T>();
    }

    public static <T extends ClassBarB> Foo<T> instance(T c) {
        return new Foo<T>();
    }

    public void foo(Class<T> c) {

    }

}

Only subclass of ClassBarA would be accepted in this case

Foo<ClassBarA> foo1 = Foo.instance(this.classBarA);
foo1.foo(ClassBarA.class); // pass
foo1.foo(ClassBar.class);  // fail