Question

I’m always struggling with generics. I don’t know why this makes my go crazy. I have an old problem I’ve got several times.

class Father {
    ...
}

class Child1 extends Father {
    ...
}

class Child2 extends Father {
    ...
}

public class TestClass {
    private static Class<? extends Father>[] myNiceClasses = new Class<? extends Father>[] {
        Child1.class,
        Child2.class
    }
}

That does not work. The compiler complains like this:

Cannot create a generic array of Class<? extends Father>

If I change the (faulty) array line to

private static Class<Father>[] myNiceClasses = new Class<Father>[] {

the same error message occurs, but also this message is added:

Type mismatch: cannot convert from Class<Child1> to Class<Father>

The only working version is this line:

private static Class<?>[] myNiceClasses = new Class<?>[] {

This solution is not satisfying, because you cannot cast so easily.

So my question is: How to solve this problem? Where’s the knot in my brain?

Answer 1

This is because java does not support generic array creation. Generics work best with Collections. So you can use ArrayList to solve your problem:

        ArrayList<Class<? extends Father>> myNiceClasses = new ArrayList<Class<? extends Father>>();
        myNiceClasses.add(Child1.class);
        myNiceClasses.add(Child2.class);