How can multiply-nested generic classes refer to each other?

Question

Examine the code below

public abstract class ClassA<ClassBType extends ClassB<ClassCType>,ClassCType extends ClassC> {
    public void method(ClassBType type) {
        type.myClassA = this;  //Error.  Type mismatch: cannot convert from TestGameMain.ClassA<ClassBType,ClassCType> to TestGameMain.ClassA<TestGameMain.ClassB<ClassCType>,ClassCType>
    }
}

public abstract class ClassB<ClassCType extends ClassC> {
    ClassA<ClassB<ClassCType>,ClassCType> myClassA;

    private void testMethod() {
        myClassA.method(this);
    }
}

public abstract class ClassC {}
}

What's the correct way to fix this problem?

Edit: I've updated the code above, which does not compile.

Answer 1

Unless I'm really missing something in your question, this does the job:

ClassA<ClassB<ClassCType>, ClassCType> myClassA;

I would have to know more about how you instantiate these animals to know if getting an instance of ClassA to have those parameters is a problem that requires solving.

Answer 2

Your example is overly complicated - you can demonstrate the same issue even without ClassC:

public abstract class ClassA<ClassBType extends ClassB> {
    public void method(ClassBType type) {
        type.myClassA = this; 
    }
}

public abstract class ClassB {
    ClassA<ClassB> myClassA;

    private void testMethod() {
        myClassA.method(this);
    }
}

The problem comes down to variance: there is no inheritance relationship between a ClassA<ClassB> and a ClassA<ClassBType> (nor should there be), so the assignment can't be made. Given the cryptic nature of this example, I'm not sure if this is actually a "solution" to your problem, but the following code does compile:

public abstract class ClassA<ClassBType extends ClassB> {
    public void method(ClassB type) {
        type.myClassA = this; 
    }
}

public abstract class ClassB {
    ClassA<? extends ClassB> myClassA;

    private void testMethod() {
        myClassA.method(this);
    }
}