[Solved] Java cooperating generic classes: can we avoid unchecked cast?

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I have come to realize that my problem really came out of stuffing two concepts into the AbstractA/ConcreteA hierarchy that didn’t belong together. Though maybe not interesting to very many, I am posting this insight for two reasons: (1) I feel I owe Chris Wohlert the answer I have found myself (2) more importantly, I’d love to inspire anyone else facing a similar tricky generics issue to review your design from a higher level than just solving the generics and/or class cast issue. It certainly helped me. The cast/generics problem was a sign that something more fundamental was not quite right.

public abstract class AbstractA {

    public void foo() {
        AbstractB aB = createB();
        aB.setA(this);
    }

    /** factory method */
    abstract public AbstractB createB();

}

public abstract class AbstractB {

    private AbstractA theA;

    public void setA(AbstractA theA) {
        this.theA = theA;
    }

    // methods that use theA

}

No generics and no class cast. Taking out the stuff that didn’t belong in the A class hierarchy into ConcreteC (with no AbstractC):

public class Client {

    public void putTheActTogether() {
        ConcreteC theC = new ConcreteC();

        // the concrete A
        AbstractA theA = new AbstractA() {
            @Override
            public AbstractB createB() {
                return new ConcreteB(theC);
            }
        };
        // call methods in theA
    }

}

public class ConcreteB extends AbstractB {

    private final ConcreteC c;

    public ConcreteB(ConcreteC c) {
        super();
        this.c = c;
    }

    public void bar() {
        c.concreteCMethod();
    }

}

public class ConcreteC {

    public void concreteCMethod() { // was concreteAMethod(); moved and renamed
        // ...
    }

}

The client needs a few more lines than before. In my real-world code I needed to duplicate one final field in AbstractA and ConcreteC, but it made sense to do. All in all I consider it a low price for a design that is otherwise pure and simple.

solved Java cooperating generic classes: can we avoid unchecked cast?