Parametrizing type in Java (GADT)

Question

I need to have so sort of GADT in Java, like

interface Action<C, O> {
    Collection<O> doAction(C<O> predicate)
}

so I can easily declare class like

class Selector<T> {
...
}

and then use it with implementation of Action accepting instance of Selector for example. And having the implementation of Action accepting Predicate as well - but type of argument must match return type of collection.

The main idea is to have one type dependent on another one. Is that possible in plain old Java 6?

Answer 1

This doesn't work because there's nothing enforcing that C in Action<C, O> is, itself, a parameterized type. You could declare an Action<String, Object> for instance -- and then what is predicate supposed to be? There's no such thing as String<Object>.

Without knowing more details, it looks to me as though what you want is something more specific, e.g., define a Predicate<T> interface:

interface Predicate<T> {
}

interface Action<C extends Predicate<O>, O> {
    Collection<O> doAction(C predicate);
}

class Selector<T> implements Predicate<T> {
}