Generics in recursive method

Question

I've been banging my head on the table over this for a few hours now.

I have this Map:
private static Map<Class<? extends BaseClass>, Predicate<? extends BaseClass>> constraints;

And I have this method:

public static <C extends BaseClass> Predicate<C> getConstraints(Class<? super C> clazz) {
    if (clazz == BaseClass.class) {
        return (Predicate<C>) constraints.getOrDefault(BaseClass.class, x -> true);
    }
    Class<? super C> superClass = clazz.getSuperclass();
    return constraints.getOrDefault(clazz, x -> true).and(getConstraints(superClass));
}

All this is supposed to do is to chain all constraints for a certain class (and the superclasses) in a Predicate<T> using the and() method. The generics seem logical to me but I still get an error when making the recursive call. getConstraints(Class<? super C>) cannot be applied to Class<capture of ? super E>. I don't get it because superClass is of the type Class<? super C> which is exactly the parameter that my method accepts. Any ideas?

Jorn Vernee · Accepted Answer · 2018-09-23 12:38:33Z

2

You could use a type variable that represents the superclass:

@SuppressWarnings("unchecked")
public static <S extends BaseClass, C extends S> Predicate<C> getConstraints(Class<C> clazz) {
    if (clazz == BaseClass.class) {
        return (Predicate<C>) constraints.getOrDefault(BaseClass.class, x -> true);
    }
    Class<S> superClass = (Class<S>) clazz.getSuperclass();
    Predicate<C> p1 = (Predicate<C>) constraints.getOrDefault(clazz, x -> true);
    Predicate<S> p2 = getConstraints(superClass);
    return p1.and(p2);
}

You will also have to cast the result of getOrDefault, since putting the Predicate in the map 'forgets' the type information, and you get back a generic Predicate<? extends BaseClass>, which you can't use directly.

answered Sep 23, 2018 at 12:38

Jorn Vernee

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1 Comment

Collin Alpert Over a year ago

Awesome, thanks, that work's great. I've discovered the parameter C can be omitted. The parameter S is enough.

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