Question

I can’t understand why I’m getting this compilation error

Bound mismatch: The generic method element(T) of type Resource is not
applicable for the arguments (Class<Chassis>). The inferred
type Class<Chassis> is not a valid substitute for the
bounded parameter <T extends Resource>.

with the following code:

public class Resource {
    protected abstract class has<T extends Resource> {
        public has(T v) {}
    }

    protected <T extends Resource> has element(T v) {
        return new has<T>(v) {};
    }
}

class Car extends Resource {
    has chassis = element(Chassis.class);
}

class Chassis extends Resource {
}

Why is this invalid? Chassis extends Resource, so why doesn’t match to <T extends Resource>?

And how could I constraint element() method to accept as an argument a Resource class object or any Resource subclass object?

Answer 1

Chassis.class is an instance of java.lang.Class, which only extends java.lang.Object.

The <T extends Resource> expects an actual instance of Resource (or one of its subclasses).

To solve this, you can change your method declaration to the following:

protected <T extends Resource> has element(Class<T> v) {
    return new has<T>(v) {};
}