如何在Sets.powerSet中使用通配符泛型

Question

I have some code like this

import com.google.common.collect.Sets;
public void handleInput(Set<Object> conditions){
    Set<Set<Object>> powerSet = Sets.powerSet(conditions);
    ...
}

This works fine. But I want to do this:

public void handleInput(Set<? extends Object> conditions){
    Set<Set<? extends Object>> powerSet = Sets.powerSet(conditions);
    ...
}

so I can get the powerset of objects that are subclasses of object. But this won't compile and I get the error:

Type mismatch: cannot convert from Set<Set<capture#1-of 
? extends Object>> to Set<Set<? extends Object>>

How can I achieve this goal?

EDIT: I guess it has something to do with the generic type getting erased at compile time, so that the compiler can't know that powerSet won't add something illegal to the sets it's creating. I've reworked the client, by casting all the inputs to Object, and removing the wildcard altogether. Is this the best way? Thanks!

Answer 1

In this case it doesn't make any sense - since all Java classes extend java.lang.Object at some point. So ? extends Object ? extends Object is redundant.

But speaking of Sets.powerSet , this works like a charm:

public class TestClass {

    public static class A {}

    public static class B extends A {}

    public static class C extends B {}


    public Set<? extends Set<? extends A>> exampleMethod(Set<? extends A> input) {
        return Sets.powerSet(input);
    }

    public static void main(String[] args) {
        final TestClass testClass = new TestClass();
        final A a = new A();
        final B b = new B();
        final C c = new C();

        System.out.println(
            testClass.exampleMethod(
                    ImmutableSet.of(a, b, c)
            )
        );
    }

}

Answer 2

as @slnowak notes, when you are extending Object, the code is really redundant.

However, to understand the Exception and avoid it...

public void handleInput(Set<? extends Object> conditions){
    Set<? extends Set<? extends Object>> powerSet = Sets.powerSet(conditions);
    ...
}

this will compile and, more usefully, you can restrict the types in your conditions argument using this method, for instance - you could have:

public void handleInput(Set<? extends Number> conditions){
    Set<? extends Set<? extends Number>> powerSet = Sets.powerSet(conditions);
    ...
}

and this would prevent you passing in sets that had non-numeric types and warn you of this at compile time.