What's the right signature for a custom matcher for an Iterable?

Question

I am trying to write a custom matcher for a class that has an Iterable field. I can't work out a way to do it so that it can accept any of the matchers everyItem, hasItem, and contains - because each of those returns a slightly different generic type. What is the best way to do this?

Here is a simple example to demonstrate the problem - how can I make it compile? I am using Hamcrest 1.3.

import static org.hamcrest.Matchers.contains;
import static org.hamcrest.Matchers.equalTo;
import static org.hamcrest.Matchers.everyItem;
import static org.hamcrest.Matchers.hasItem;
import static org.junit.Assert.assertThat;

import java.util.Collection;

import org.hamcrest.Description;
import org.hamcrest.Matcher;
import org.hamcrest.TypeSafeDiagnosingMatcher;

    public class MatcherExample {

        public static class Band {
            public Collection<Member> getMembers() {
                return null;
            }
        };

        public static class Member {
        };

        public static Matcher<Band> hasMembers(final Matcher<Iterable<Member>> matcher) {
            return new TypeSafeDiagnosingMatcher<Band>() {
                @Override
                public void describeTo(Description description) {
                }

                @Override
                protected boolean matchesSafely(Band item, Description mismatchDescription) {
                    return matcher.matches(item.getMembers());
                }
            };

        }

        public static void main(String[] args) {
            Band band = new Band();
            assertThat(band, hasMembers(everyItem(equalTo(new Member())))); // works with signature Matcher<Iterable<Member>>
            assertThat(band, hasMembers(hasItem(equalTo(new Member()))));   // works with signature Matcher<Iterable<? super Member>>
            assertThat(band, hasMembers(contains(equalTo(new Member()))));  // works with signature Matcher<Iterable<? extends Member>>
        }
    }

Answer 1

The following signature is working for me:

public static Matcher<Band> hasMembers(final Matcher<? super Iterable<Member>> matcher) {
    // ...
}

---

Side note:

Hamcrest has the FeatureMatcher class to easily create matchers for a specific property:

public static Matcher<Band> hasMembers(final Matcher<? super Iterable<Member>> matcher) {
    return new FeatureMatcher<Band, Iterable<Member>>(matcher, "a band with members", "members") {

        @Override
        protected Iterable<Member> featureValueOf(Band actual) {
            return actual.getMembers();
        }
    };
};