How to assertThat superclass list equal to subclass matcher using hamcrest and mockito

Question

I am trying to do the following.

final Matcher<SuperClass> matcher1 = Matchers.hasProperty("a", equalTo("b"));
final Matcher<SuperClass> matcher2 = Matchers.hasProperty("c", equalTo("d"));
final Matcher<SuperClass> matchers = Matchers.allOf(matcher1, matcher2);

List<SubClass> list = someStuff();

assertThat(list, everyItem(matchers));

I am getting a compilation error on the assert line, is there any easy way to get rid of this.

Answer 1

Option 1:

@SuppressWarnings({"unchecked", "rawtypes"})
@Test public void yourTest() {
    final Matcher<SuperClass> matcher1 = Matchers.hasProperty("a", equalTo("b"));
    final Matcher<SuperClass> matcher2 = Matchers.hasProperty("c", equalTo("d"));
    final Matcher<SuperClass> matchers = Matchers.allOf(matcher1, matcher2);
    List<SubClass> list = someStuff();
    // Note cast to raw type here, corresponding to the suppressed warnings.
    assertThat(list, (Matcher) everyItem(matchers));
}

Option 2:

// These are all of type Matcher<SubClass> instead.
final Matcher<SubClass> matcher1 = Matchers.hasProperty("a", equalTo("b"));
final Matcher<SubClass> matcher2 = Matchers.hasProperty("c", equalTo("d"));
final Matcher<SubClass> matchers = Matchers.allOf(matcher1, matcher2);
List<SubClass> list = someStuff();
assertThat(list, everyItem(matchers));

Why? Java's generics handling is too clever for Hamcrest. everyItem(matchers) will return a Matcher<Iterable<SuperClass>>, but you don't have an Iterable<SuperClass>, you have an Iterable<SubClass>. It would all be fine if you actually had your Matcher methods produce a Matcher<Iterable<? extends SuperClass>>, but it's nearly impossible for you to convince Java that your matcher1 and matcher2 are compatible with one another.

Answer 2

Generics.

The problem with your code is that you try to assert that a list with type List<SubClass> matches every item of a matcher with type Matcher<SuperClass>.

Even though SuperClass extends SubClass, the assertThat method requires the two to be the same.

So if you can coerce your someStuff() method to return a list of of SuperClass, you should be home free. I.e. something like this

    List<SuperClass> list = someStuff();
    assertThat(list, everyItem(matchers));

would get rid of the compilation error.