Hamcrest matcher for checking return value of method in collection

Question

hasProperty can be used with hasItem to check for the value of a given property, eg:

Matcher hasName = Matchers.<Person>hasProperty("name", is("Winkleburger"));
assertThat(names, hasItem(hasName));

This is fine when name is a property, ie: there is a method called getName().

Is there a matcher that will check a method that isn't a property? ie: in this case, it will check the return value of the method name() rather than getName(), for items in the collection.

Crescendo

so, you want to call the name() method on all items of a collection? – Regional

Answer 1

You can use another Hamcrest built-in for this, a FeatureMatcher. These are designed to be combined with other matchers after they transform your input into something else. So in your case you would do something like this:

@Test
public void test1() {
    List<Person> names = new ArrayList<>();
    names.add(new Person("Bob"));
    names.add(new Person("i"));

    assertThat(names, hasItem(name(equalTo("Winkleburger"))));
}

private FeatureMatcher<Person, String> name(Matcher<String> matcher) {
    return new FeatureMatcher<Person, String>(matcher, "name", "name") {
        @Override
        protected String featureValueOf(Person actual) {
            return actual.name();
        }
    };
}

The benefit you'll get with this over a custom matcher is that it's fully reusable and composable with other matchers as all it does is the data-extraction then defers to whatever other matcher you want. You're also going to get appropriate diagnostics, e.g. in the above example you will if you change the assert to a value that's not present you will receive:

java.lang.AssertionError: 
    Expected: a collection containing name "Batman"
    but: name was "Bob", name was "Winkleburger"

Neurocoele

Answer 2

You can write one yourself:

public class HasName extends TypeSafeMatcher<MyClass> {
    private String expectedName;

    private HasName(String expectedName) {
        this.expectedName = expectedName;
    }

    @Override
    public boolean matchesSafely(MyClass obj) {
        return expectedName.equals(obj.name());
    }

    @Factory
    public static Matcher<MyClass> hasName(String expectedName) {
        return new HasName(expectedName);
    }
}

Where MyClass is a class or interface which defines the name() method.

Humic

Answer 3

Here's a generic primitive for specifying a property as a method reference:

static <T, U> Matcher<T> hasProperty( Function<T, U> getter, 
                                      String name,
                                      Matcher<? super U> subMatcher ) {

  return new FeatureMatcher<T, U>( subMatcher, name, name ) {
    @Override
    protected U featureValueOf( T actual ) {
      return getter.apply( actual );
    }
  };
}

Which you could use as:

assertThat(person, hasProperty(Person::getName, "name", is("Winkleburger")));

Hindoo