Hamcrest IsNot matcher together with a wrapped custom matcher - describeMismatch doesn't work as expected

Question

Recently I've made a custom matcher for the jaxb generated element and came upon this scenario:

Preconditions:

• I have a custom Matcher extending BaseMatcher with overriden methods describeTo and describeMismatch (and of course matches..)
• I'm using assertThat( actualObject, not( myMatchersStaticRunMethod( expectedObject ) )

When assertion fails, in result I have:

Expected: not myMatchersDescribeToDescription
but: isNotCoreMatcherDescribeMismatchDescription

Digging in the code of org.hamcrest.core.IsNot class I can see that describeTo is implemented properly (ie. delegates gathering of description to a wrapped matcher), but describeMismatch is not overriden hence the BaseMatcher's version is used.

Imho it's a faulty behavior, because mismatch should be taken from the wrapped matcher as well. What do you think, guys?

Answer 1

I don't know why this was voted down. I agree it is faulty behaviour. Seems similar to this issue

You can fix it only by creating your own custom "notD" matcher, its a copy of the IsNot matcher adding the Override for describeMismatch:

import org.hamcrest.BaseMatcher;
import org.hamcrest.Description;
import org.hamcrest.Factory;
import org.hamcrest.Matcher;

import static org.hamcrest.core.IsEqual.equalTo;


/**
 * Calculates the logical negation of a matcher.
 */
public class IsNotDescribing<T> extends BaseMatcher<T>  {
    private final Matcher<T> matcher;

    public IsNotDescribing(Matcher<T> matcher) {
        this.matcher = matcher;
    }

    @Override
    public boolean matches(Object arg) {
        return !matcher.matches(arg);
    }

    @Override
    public void describeTo(Description description) {
        description.appendText("not ").appendDescriptionOf(matcher);
    }

    // use the matcher to describe its mismatch
    @Override
    public void describeMismatch(Object item, Description description) {
        matcher.describeMismatch(item, description);
    }

    @Factory
    public static <T> Matcher<T> notD(Matcher<T> matcher) {
        return new IsNotDescribing<T>(matcher);
    }

    @Factory
    public static <T> Matcher<T> notD(T value) {
        return notD(equalTo(value));
    }
}