Be able to write Hamcrest Matchers in lambda expressions

If you are implementing Java testing with JUnit + Hamcrest, you may want to implement the second argument of the assertThat method in a lambda expression. However, since the type of the second argument of assertThat is org.hamcrest.Matcher, it does not correspond to Java 8 lambda expressions. (Because Hamcrest 2.1 is built with Java 7)

I thought about how to implement value validation with a lambda expression.

Test implementation image

The following program is a test to check if the method to be verified is 0 or less. Matcher when it is smaller than the specified value is provided by hamcrest, but here it is just a sample ...

@Test
public void testSum() {
 // Test if the value is less than or equal to 0
    assertThat(new App().sum(10, -20), is(matches(i -> i <= 0, "negative value")));
}

Effective usage

--When there are several comparison conditions. For example, verifying whether it matches any of 1,2,3,4, etc. --If you want to implement property comparison in an object with a method other than the equals method --The verification method is simple, but when there are a large number of classes of objects to verify. When creating a custom Matcher class for each object is expected to take too much effort.

Realization method

Implement a custom Matcher like the one below. You can implement code like the one above by statically importing that custom Matcher.

import java.util.function.Predicate;

import org.hamcrest.Description;
import org.hamcrest.Matcher;
import org.hamcrest.TypeSafeMatcher;

public class LambdaMatcher<T> extends TypeSafeMatcher<T> {

	private Predicate<T> predicate;
	private String message;

	public LambdaMatcher(Predicate<T> predicate, String message) {
		this.predicate = predicate;
		this.message = message;
	}

	@Override
	public void describeTo(Description description) {
		description.appendText(this.message);
	}

	@Override
	protected boolean matchesSafely(T item) {
		return this.predicate.test(item);
	}

	public static <T> Matcher<T> matches(Predicate<T> predicate, String message) {
		return new LambdaMatcher<>(predicate, message);
	}
}

In addition, although the method is to receive the message from the caller as a String type, it can also be used like the Assertions class of JUnit5 by using the Supplier \ <String > type.

I hope it helps if you want to test with a lambda expression in Hamcrest.

Remarks

--2018/12/31 Since it was pointed out in the comment, the code of the implementation method was corrected. Thank you very much.