[FIXED] Assert expected exception with specific criteria in custom fields

Issue

I want to verify that an expected exception meets certain criteria. Take this as a starting point:

class MyException extends RuntimeException {
    int n;
    public MyException(String message, int n) {
        super(message);
        this.n = n;
    }
}

public class HowDoIDoThis {
    @Rule
    public ExpectedException thrown = ExpectedException.none();

    @Test
    public void test1() {
        thrown.expect(MyException.class);
        throw new MyException("x", 10);
    }
}

How do I assert, for example, that the thrown exception has n > 1 and message contains only lowercase letters? I was thinking of using thrown.expect(Matcher) but can't figure out how to get a Hamcrest matcher to check arbitrary fields of an object.

Solution

You can use TypeSafeMatcher where you can provide your MyException class, and then an IntPredicate to check the n value against a condition:

public class MyExceptionMatcher extends TypeSafeMatcher<MyException> {
    private final IntPredicate predicate;

    public MyExceptionMatcher(IntPredicate predicate) {
        this.predicate = predicate;
    }

    @Override
    protected boolean matchesSafely(MyException item) {
        return predicate.test(item.n);
    }

    @Override
    public void describeTo(Description description) {
        description.appendText("my exception which matches predicate");
    }
}

Then you can expect like so:

thrown.expect(new MyExceptionMatcher(i -> i > 1));

Answered By - Lino
Answer Checked By - Mildred Charles (JavaFixing Admin)