Unable to expect multiple exceptions with JUnit's ExpectedException

Question

• Java 7
• JUnit 4.11

I'm writing tests to validate my methods' contracts, e.g., a parameter cannot be null, an int param must be positive, etc. When a contract is violated, the thrown exception type reflects why it failed, such as NullPointerException or IllegalArgumentException. Sometimes I don't care which of these two is thrown, only that one of them is thrown. (Yes, I should be testing for the exact exception type, but bear with me...)

I could use JUnit's @Test(expected = RuntimeException.class) (since RuntimeException is the closest common parent of IAE and NPE), but this is overly generic as the method under test might throw some other RuntimeException.

Instead, I'm trying to use JUnit's ExpectedExceptions class.

Here is a complete example:

import org.hamcrest.CoreMatchers;
import org.junit.Rule;
import org.junit.Test;
import org.junit.rules.ExpectedException;
import static org.hamcrest.CoreMatchers.anyOf;

public class ParameterViolationTest {
    private Target target = new Target();

    @Rule
    public ExpectedException expected = ExpectedException.none();

    @Test
    public void parameterViolation() {
        expected.expect(anyOf(
                CoreMatchers.<Class<? extends Exception>>
                        equalTo(NullPointerException.class),
                CoreMatchers.<Class<? extends Exception>>
                        equalTo(IllegalArgumentException.class)));

        // Null parameter violates contract, throws some exception.
        target.methodUnderTest(null);
    }

    private static class Target {
        public void methodUnderTest(Object o) {
            if (o == null) {
                // throw new IllegalArgumentException();
                throw new NullPointerException();
            }
        }
    }
}

I would expect this test to pass, but instead it fails:

java.lang.AssertionError: 
Expected: (<class java.lang.NullPointerException> or <class java.lang.IllegalArgumentException>)
     but: was <java.lang.NullPointerException>
Stacktrace was: java.lang.NullPointerException
    at ParameterViolationTest$Target.methodUnderTest(ParameterViolationTest.java:35)
    at ParameterViolationTest.parameterViolation(ParameterViolationTest.java:25)
    < internal calls... >
    at org.junit.rules.ExpectedException$ExpectedExceptionStatement.evaluate(ExpectedException.java:168)
    at org.junit.rules.RunRules.evaluate(RunRules.java:20)
    < internal calls... >
java.lang.AssertionError: 
Expected: (<class java.lang.NullPointerException> or <class java.lang.IllegalArgumentException>)
     but: was <java.lang.NullPointerException>
Stacktrace was: java.lang.NullPointerException
    at ParameterViolationTest$Target.methodUnderTest(ParameterViolationTest.java:35)
    at ParameterViolationTest.parameterViolation(ParameterViolationTest.java:25)
    < internal calls... >
    at org.junit.rules.ExpectedException$ExpectedExceptionStatement.evaluate(ExpectedException.java:168)
    at org.junit.rules.RunRules.evaluate(RunRules.java:20)
    < internal calls... >
    at org.hamcrest.MatcherAssert.assertThat(MatcherAssert.java:20)
    at org.junit.Assert.assertThat(Assert.java:865)
    at org.junit.Assert.assertThat(Assert.java:832)
    at org.junit.rules.ExpectedException.handleException(ExpectedException.java:198)
    at org.junit.rules.ExpectedException.access$500(ExpectedException.java:85)
    at org.junit.rules.ExpectedException$ExpectedExceptionStatement.evaluate(ExpectedException.java:177)
    at org.junit.rules.RunRules.evaluate(RunRules.java:20)
    < internal calls... >

What's going on here?

Answer 1

You have to use Matchers.instanceOf instead of CoreMatchers.equalTo, because you're comparing instances of XXXException.

expected.expect(anyOf(
  instanceOf(NullPointerException.class),
  instanceOf(IllegalArgumentException.class)));