Asserting exceptions in Java, how? [duplicate]

Question

This might be a conceptually stupid question, but it also might not and since I am still a student I think I should I have no problem asking.

Imagine you have a method that if given certain conditions it will throw an NumberFormatException. I want to write a Unit Test to see if the exception is being correctly thorwn. How can I achieve this?

P.S. I am using JUnit to write the Unit Tests.

Thanks.

Answer 1

As other posters suggested, if you are using JUnit4, then you can use the annotation:

@Test(expected=NumberFormatException.class);

However, if you are using an older version of JUnit, or if you want to do multiple "Exception" assertions in the same test method, then the standard idiom is:

try {
   formatNumber("notAnumber");
   fail("Expected NumberFormatException");
catch(NumberFormatException e) {
  // no-op (pass)
}

Answer 2

Assuming you are using JUnit 4, call the method in your test in a way that causes it to throw the exception, and use the JUnit annotation

@Test(expected = NumberFormatException.class)

If the exception is thrown, the test will pass.

Answer 3

If you can use JUnit 4.7, you can use the ExpectedException Rule

@RunWith(JUnit4.class)
public class FooTest {
  @Rule
  public ExpectedException exception = ExpectedException.none();

  @Test
  public void doStuffThrowsIndexOutOfBoundsException() {
    Foo foo = new Foo();

    exception.expect(IndexOutOfBoundsException.class);
    exception.expectMessage("happened?");
    exception.expectMessage(startsWith("What"));
    foo.doStuff();
  }
}

This is much better than @Test(expected=IndexOutOfBoundsException.class) because the test will fail if IndexOutOfBoundsException is thrown before foo.doStuff()

See this article and the ExpectedException JavaDoc for details