Asserting exceptions in Java, how?

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

Use @Test(expected=IOException.class)

http://junit.sourceforge.net/doc/faq/faq.htm#tests_7

This is fine if you have one expected exception. An alternative strategy is to add an Assert.fail() at the end of the test method. If an exception isn't thrown then the test will fail accordingly. e.g.

@Test
public void testIOExceptionThrown() {      
   ftp.write(); // will throw IOException      
   fail();
}

Answer 2

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 3

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 4

You can do this:

 @Test(expected=IndexOutOfBoundsException.class)
    public void testIndexOutOfBoundsException() {
        ArrayList emptyList = new ArrayList();
        Object o = emptyList.get(0);
    }

Answer 5

@Test(expected="java.lang.NumberFormatException")

This annotation will do the trick.

Answer 6

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