Exception testing

https://github.com/junit-team/junit/wiki/Exception-testing
Exception testing
Expected Exceptions
How do you verify that code throws exceptions as expected? Verifying that code completes normally is important, but making sure the code behaves as expected in exceptional situations is vital too. For example:

    new ArrayList<Object>().get(0);

This code should throw an IndexOutOfBoundsException. The  @Test  annotation has an optional parameter "expected "that takes as values subclasses of  Throwable . If we wanted to verify that  ArrayList  throws the correct exception, we would write:

    @Test(expected= IndexOutOfBoundsException.class) 

    public void empty() { 

         new ArrayList<Object>().get(0); 

    }

The  expected  parameter should be used with care. The above test will pass if any code in the method throws  IndexOutOfBoundsException . For longer tests, it's recommended to use the  ExpectedException  rule, which is described below.
Deeper Testing of the Exception
The above approach is useful for simple cases, but it has its limits. For example, you can't test the value of the message in the exception, or the state of a domain object after the exception has been thrown.
Try/Catch Idiom
To address this you can use the try/catch idiom which prevailed in JUnit 3.x:

    @Test

    public void testExceptionMessage() {

        try {

            new ArrayList<Object>().get(0);

            fail("Expected an IndexOutOfBoundsException to be thrown");

        } catch (IndexOutOfBoundsException anIndexOutOfBoundsException) {

            assertThat(anIndexOutOfBoundsException.getMessage(), is("Index: 0, Size: 0"));

        }

    }

ExpectedException Rule
Alternatively, use the  ExpectedException  rule. This rule lets you indicate not only what exception you are expecting, but also the exception message you are expecting:

    @Rule

    public ExpectedException thrown = ExpectedException.none();



    @Test

    public void shouldTestExceptionMessage() throws IndexOutOfBoundsException {

        List<Object> list = new ArrayList<Object>();



        thrown.expect(IndexOutOfBoundsException.class);

        thrown.expectMessage("Index: 0, Size: 0");

        list.get(0); // execution will never get past this line

    }

The expectMessage also lets you use Matchers, which gives you a bit more flexibility in your tests. An example: thrown.expectMessage(JUnitMatchers.containsString("Size: 0"));
For an expanded discussion of the  ExpectedException  rule, see this  blog post .