What's the actual use of 'fail' in JUnit test case?

This is how I use the Fail method.

There are three states that your test case can end up in

1. Passed : The function under test executed successfully and returned data as expected
2. Not Passed : The function under test executed successfully but the returned data was not as expected
3. Failed : The function did not execute successfully and this was not

intended (Unlike negative test cases that expect a exception to occur).

If you are using eclipse there three states are indicated by a Green, Blue and red marker respectively.

I use the fail operation for the the third scenario.

e.g. : public Integer add(integer a, Integer b) { return new Integer(a.intValue() + b.intValue())}

1. Passed Case : a = new Interger(1), b= new Integer(2) and the function returned 3
2. Not Passed Case: a = new Interger(1), b= new Integer(2) and the function returned soem value other than 3
3. Failed Case : a =null , b= null and the function throws a NullPointerException

In concurrent and/or asynchronous settings, you may want to verify that certain methods (e.g. delegates, event listeners, response handlers, you name it) are not called. Mocking frameworks aside, you can call fail() in those methods to fail the tests. Expired timeouts are another natural failure condition in such scenarios.

For example:

final CountDownLatch latch = new CountDownLatch(1);

service.asyncCall(someParameter, new ResponseHandler<SomeType>() {
    @Override
    public void onSuccess(SomeType result) {
        assertNotNull(result);
        // Further test assertions on the result
        latch.countDown();
    }

    @Override
    public void onError(Exception e) {
        fail(exception.getMessage());
        latch.countDown();
    }
});

if ( !latch.await(5, TimeUnit.SECONDS) ) {
    fail("No response after 5s");
}