Question

I’m pretty new to JUnit, and I don’t really know what best practices are for exceptions and exception handling.

For example, let’s say I’m writing tests for an IPAddress class. It has a constructor IPAddress(String addr) that will throw an InvalidIPAddressException if addr is null. As far as I can tell from googling around, the test for the null parameter will look like this.

@Test
public void testNullParameter()
{
    try
    {
        IPAddress addr = new IPAddress(null);
        assertTrue(addr.getOctets() == null);
    }
    catch(InvalidIPAddressException e)
    {
        return;
    }

    fail("InvalidIPAddressException not thrown.");
}

In this case, try/catch makes sense because I know the exception is coming.

But now if I want to write testValidIPAddress(), there’s a couple of ways to do it:

Way #1:

@Test
public void testValidIPAddress() throws InvalidIPAddressException
{
    IPAddress addr = new IPAddress("127.0.0.1");
    byte[] octets = addr.getOctets();

    assertTrue(octets[0] == 127);
    assertTrue(octets[1] == 0);
    assertTrue(octets[2] == 0);
    assertTrue(octets[3] == 1);
}

Way #2:

@Test
public void testValidIPAddress()
{
    try
    {
        IPAddress addr = new IPAddress("127.0.0.1");
        byte[] octets = addr.getOctets();

        assertTrue(octets[0] == 127);
        assertTrue(octets[1] == 0);
        assertTrue(octets[2] == 0);
        assertTrue(octets[3] == 1);
    }
    catch (InvalidIPAddressException e)
    {
        fail("InvalidIPAddressException: " + e.getMessage());
    }
}

Is is standard practice to throw unexpected exceptions to JUnit or just deal with them yourself?

Thanks for the help.

Answer 1

Actually, the old style of exception testing is to wrap a try block around the code that throws the exception and then add a fail() statement at the end of the try block. Something like this:

public void testNullParameter() {
    try {
        IPAddress addr = new IPAddress(null);
        fail("InvalidIPAddressException not thrown.");
    } catch(InvalidIPAddressException e) {
        assertNotNull(e.getMessage());
    }
}

This isn’t much different from what you wrote but:

1. Your assertTrue(addr.getOctets() == null); is useless.
2. The intend and the syntax are clearer IMO and thus easier to read.

Still, this is a bit ugly. But this is where JUnit 4 comes to the rescue as exception testing is one of the biggest improvements in JUnit 4. With JUnit 4, you can now write your test like this:

@Test (expected=InvalidIPAddressException.class) 
public void testNullParameter() throws InvalidIPAddressException {
    IPAddress addr = new IPAddress(null);
}

Nice, isn’t it?

Now, regarding the real question, if I don’t expect an exception to be thrown, I’d definitely go for way #1 (because it’s less verbose) and let JUnit handle the exception and fail the test as expected.