JUnit Test against an interface without having the implementation yet - testing

Question

I try to write a test for a given interface like that with JUnit and have no idea how to do that:
public interface ShortMessageService {
/**
* Creates a message. A message is related to a topic
* Creates a date for the message
* #throws IllegalArgumentException, if the message is longer then 255 characters.
* #throws IllegalArgumentException, if the message ist shorter then 10 characters.
* #throws IllegalArgumentException, if the user doesn't exist
* #throws IllegalArgumentException, if the topic doesn't exist
* #throws NullPointerException, if one argument is null.
* #param userName
* #param message
* #return ID of the new created message
*/
Long createMessage(String userName, String message, String topic);
[...]
}
I tried to mock the interface after I realized that it doesn't make sense at all so I am a bit lost. Maybe someone can give me a good approach I can work with. I also heard about junit parameterized tests but I am not sure if that is what I am looking for.
Many thanks!

Answer 1

I use the following pattern to write abstract tests against my interface APIs without having any implementations available. You can write whatever tests you require in AbstractShortMessageServiceTest without having to implement them at that point in time.
public abstract class AbstractShortMessageServiceTest
{
/**
* #return A new empty instance of an implementation of FooManager.
*/
protected abstract ShortMessageService getNewShortMessageService();
private ShortMessageService testService;
#Before
public void setUp() throws Exception
{
testService = getNewShortMessageService();
}
#Test
public void testFooBar() throws Exception
{
assertEquals("question", testService.createMessage(
"DeepThought", "42", "everything"));
}
}
When you have an implementation, you can use the test simply by defining a new test class that overrides AbstractShortMessageServiceTest and implements the getNewShortMessageService method.
public class MyShortMessageServiceTest extends AbstractShortMessageServiceTest
{
protected ShortMessageService getNewShortMessageService()
{
return new MyShortMessageService();
}
}
In addition, if you need the test to be parameterized, you can do that in AbstractShortMessageServiceTest without doing it in each of the concrete tests.

Answer 2

Usually test is prepared for class that implements the interface and mocks are used for cooperating classes but you can test your test by mock if the class is not ready yet. It is unusual and you should use thenAnsfer with implemented logic of possible cases:
Better way is simply prepare tests for the implementation class and start to improve it till all test passes:
Implementing class can be in field and initialized before tests
private ShortMessageService testedClasOrMock;
//version with implementing class
#Before
public void setUp(){
testedClasOrMock = new ShortMessageServiceImpl0();
}
#Before
public void setUp(){
testedClasOrMock = mock(ShortMessageService.class);
when(testedClasOrMock).thenAnswer(new Answer<Long>(){
#Override
public Long answer(InvocationOnMock invocation) throws Throwable {
String message =(String) invocation.getArguments()[1];
if (message.length() > 256){
throw new IllegalArgumentException("msg is too long");
}
//other exception throwing cases
…...
return new Long(44);
}});
}
so you will have several test with expected exceptions like
#Test (expected= IllegalArgumentException.class)
public void testTooLongMsg(){
testedClasOrMock.createMessage(USER, TOO_LONG_MSG, TOPIC);
}
and one that simply should not throw exception and for instance check that msg ids are different
#Test
public void testTooLongMsg(){
long id0 = testedClasOrMock.createMessage(USER, TOO_LONG_MSG, TOPIC);
long id1 = testedClasOrMock.createMessage(USER, TOO_LONG_MSG, TOPIC);
assertTrue(id0 != id1);
}
If you insist on testing your test by mock let me know and I will add example for one test case.