How to get started with testing(jMock)

Question

I'm trying to learn how to write tests. I'm also learning Java, I was told I should learn/use/practice jMock, I've found some articles online that help to certain extend like :

http://www.theserverside.com/news/1365050/Using-JMock-in-Test-Driven-Development

http://jeantessier.com/SoftwareEngineering/Mocking.html#jMock

And most articles I found was about test driven development, write tests first then write code to make the test pass. I'm not looking for that at the moment, I'm trying to write tests for already existing code with jMock.

The official documentation is vague to say the least and just too hard for me. Does anybody have better way to learn this. Good books/links/tutorials would help me a lot. thank you

EDIT - more concrete question :

http://jeantessier.com/SoftwareEngineering/Mocking.html#jMock - from this article

Tried this to mock this simple class :

import java.util.Map;
    public class Cache {
        private Map<Integer, String> underlyingStorage;
        public Cache(Map<Integer, String> underlyingStorage) {
            this.underlyingStorage = underlyingStorage;
        }
        public String get(int key) {
            return underlyingStorage.get(key);
        }
        public void add(int key, String value) {
            underlyingStorage.put(key, value);
        }
        public void remove(int key) {
            underlyingStorage.remove(key);
        }
        public int size() {
            return underlyingStorage.size();
        }
        public void clear() {
            underlyingStorage.clear();
        }
    }

Here is how I tried to create a test/mock :

public class CacheTest extends TestCase {

    private Mockery context;
    private Map mockMap;
    private Cache cache;

    @Override
    @Before
    public void setUp() {
        context = new Mockery() {
            {
                setImposteriser(ClassImposteriser.INSTANCE);
            }
        };

        mockMap = context.mock(Map.class);
        cache = new Cache(mockMap);
    }

    public void testCache() {
        context.checking(new Expectations() {{
            atLeast(1).of(mockMap).size(); 
            will(returnValue(int.class));
        }});

    }
}

It passes the test and basically does nothing, what I wanted is to create a map and check its size, and you know work some variations try to get a grip on this. Understand better trough examples, what else could I test here or any other exercises would help me a lot. tnx

Answer 1

Here is a tutorial about using JUnit and EasyMock (a mocking library I personally find far easier to use than JMock): http://www.michaelminella.com/testing/unit-testing-with-junit-and-easymock.html

Even if you are 100% dedicated to using JMock, the concepts between the two are the same and this should help you understand them better.

The purpose of mocking is that when you test Class A, which depends on B and C, your test of A uses mock versions of B and C to be able to specify their exact behavior rather than using the real implementations of B and C in your test of A. Otherwise you are not testing just the single unit of A, you are implicitly testing B and C as well.