Easymock throws NPE when calling methods on mocked object

Question

I'm trying to use this mock in a test class:

    @Mock
    private IDPool idPool;

    @Before
    private void before() {
        expect(idPool.getNextFree()).andReturn(TRACK_ID).anyTimes(); //line 63
    }

When I run it I get NPE:

java.lang.NullPointerException
    at com.mycompany.IDPool.getNextFree(IDPool.java:68)
    at com.mycompany.Test.before(Test.java:63)

It looks like it tries to run the production code instead of mock? Because the NPE is thrown here in IDPool class:

    public final Integer getNextFree() {
        int nextFree;
        synchronized (this.usedIDs) { //line 68
            //...
        }
    }

I thought mocks don't use actual implementations? can I mock this object so that it doesn't?

Answer 1

I would comment, but I don't have enough reputation.
Your suspicion about EasyMock not being able to mock the final method (or class) is right. However, you can use PowerMock. I cannot verify it now, but initializing the mock with PowerMock.createMock(IDPool.class) should enable final methods mocking.
This answer and documentation should help you.