Java - HashCode - Equals

I am beginner on Java and I have a little problem with UnitTests with Java.

I think my problem we will be simple for you.

I put here my code and my error. On internet I found this is a problem with hashcode. I need to recreate them but I don't know why and how.. ^^

My method :

public void setGroupModel(GroupModel groupModel) {
        this.groupModel = groupModel;
        this.groupModel.add(this);
    }

My UnitTest :

public class CellControllerTest {
    protected CellController cellController;
    @Before
    public void setUp() throws Exception {
        cellController = new CellController();
    }

    @After
    public void tearDown() throws Exception{
    }

    @Test
    public void testClick() throws Exception{
        GroupModel groupModel = new GroupModel();
        CellModel cellModel = new CellModel()
        cellController.click(cellModel);
        assertEquals(groupModel, cellModel.getGroupModel());
    }
}

My error

java.lang.AssertionError: expected:<model.GroupModel@6d5380c2> but was:<model.GroupModel@45ff54e6>

I think the problem is : this.groupModel = groupModel;

Please can you help me and tell me what I have to write and why? :)

Thx very much !