How do I create a Mock of a JFrame/JPanel that supports repaint functionality

Question

I have a little problem with my unit tests for a Swing application. What I want to do is pass a paintable object to my JPanel and everytime I do that it should repaint itself.

So much for the basic scenario, now on to my unit tests:

public class GraphicViewImplTest {

    private JFrame frame;
    private GraphicViewImpl view; //This is my JPanel
    private GraphicLineSpy firstGraphicLine;
    private Line firstLine;

    @Before
    public void setUp() throws Exception {
        frame = new JFrame();
        view = new GraphicViewImpl();
        frame.add(view);
        frame.setVisible(true);
        firstLine = new Line();
        firstLine.setStart(new Point(11, 12));
        firstLine.setEnd(new Point(21, 22));
        firstGraphicLine = new GraphicLineSpy(firstLine);
    }

    @Test
    public void whenReceivingLine_shouldPaintLine() {
        view.receiveShape(firstGraphicLine);
        assertTrue(firstGraphicLine.wasPainted());
    }

}

As you can see I'm passing GraphicLineSpy to my view. The GraphicLine class is basically a decorator for the Line class that knows how to paint a line in Swing. The GraphicLineSpy overrides the paint method of the GraphicLine and just sets a flag to true, so I can check if the paint method was called.

Now on to the implemenation of my GraphicView JPanel:

public class GraphicViewImpl extends JPanel implements GraphicView, Observer {

    protected GraphicViewPresenter presenter;
    protected List<GraphicShape> graphicShapeList = new LinkedList<>();

    @Override
    public void receiveShape(GraphicShape graphicShape) {
        graphicShapeList.add(graphicShape);
        graphicShape.addObserver(this);
        repaint();
    }

    @Override
    public void removeShape(GraphicShape graphicShape) {
        graphicShapeList.remove(graphicShape);
        graphicShape.removeObserver(this);
        repaint();
    }

    public void setPresenter(GraphicViewPresenter presenter) {
        this.presenter = presenter;
    }

    @Override
    public void update() {
        repaint();
    }

    @Override
    public void paintComponent(Graphics graphics) {
        super.paintComponent(graphics);
        for (GraphicShape graphicShape : graphicShapeList)
            graphicShape.paint(graphics);
    }
}

Now, my problem is that when I run these tests they say that my GraphicLine was not painted. However, when I actually run the program and add a new GraphicLine it works perfectly fine, all my Shapes get painted. Am I missing something in the test setup?

Moreover, and this is probably the most important part, I guess it's not really an optimal solution to start up a whole JFrame everytime I run my tests, so I was wondering how I would best go about creating a test double that doesn't break the whole repaint functionality.

Thanks in advance for any hints!

Answer 1

I think you should concentrate on testing your code instead of the JPanel implementation, therefore you should mock out these other dependencies using the Mockito framework (or any other):

public class GraphicViewImplTest {

    @Rule
    public MockitoRule rule = MockitoJUnit.rule();
    @Mock
    private Graphics2D graphics; // not tested dependency
    @Mock
    private GraphicShape firstLine; // not tested dependency

    private GraphicViewImpl view; //This is my JPanel

    @Before
    public void setUp() throws Exception {
        view = spy(new GraphicViewImpl());
        doNothing().when(view).repaint();
    }

    @Test
    public void whenReceivingLine_shouldPaintLine() {
        view.receiveShape(firstGraphicLine);
        verify(view).repaint();
        verify(firstLine,never()).paint(graphics);

        view.paintComponent(graphics);
        verify(firstLine).paint(graphics);
    }
}