@Test
 public void TestClickOnGridAt() {
   System.out.println("clickOnGridAt");
   instance.clickOnGridAt(0, 0, "X");
   Object expResult = "X";
   Object result = instance.getCellAt(0, 0);
   assertEquals(expResult, result);
 }
  @Test
  public void testClear() {
    System.out.println("clear");
    instance.setCellAt("X", 0, 0);

    Object expResult = null;
    instance.clear();
    Object result = instance.getCellAt(0, 0);
    assertEquals(expResult, result);
  }
 @Test
 public void TestGetCellAt() {
   System.out.println("getCellAt");
   Image image =
       Toolkit.getDefaultToolkit().createImage(getClass().getResource("resources/x-icon.gif"));
   ImageIcon icon = new ImageIcon(image);
   icon.setDescription("X");
   instance.setValueAt(icon, 0, 0);
   String expResult = "X";
   String result = instance.getCellAt(0, 0);
   assertEquals(expResult, result);
 }