@Test
public void testStringValue() {

    assertEquals("9999", new FourDigitNumber(mockSlot, 9, 9, 9, 9).stringValue());
    assertEquals("0999", new FourDigitNumber(mockSlot, 0, 9, 9, 9).stringValue());
    assertEquals("0099", new FourDigitNumber(mockSlot, 0, 0, 9, 9).stringValue());
    assertEquals("0009", new FourDigitNumber(mockSlot, 0, 0, 0, 9).stringValue());
    assertEquals("1111", new FourDigitNumber(mockSlot, 1, 1, 1, 1).stringValue());
    assertEquals("0111", new FourDigitNumber(mockSlot, 0, 1, 1, 1).stringValue());
    assertEquals("0011", new FourDigitNumber(mockSlot, 0, 0, 1, 1).stringValue());
    assertEquals("0001", new FourDigitNumber(mockSlot, 0, 0, 0, 1).stringValue());
    assertEquals("0000", new FourDigitNumber(mockSlot, 0, 0, 0, 0).stringValue());
}