.add("field2", "value2");
    final String expected = "TestClass{field1=1, value1, field2=value2}";

    assertEquals(expected, helper.toString());
    // Call toString again
    assertEquals(expected, helper.toString());

    // Make sure the cached value is reset when we modify the helper at all
    final String expected2 = "TestClass{field1=1, value1, field2=value2, 2}";
    helper.addValue(2);