@Test
public void testConvert() {

    // przypadki testowe: [argument][oczekiwana wartosc holdera]
    Object[][] testCases = {
        {new Integer(17), "17"},
        {new Float(1. / 3), "0.33333334"},
        {(byte) 100, "100"},
        {null, null},
        {"123", "123"},};
    // wykonanie testow
    for (Object[] testCase : testCases) {
        Object obj = testCase[0];
        String expectedValue = (String) testCase[1];
        StringHolder holder = type.convert(obj);
        assertEquals(expectedValue, holder.getValue());
    }
}