@Override
    public Character convert(String source) {
        int length = length(source);
        if (length == 0) {
            return null;
        }
        if (length > 1) {
            throw new IllegalArgumentException("The source String is more than one character!");
        }
        return source.charAt(0);
    }
@Test
    void testConvert() {
        assertEquals('t', converter.convert("t"));
        assertNull(converter.convert(null));
        assertThrows(IllegalArgumentException.class, () -> {
            converter.convert("ttt");
        });
    }