public static String prependHexPrefix(String input) {
        if (!containsHexPrefix(input)) {
            return HEX_PREFIX + input;
        } else {
            return input;
        }
    }
@Test
    public void testPrependHexPrefix() {
        assertEquals(Numeric.prependHexPrefix(""), ("0x"));
        assertEquals(Numeric.prependHexPrefix("0x0123456789abcdef"), ("0x0123456789abcdef"));
        assertEquals(Numeric.prependHexPrefix("0x"), ("0x"));
        assertEquals(Numeric.prependHexPrefix("0123456789abcdef"), ("0x0123456789abcdef"));
    }