public static boolean isValidPrivateKey(String privateKey) {
        String cleanPrivateKey = Numeric.cleanHexPrefix(privateKey);
        return cleanPrivateKey.length() == PRIVATE_KEY_LENGTH_IN_HEX;
    }
@Test
    public void testIsValidPrivateKey() {
        assertTrue(isValidPrivateKey(SampleKeys.PRIVATE_KEY_STRING));
        assertTrue(isValidPrivateKey(Numeric.prependHexPrefix(SampleKeys.PRIVATE_KEY_STRING)));

        assertFalse(isValidPrivateKey(""));
        assertFalse(isValidPrivateKey(SampleKeys.PRIVATE_KEY_STRING + "a"));
        assertFalse(isValidPrivateKey(SampleKeys.PRIVATE_KEY_STRING.substring(1)));
    }