### Question:
WalletUtils { public static boolean isValidPrivateKey(String privateKey) { String cleanPrivateKey = Numeric.cleanHexPrefix(privateKey); return cleanPrivateKey.length() == PRIVATE_KEY_LENGTH_IN_HEX; }  static String generateFullNewWalletFile(String password, File destinationDirectory); static String generateLightNewWalletFile(String password, File destinationDirectory); static String generateNewWalletFile(String password, File destinationDirectory); static String generateNewWalletFile(
            String password, File destinationDirectory, boolean useFullScrypt); static String generateWalletFile(
            String password, ECKeyPair ecKeyPair, File destinationDirectory, boolean useFullScrypt); static Bip39Wallet generateBip39Wallet(String password, File destinationDirectory); static Credentials loadCredentials(String password, String source); static Credentials loadCredentials(String password, File source); static Credentials loadBip39Credentials(String password, String mnemonic); static String getDefaultKeyDirectory(); static String getTestnetKeyDirectory(); static String getMainnetKeyDirectory(); static boolean isValidPrivateKey(String privateKey); static boolean isValidAddress(String input);  }### Answer:
@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))); }