i used web3j to Transacting with a smart contract. this is my Contract function which i want to intertact with.

function usersRegister(string _id, string _password){
    Equipment e = equipments[msg.sender];
    e.users[e.usersNum++] = User({id: _id, password: _password, registerTime: now});

and this is my code in Android using web3j

Web3j web3j = Web3jFactory.build(new HttpService(url));
EthGetTransactionCount ethGetTransactionCount = web3j.ethGetTransactionCount(
                        equipmentAddress, DefaultBlockParameterName.LATEST).sendAsync().get();
                BigInteger nonce = ethGetTransactionCount.getTransactionCount();
BigInteger gasPrice = Contract.GAS_PRICE;
BigInteger gasLimit = Contract.GAS_LIMIT;

List<Type> inputParameters = new ArrayList<>();
Type usersId = new Utf8String(_usersId);
Type usersPassword = new Utf8String(_usersPassword);
Function function = new Function("usersRegister",
String functionEncoder = FunctionEncoder.encode(function);
Transaction transaction = Transaction.createFunctionCallTransaction(
                        equipmentAddress, nonce, gasPrice,
                        gasLimit, contractAddress, new BigInteger("0"),
EthSendTransaction transactionResponse =

but i get "null" in transactionResponse.getTransactionHash()

someone know what's wrong with my code?thx for help :x

  • Transactions take some time to validate. It might take up to a few minutes. Are you doing your transaction in the background?
    – noev
    Aug 24 '17 at 10:46
  • I created an example project, how to read and write to smart contracts. You can find the code on github and some explanation here
    – noev
    Aug 24 '17 at 10:49

please check the credential. because the the key file in keystore folder is wrong file. an account of credential have to be registered in eth.accounts[] list. I solved the problem. ^^

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