How to make transaction using private key and address using web3j

Question

I have a private key and address . Is it possible to make transaction with these parameter using web3j.

Answer 1

Web3j web3j = Web3j.build(new HttpService(""));
    Credentials credentials = Credentials.create("privateKey");
    BigInteger nonce = web3j.ethGetTransactionCount(credentials.getAddress(), DefaultBlockParameterName.PENDING).sendAsync().get().getTransactionCount();
    BigInteger gasPrice = web3j.ethGasPrice().sendAsync().get().getGasPrice();
    RawTransaction rawTransaction = RawTransaction.createEtherTransaction(nonce, gasPrice, Constants.ETH_GAS_LIMIT, "address", Convert.toWei("amount", Convert.Unit.ETHER).toBigIntegerExact());
    EthSendTransaction response = web3j.ethSendRawTransaction(Numeric.toHexString(TransactionEncoder.signMessage(rawTransaction, credentials))).send();
    String hash = response.getTransactionHash();

Answer 2

If you want to send custom tokens as the contract owner, check out my answer here: Send Custom ERC20 Tokens from one Address to another

If you want to send tokens on behalf of another user, the idea is similiar.

First retrieve user's credentials from the privatekey , then do transaction.

Credentials getUserInfo (String privateKeyInHex){

    BigInteger privateKeyInBT = new BigInteger(privateKeyInHex, 16);

    ECKeyPair aPair = ECKeyPair.create(privateKeyInBT);
    Credentials aCredential = Credentials.create(aPair);

    return aCredential;
}

send tokens:

process(){
    ... 

    Credentials newUser =  getUserInfo(privateKeyInHex);

    TransactionReceiptProcessor transactionReceiptProcessor = new NoOpProcessor(web3);
    TransactionManager transactionManager = new RawTransactionManager(
                            web3, newUser, ChainId.MAINNET, transactionReceiptProcessor);
                    //if testing, use ChainId.ROPSTEN
    ...

Answer 3

Yes it's possible to send transaction. Use web3.eth.sendTransaction({to:'0xaddress', from:'0xaddress', value:web3.toWei(.033,'ether'),gas:30000})