A bit confused. How do I send ether using Web3j to an address (Android)

Question

EDIT: I am using android platform

I think I'm a bit confused conceptually. I would appreciate it if someone could answer these few questions for me as I'm trying to make an app so that I can be clear on exactly what's going on here. I'm trying to read and decipher the documentation at the same time.

-I have generated my address and private key using web3j ECKeyPair class

Now I need to be able to send that ether to an address using that data and I'm not sure how I'm supposed to go about doing it

• I think I'm supposed to use something called a rawtransaction but again my understanding is hazy

• I would also like to be able to encrypt my private key however I saw that it encrypts the data into a keystore file. Is there any way to clear out the unneccesary data here as I'm only trying to encrypt the private key using a password.

If a more knowledgeable member than myself could clear up the steps involved in "signing a transaction" so that ether can be sent from one address to another, you'd be a huge help to my design project.

I also don't understand how transactions can be signed offline as I thought the point in signing a transaction was so it could be verified by the network

Thank you.

Answer 1

You basically need to to three steps:

1. Construct transaction object
2. Sign it with private key
3. Publish signed transaction

For step 1 you do something like this:

var from = "0x..."; // The address you are sending ether from
var tx = {
  nonce: web3.eth.getTransactionCount (from),
  chainId: 1, // 1 means Mainnet, more chain IDs here: https://chainid.network/
  to: "0x...", // The address you are sending ether to
  data: "",
  value: value, // The amount of ether to send (in Wei)
  gasPrice: web3.eth.getGasPrice(),
  gas: 21000 // Enough for simple transfer
};

For step 2 you do:

var signedTx = web3.eth.accounts.signTransaction (tx, privateKey);

For step 3 you do:

web3.eth.sendSignedTransaction(signedTx);

See Web3js documentation for more info.

Sorry, my answer is about Web3js, not Web3j. But, for Web3j steps are basically the same:

1. Prepare transaction object (RawTransaction)
2. Sign it (TransactionEncoder)
3. Publish it (ethSendRawTransaction)

See official documentation for details.

Answer 2

If i understand your question correctly, you should be able to just use web3.eth.sendTransaction by passing in a transaction object.

var transactionObject = {
    from: web3.defaultAccount,
    to: addressToSendTo,
    value: web3.toWei(1.0, "ether"),
    gas: gasAmountInWei
};

web3.eth.sendTransaction(transactionObject, function(err, data) {
    // if no err, transaction was success
});

For the gas value, you can estimate the gas needed like this

// note that `transactionObject` is the same as above but without the gas value
web3.eth.estimateGas(transactionObject, function(err, estimate) {
    transactionObject.gas = estimate;
});

If you have issues with gas not being enough, experiment with adding more. I generally multiply by 2, since it only uses what is needed and not the entire amount (since it is supposed to just be an estimate).