Interacting with a Node

Start sending requests

To send synchronous requests:

Web3j web3 = Web3j.build(new HttpService());  // defaults to http://localhost:8545/
Web3ClientVersion web3ClientVersion = web3.web3ClientVersion().send();
String clientVersion = web3ClientVersion.getWeb3ClientVersion();

To send asynchronous requests using a CompletableFuture (Future on Android):

Web3j web3 = Web3j.build(new HttpService());  // defaults to http://localhost:8545/
Web3ClientVersion web3ClientVersion = web3.web3ClientVersion().sendAsync().get();
String clientVersion = web3ClientVersion.getWeb3ClientVersion();

To use an RxJava Flowable:

Web3j web3 = Web3j.build(new HttpService());  // defaults to http://localhost:8545/
web3.web3ClientVersion().flowable().subscribe(x -> {
    String clientVersion = x.getWeb3ClientVersion();
    ...
});

IPC

Web3j also supports fast inter-process communication (IPC) via file sockets to clients running on the same host as web3j. To connect simply use the relevant IpcService implementation instead of HttpService when you create your service:

// OS X/Linux/Unix:
Web3j web3 = Web3j.build(new UnixIpcService("/path/to/socketfile"));
...

// Windows
Web3j web3 = Web3j.build(new WindowsIpcService("/path/to/namedpipefile"));
...

Note: IPC is not available on web3j-android.

Transactions

Web3j provides support for both working with Ethereum wallet files (recommended) and Ethereum client admin commands for sending transactions.

Send Ether

To send Ether to another party using your Ethereum wallet file:

Web3j web3 = Web3j.build(new HttpService());  // defaults to http://localhost:8545/
Credentials credentials = WalletUtils.loadCredentials("password", "/path/to/walletfile");
TransactionReceipt transactionReceipt = Transfer.sendFunds(
        web3, credentials, "0x<address>|<ensName>",
        BigDecimal.valueOf(1.0), Convert.Unit.ETHER)
        .send();

Custom Transaction

Or if you wish to create your own custom transaction:

Web3j web3 = Web3j.build(new HttpService());  // defaults to http://localhost:8545/
Credentials credentials = WalletUtils.loadCredentials("password", "/path/to/walletfile");

// get the next available nonce
EthGetTransactionCount ethGetTransactionCount = web3j.ethGetTransactionCount(
             address, DefaultBlockParameterName.LATEST).send();
BigInteger nonce = ethGetTransactionCount.getTransactionCount();

// create our transaction
RawTransaction rawTransaction  = RawTransaction.createEtherTransaction(
             nonce, <gas price>, <gas limit>, <toAddress>, <value>);

// sign & send our transaction
byte[] signedMessage = TransactionEncoder.signMessage(rawTransaction, credentials);
String hexValue = Numeric.toHexString(signedMessage);
EthSendTransaction ethSendTransaction = web3j.ethSendRawTransaction(hexValue).send();
// ...

Although it's far simpler using Web3j's Transfer for transacting with Ether.

Using an Ethereum client's admin commands (make sure you have your wallet in the client's keystore):

Admin web3j = Admin.build(new HttpService());  // defaults to http://localhost:8545/
PersonalUnlockAccount personalUnlockAccount = web3j.personalUnlockAccount("0x000...", "a password").sendAsync().get();
if (personalUnlockAccount.accountUnlocked()) {
    // send a transaction
}

If you want to make use of OpenEthereum's Personal or Trace, or Geth's Personal client APIs, you can use the org.web3j:parity and org.web3j:geth modules respectively.