Java / Kotlin

Web3j is the primary JVM library for working with Arbitrum One, supporting Java 8+ and Kotlin coroutines out of the box. Because Arbitrum One exposes the standard Ethereum JSON-RPC interface, Web3j connects to https://arbitrum.therpc.io/YOUR_API_KEY with no special configuration. For Android projects, use the web3j-android artifact, which strips desktop-only dependencies that would otherwise bloat your APK and trip up the dex compiler. Add the core library through Gradle with implementation "org.web3j:core:4.x.x".

Web3j supports Java 8+ and integrates cleanly with Kotlin coroutines, so an Arbitrum One backend service and an Android client can share the same domain logic. For mobile builds, depend on the web3j-android artifact to exclude desktop-only dependencies and keep the app slim.

Web3j

Create the client with Web3j.build(new HttpService("https://arbitrum.therpc.io/YOUR_API_KEY")) and hold a single instance for the life of your application. The same builder call works identically from Java and Kotlin; the snippets below show balance reads and signed transfers in both languages against Arbitrum One.

import org.web3j.protocol.Web3j;
import org.web3j.protocol.http.HttpService;
import org.web3j.utils.Convert;
import java.math.BigDecimal;
 
public class EthereumClient {
    private final Web3j web3j;
 
    public EthereumClient(String nodeUrl) {
        this.web3j = Web3j.build(new HttpService(nodeUrl));
    }
 
    public BigDecimal getBalance(String address) throws Exception {
        BigInteger balanceWei = web3j.ethGetBalance(address, DefaultBlockParameterName.LATEST)
            .send()
            .getBalance();
        return Convert.fromWei(balanceWei.toString(), Convert.Unit.ETHER);
    }
 
    public String sendTransaction(Credentials credentials, String toAddress, BigDecimal ethAmount) {
        BigInteger gasPrice = web3j.ethGasPrice().send().getGasPrice();
        BigInteger value = Convert.toWei(ethAmount, Convert.Unit.ETHER).toBigInteger();
 
        RawTransaction rawTransaction = RawTransaction.createEtherTransaction(
            nonce, gasPrice, GAS_LIMIT, toAddress, value);
 
        byte[] signedMessage = TransactionEncoder.signMessage(rawTransaction, credentials);
        String hexValue = Numeric.toHexString(signedMessage);
 
        return web3j.ethSendRawTransaction(hexValue).send().getTransactionHash();
    }
}

import org.web3j.protocol.Web3j
import org.web3j.protocol.http.HttpService
import org.web3j.utils.Convert
import java.math.BigDecimal
 
class EthereumClient(nodeUrl: String) {
    private val web3j = Web3j.build(HttpService(nodeUrl))
 
    suspend fun getBalance(address: String): BigDecimal {
        val balanceWei = web3j.ethGetBalance(address, DefaultBlockParameterName.LATEST)
            .send()
            .balance
        return Convert.fromWei(balanceWei.toString(), Convert.Unit.ETHER)
    }
 
    suspend fun sendTransaction(
        credentials: Credentials,
        toAddress: String,
        ethAmount: BigDecimal
    ): String {
        val gasPrice = web3j.ethGasPrice().send().gasPrice
        val value = Convert.toWei(ethAmount, Convert.Unit.ETHER).toBigInteger()
 
        val rawTransaction = RawTransaction.createEtherTransaction(
            nonce, gasPrice, GAS_LIMIT, toAddress, value)
 
        val signedMessage = TransactionEncoder.signMessage(rawTransaction, credentials)
        val hexValue = Numeric.toHexString(signedMessage)
 
        return web3j.ethSendRawTransaction(hexValue).send().transactionHash
    }
}

GitHub: https://github.com/web3j/web3j
Docs: https://docs.web3j.io
Key features for Arbitrum One: Java 8+ support, Kotlin coroutines, Android compatibility via the dedicated artifact, generated smart contract wrappers, CLI tooling, and wallet management.

Android Support

On Android, never run RPC calls on the main thread — launch them with viewModelScope.launch and switch to Dispatchers.IO so the UI stays responsive while a request to Arbitrum One is in flight. The client setup is identical to the desktop case: Web3j.build(HttpService("https://arbitrum.therpc.io/YOUR_API_KEY")). Pairing coroutines with the sequencer's sub-second confirmations lets a wallet screen refresh balances almost as fast as the user taps.

// Android example
class EthereumViewModel : ViewModel() {
    private val web3j = Web3j.build(HttpService("https://arbitrum.therpc.io/YOUR_API_KEY"))
 
    fun getBalance(address: String) = viewModelScope.launch {
        withContext(Dispatchers.IO) {
            val balance = web3j.ethGetBalance(address, DefaultBlockParameterName.LATEST)
                .send()
                .balance
            // Update UI
        }
    }
}

Smart Contract Integration

Use the web3j CLI to generate Java or Kotlin wrapper classes from a Solidity ABI — the recommended way to integrate an Arbitrum One contract such as a Camelot router or an Aave pool. The generated wrapper exposes each contract method as a type-safe call, so the compiler catches argument and return-type mistakes before they ever reach the network. Grab verified ABIs from https://arbiscan.io to feed the generator.

1web3j solidity generate /path/to/contract.sol -o /path/to/output -p com.your.package

val contract = YourContract.load(
    contractAddress,
    web3j,
    credentials,
    gasPrice,
    gasLimit
)
 
// Call contract method
val result = contract.someMethod().send()

Java / Kotlin

Web3j

import org.web3j.protocol.Web3j;
import org.web3j.protocol.http.HttpService;
import org.web3j.utils.Convert;
import java.math.BigDecimal;
 
public class EthereumClient {
    private final Web3j web3j;
 
    public EthereumClient(String nodeUrl) {
        this.web3j = Web3j.build(new HttpService(nodeUrl));
    }
 
    public BigDecimal getBalance(String address) throws Exception {
        BigInteger balanceWei = web3j.ethGetBalance(address, DefaultBlockParameterName.LATEST)
            .send()
            .getBalance();
        return Convert.fromWei(balanceWei.toString(), Convert.Unit.ETHER);
    }
 
    public String sendTransaction(Credentials credentials, String toAddress, BigDecimal ethAmount) {
        BigInteger gasPrice = web3j.ethGasPrice().send().getGasPrice();
        BigInteger value = Convert.toWei(ethAmount, Convert.Unit.ETHER).toBigInteger();
 
        RawTransaction rawTransaction = RawTransaction.createEtherTransaction(
            nonce, gasPrice, GAS_LIMIT, toAddress, value);
 
        byte[] signedMessage = TransactionEncoder.signMessage(rawTransaction, credentials);
        String hexValue = Numeric.toHexString(signedMessage);
 
        return web3j.ethSendRawTransaction(hexValue).send().getTransactionHash();
    }
}

import org.web3j.protocol.Web3j
import org.web3j.protocol.http.HttpService
import org.web3j.utils.Convert
import java.math.BigDecimal
 
class EthereumClient(nodeUrl: String) {
    private val web3j = Web3j.build(HttpService(nodeUrl))
 
    suspend fun getBalance(address: String): BigDecimal {
        val balanceWei = web3j.ethGetBalance(address, DefaultBlockParameterName.LATEST)
            .send()
            .balance
        return Convert.fromWei(balanceWei.toString(), Convert.Unit.ETHER)
    }
 
    suspend fun sendTransaction(
        credentials: Credentials,
        toAddress: String,
        ethAmount: BigDecimal
    ): String {
        val gasPrice = web3j.ethGasPrice().send().gasPrice
        val value = Convert.toWei(ethAmount, Convert.Unit.ETHER).toBigInteger()
 
        val rawTransaction = RawTransaction.createEtherTransaction(
            nonce, gasPrice, GAS_LIMIT, toAddress, value)
 
        val signedMessage = TransactionEncoder.signMessage(rawTransaction, credentials)
        val hexValue = Numeric.toHexString(signedMessage)
 
        return web3j.ethSendRawTransaction(hexValue).send().transactionHash
    }
}

GitHub: https://github.com/web3j/web3j

Docs: https://docs.web3j.io

Key features for Arbitrum One: Java 8+ support, Kotlin coroutines, Android compatibility via the dedicated artifact, generated smart contract wrappers, CLI tooling, and wallet management.

Android Support

// Android example
class EthereumViewModel : ViewModel() {
    private val web3j = Web3j.build(HttpService("https://arbitrum.therpc.io/YOUR_API_KEY"))
 
    fun getBalance(address: String) = viewModelScope.launch {
        withContext(Dispatchers.IO) {
            val balance = web3j.ethGetBalance(address, DefaultBlockParameterName.LATEST)
                .send()
                .balance
            // Update UI
        }
    }
}

Smart Contract Integration

1web3j solidity generate /path/to/contract.sol -o /path/to/output -p com.your.package

val contract = YourContract.load(
    contractAddress,
    web3j,
    credentials,
    gasPrice,
    gasLimit
)
 
// Call contract method
val result = contract.someMethod().send()

Java / Kotlin

Web3j

Android Support

Smart Contract Integration

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Java / Kotlin

Web3j

Android Support

Smart Contract Integration