Java / Kotlin

Web3j is the primary JVM library for working with Blast, the ETH-gas Optimistic Rollup with chain ID 81457. It works with Java 8+ and integrates cleanly with Kotlin coroutines out of the box. For Android projects, depend on the web3j-android artifact instead, which excludes desktop-only dependencies that bloat or break an APK build. Add it through Gradle with implementation "org.web3j:core:4.x.x" and point the HttpService at https://blast.therpc.io/YOUR_API_KEY.

Web3j supports Java 8+ and pairs naturally with Kotlin coroutines, so suspending balance and transaction calls against Blast read cleanly. For Android builds, use the web3j-android artifact to drop desktop-only dependencies that would otherwise inflate your APK or trigger packaging conflicts.

Web3j

Create the client with Web3j.build(new HttpService(nodeUrl)), passing your Blast endpoint https://blast.therpc.io/YOUR_API_KEY as nodeUrl. The same call works identically from Java and Kotlin; the examples below show both, reading an ETH balance and signing a raw transfer transaction.

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: Java 8+ support, Kotlin coroutines, Android compatibility, generated smart contract wrappers for Blast, CLI tooling, and wallet management

Android Support

On Android, never run Blast RPC calls on the main thread — launch them in viewModelScope.launch and switch to Dispatchers.IO so network I/O stays off the UI thread and keeps the app responsive. The client construction is identical to the desktop case: Web3j.build(HttpService("https://blast.therpc.io/YOUR_API_KEY")). The ViewModel example below fetches a balance and is ready to push the result back to your UI state.

// Android example
class EthereumViewModel : ViewModel() {
    private val web3j = Web3j.build(HttpService("https://blast.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 directly from a Solidity ABI, as shown below. The generated wrapper exposes each contract method as a type-safe call, so interacting with a Blast contract — say, a yield-bearing vault — becomes a strongly typed method invocation rather than manual ABI encoding.

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

Ready to call this in production?