Java / Kotlin

Web3j is the primary JVM library for working with Polygon, and because Polygon PoS is EVM-equivalent it speaks to https://polygon.therpc.io/YOUR_API_KEY over the same standard Ethereum JSON-RPC as any other EVM chain. It works with Java 8+ and integrates cleanly with Kotlin coroutines out of the box. For Android projects, depend on the web3j-android artifact, which excludes desktop-only dependencies that would otherwise bloat or break your APK. Add it through Gradle with implementation "org.web3j:core:4.x.x".

Web3j supports Java 8+ and Kotlin coroutines out of the box, so the same Polygon code reads naturally whether you write it in Java or idiomatic suspend functions. On Android, use the web3j-android artifact to drop desktop-only dependencies and keep your app's footprint small.

Web3j

Create the client with Web3j.build(new HttpService(nodeUrl)), passing your Polygon endpoint https://polygon.therpc.io/YOUR_API_KEY as nodeUrl. Reuse that single Web3j instance across your application, and remember that balances and transfer amounts come back in wei — convert them to MATIC with Convert.fromWei(...) before showing them to users.

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 coroutine compatibility, Android builds for Polygon mobile dApps, generated smart contract wrappers, CLI tooling, and wallet management

Android Support

On Android, network calls to Polygon must never run on the main thread or the app will freeze and risk an ANR. Launch them from viewModelScope.launch and switch to Dispatchers.IO for the blocking RPC work, as shown below. The connection itself is identical to any other platform — Web3j.build(HttpService("https://polygon.therpc.io/YOUR_API_KEY")) is the same call inside a ViewModel as it is in a backend service.

// Android example
class EthereumViewModel : ViewModel() {
    private val web3j = Web3j.build(HttpService("https://polygon.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

Rather than encoding calls by hand, use the web3j CLI to generate Java or Kotlin wrapper classes directly from a contract's Solidity ABI. The generated wrapper exposes each contract method as a strongly typed function — so loading a contract deployed on Polygon (chain ID 137) and calling its methods becomes ordinary, compile-time-checked code with no 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

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 coroutine compatibility, Android builds for Polygon mobile dApps, generated smart contract wrappers, CLI tooling, and wallet management

Android Support

// Android example
class EthereumViewModel : ViewModel() {
    private val web3j = Web3j.build(HttpService("https://polygon.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