Write

You can make a "transaction" interacting with a Smart Contract when you **need to update some state ** in the contract.

Import necessary classes from the Web3j and kaia libraries(web3j-ext).

Connect to the kaia blockchain using Web3j and specify the URL

Create KlayCredentials using the private key

Define smart contract address to interact with

Define gas price and gas limit for contract transactions

Create a gas provider with static gas price and gas limit

Load the contract using contact address, Web3j instance, credentials, and gas provider. You can read and write the contract through this instance

Get the current number stored in the contract before writing

Increment the number stored in the contract by calling counter.increment().send()

Get the updated number stored in the contract after writing

WriteContractExample.java

package org.web3j.example.contracts;

import java.math.BigInteger;

import org.web3j.crypto.KlayCredentials;

import org.web3j.tx.response.PollingTransactionReceiptProcessor;

import org.web3j.tx.response.TransactionReceiptProcessor;

import org.web3j.example.keySample;

import org.web3j.protocol.http.HttpService;

import org.web3j.protocol.kaia.Web3j;

import org.web3j.tx.gas.StaticGasProvider;

public class WriteContractExample {

/**

* @throws Exception

*

*/

public static void run() throws Exception {

Web3j web3j = Web3j.build(new HttpService(keySample.BAOBAB_URL));

KlayCredentials credentials = KlayCredentials.create(keySample.LEGACY_KEY_privkey);

String contractAddr = "0x95Be48607498109030592C08aDC9577c7C2dD505";

BigInteger GAS_PRICE = BigInteger.valueOf(50000000000L);

BigInteger GAS_LIMIT = BigInteger.valueOf(6721950);

StaticGasProvider gasProvider = new StaticGasProvider(GAS_PRICE, GAS_LIMIT);

Counter counter = Counter.load(contractAddr, web3j, credentials.convertToCredentials(), gasProvider);

// Get number before Contract Write

System.out.println("number before : " + counter.number().send());

// Contract Write (increment function)

counter.increment().send();

// Get number after Contract Write

System.out.println("number after : " + counter.number().send());

}

}

output

❯ java WriteContractExample.java

number before : 297

number after : 298