For the purpose of this, we’ll be using a basic ERC20 smart contract I created called JavaToken you can find . We’ll be using it and the repository it’s in for demonstration purposes, so feel free to clone it and follow along! The network we will be deploying and running it on will be a local Ganache Testnet using Truffle. here I recently wrote an article about getting started with where I demonstrated how to set up web3j in Android and how to transfer ether. Upon publishing it, I got a request to write a piece about interacting with custom ERC20 tokens using web3j which lead to the creation of this article. Enjoy! Ethereum development using Android Step 0.9: Generate a smart contract java wrapper The 0.9th step is getting a java wrapper for our smart contract. It’s step 0.9 because it’s not mandatory if your contract is already deployed, but it will make things a lot easier later on when interacting with the contract. Do this by first compiling your solidity contract using a solidity compiler. If you’re using Truffle you can do this by simply running the following command inside the Truffle directory of the JavaToken repository: technically truffle compile Next, install the web3j command line interface. Mac/Linux: curl -L get.web3j.io | sh Windows: Set-ExecutionPolicy Bypass -Scope Process -Force; iex ((New-Object System.Net.WebClient).DownloadString( )) 'https://raw.githubusercontent.com/web3j/web3j-installer/master/installer.ps1' Now that we’ve got all that installed, simply run web3j truffle generate Truffle/build/contracts/JavaToken.json -o src -p com.javaToken from within the JavaToken directory. This will generate a wrapper for the JavaToken smart contract to make interacting with it in Ethereum easier. Step 1: Deploying your contract To deploy our token, we set up some credentials and simply call the deploy() method with them. Web3j w3 = Web3j.build( HttpService( ));
String privateKey = ;
Credentials credentials = Credentials.create(privateKey); BigInteger initialSupply = BigInteger.valueOf( );

JavaToken javaToken = JavaToken.deploy(w3, credentials, DefaultGasProvider(), initialSupply).send() // The Ganache TestNet listens at port 7545 new "HTTP://127.0.0.1:7545" "YourPrivateKeyHere" // Sets the initial supply of JavaTokens in existence 100000 new And there you go, you just deployed a brand new Ethereum token! Alternatively, if your contract is already deployed and you’re just looking to interact with it, you can simply load it using: Web3j w3 = Web3j.build( HttpService( ));
String privateKey = ;
Credentials credentials = Credentials.create(privateKey);
String contractAddress = ;

JavaToken javaToken = JavaToken.load(contractAddress,w3,credentials, DefaultGasProvider()); // The Ganache TestNet listens at port 7545 new "HTTP://127.0.0.1:7545" "YourPrivateKeyHere" "YourContractAddress" new Step 2: Getting your balance The contract is configured to give its deployer the whole initial supply. Let’s check if we really got everything. If we have our wrapper from step 0.9, this is as easy as: BigInteger balance = javaToken.balanceOf(credentials.getAddress()).send() String contractAddress = ; Function function = Function( , Arrays.asList( Address(credentials.getAddress())),
                                 Arrays.asList( TypeReference<Uint256>() {})); String encodedFunction = FunctionEncoder.encode(function); EthCall response = w3.ethCall(
  Transaction.createEthCallTransaction(credentials.getAddress(), contractAddress, encodedFunction),
  DefaultBlockParameterName.LATEST).send();

balance = Numeric.toBigInt(response.getValue()); // With java wrapper // Without java wrapper "YourDeployedContractAddress" // Define the function we want to invoke from the smart contract new "balanceOf" new new // Encode it for the contract to understand /*
Send the request and wait for the response using eth call since
it's a read only transaction with no cost associated
*/ Otherwise, it’s a bit more complicated: First we need to manually input in our transaction the details of the function we want to invoke in our contract, in this case it’s name is balanceOf, the parameter we are providing is an address, and we want the function to return a uint256. Then, we encode this data and put it into a transactions along with the contract we wish to send it to and from who the call comes (our address). Finally, we decode the hexadecimal result into a BigInteger. Now we’ve got 100.000 JavaTokens to our name. Let’s send some to a friend! Step 3: Transferring some tokens Again, the difference between having the wrapper and not having the wrapper is rather significant: String addressTo = ;
BigInteger amount = BigInteger.valueOf( ); TransactionReceipt receipt = javaToken.transfer(addressTo,amount).send(); Function function = Function( , Arrays.asList( Address(addressTo), Uint256(amount)),
                                 Collections.emptyList()); String encodedFunction = FunctionEncoder.encode(function); TransactionManager transactionManager = FastRawTransactionManager(w3, credentials); String transactionHash = transactionManager.sendTransaction(DefaultGasProvider.GAS_PRICE,
                                                            DefaultGasProvider.GAS_LIMIT, contractAddress, encodedFunction,
                                                            BigInteger.ZERO).getTransactionHash(); Optional<TransactionReceipt> transactionReceipt =
  w3.ethGetTransactionReceipt(transactionHash).send().getTransactionReceipt(); (transactionReceipt.isEmpty())
  receipt = ; receipt = transactionReceipt.get(); "YourFriendsAddress" 500 // With java wrapper // Without java wrapper // Define the function we want to invoke from the smart contract new "transfer" new new // Encode it for the contract to understand /*
Need to use a TransactionManager here since transfer actually alters the state of the blockchain
and credentials are therefore relevant
*/ new // Send the transaction off using transactionManager and wait for the hash // Fetch the transaction receipt if null else Without the wrapper is a little bit different this time, as we need to actually prove this transaction comes from our account because we are altering the state of the blockchain by transferring tokens rather than just reading data like previously. We start off the same way by creating a function object. Though the smart contract function returns a Boolean, we don’t specify it here because we don’t need it, as we’re checking whether the transaction was successful via the transaction receipt, so we specify the return as simply an empty list. Next, we create a transaction manager. This distinction is important, as said manager takes our credentials as a parameter, meaning transactions sent through it are attributed to us (unlike the balanceOf call above). Finally, we send off the transaction and verify if it was successful by checking the receipt. Conclusion And that’s all! You just learned how to deploy your own ERC20 token using web3j. Congratulations! The code for all this can be found . here Thanks for following along! Nicolas If you liked this, give this story a couple of claps so more people can see it! If you LOVED this, buy me a coffee :) Ethereum: 0x190d71ba3738f43dc6075f5561e58ac9d4e3dfc2 Bitcoin: 1BRucWzs2vnrkfmbfss14ZQErW74A1H1KE Litecoin: LbNfoJfTFVCj7GFaAUtWqhitHJcJ3m8y3V (Originally published ) here

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