Interacting with the Blockchain via Android Cristian Serje Native TL @Check24 @SerjeCristian @serjec 

Personal Motivation 

Why Blockchain? • >$268B • Disrupted technology • Digital Freedom • Keep ID and personal info secure • Goverment and companies taking it „serious“ 

What it is Blockchain? Blockchain is a technology that allows to make instantaneous transactions on a network without any middlemen. Keeping a record of what happens 

What it is Blockchain? Is a record of transactions 

Why is it called Blockchain? Transactions Transactions Transactions Previous Hash Root Hash Nonce Timestamp Previous Hash Root Hash Nonce Timestamp Previous Hash Root Hash Nonce Timestamp 

How does it work? A User request a transaction A block representing the transaction is created The block it‘s broadcasted to all the nodes of the network All the nodes validate the block and the transaction The block is added to the chain The transaction is verified and executed 

Advantages of Blockchain • Transparency • Security • Instanteneous Transactions • Decentralized • Immutable • Double spend problem solved 

Are there any disadvantages? • Everyone needs to cooperate. • Not regulated • Security issues 

Brief History 1990 2009 2011/12 2012/13 The concept of distributed computing Bitcoin Currency payments Smart Contracts 2013/14 2014/15 Finantial markets using blockchain beyond cash transaction Contracts Origin Transactions 2015/16 2016/17 2018/19 NASDAQ blockchain trials Market consolidation and more developments Year of ICO and Adoption Application 

Brief History 

Bitcoin • Created in 2009 • Market Cap of >$180B • All Digital „Cryptocurrency“ 

The Rise of Ethereum • Decentralization Application Platform • The world of computer • Turing-complete virtual machine • Other assets • Smart Contract • Public blockchain (main & test) • Ether • 2nd largest crypto 

Smart Contracts • Smartcontract = human being • Computarized contract • Immutable code which lives on the blockchain in certain address 

Power of Smart Contracts 

Power of Smart Contracts 1. Trust each other 2. Sign a legal agreement 3. Get help from a mutual friend 

Smart contracts to the rescue 

Transactions 

Transactions Regular transfers of ether from one user to a human user 

Transactions Transfers of ether from one user to noone 

Smart Contract Transfers of ether from one user to a smart contract 

Ethereum Transaction 

Interaction with the blockchain 

What do we need? • Web3j • Infura • Metamask • Web3j Gradle plugin • Remix IDE 

Web3j • Type safe Java/Android library • Command line tools • Ethereum wallet support • Smart contract wrappers • Easier integration with nodes on the Ethereum Network 

Setting up web3J implementation 'org.web3j:core:4.2.0-android' 

Ethereum Transaction 

Connecting to Ethereum Network web3 = Web3j.build(HttpService("https://ropsten.infura.io/v3/YOUR_KEY}")) try { val clientVersion = web3.web3ClientVersion().sendAsync().get() if (!clientVersion.hasError()) { toast("Connected!") } else { toast(clientVersion.error.message) } } catch (e: Exception) { toast(e.message!!) } 

Create a Wallet try { lastWalletFile = WalletUtils.generateNewWalletFile(PASSWORD, walletDir) toast("Wallet created!") } catch (e: Exception) { toast("Error creating wallet!") e.printStackTrace() } 

Send Ether val credentials = WalletUtils.loadCredentials(PASSWORD, File(walletPath + "/${lastWalletFile}")) val receipt = sendFunds( web3, credentials, “to-adddress“ BigDecimal(“how much ether?“), Convert.Unit.ETHER ).sendAsync().get() 

Get Balance val ethGetBalance = web3 .ethGetBalance(walletAddressText.text.toString(), DefaultBlockParameterName.LATEST) .sendAsync() .get() // 1 wei = 10^-18 Ether toast("Balance ${Convert.fromWei(ethGetBalance.balance.toString(), Convert.Unit.ETHER)} ether") 

Smart Contract contract Mortal { /* Define variable owner of the type address */ address owner; /* This constructor is executed at initialization and sets the owner of the contract */ constructor() public { owner = msg.sender; } /* Function to recover the funds on the contract */ function kill() public { if (msg.sender == owner) selfdestruct(msg.sender); } } 

Smart Contract contract Greeter is Mortal { /* Define variable greeting of the type string */ string greeting; /* This runs when the contract is executed */ constructor(string _greeting) public { greeting = _greeting; } /* change greeting */ function changeGreeting(string _greeting) public { greeting = _greeting; } /* Main function */ function greet() public view returns (string) { return greeting; } } 

Deploy Smart Contract 

Check Smart Contract 

Verify Smart Contract 

Interacting with a Smart Contract buildscript { repositories { mavenCentral() } dependencies { classpath ''org.web3j:web3j-gradle-plugin:4.3.0‘ } } apply plugin: 'org.web3j' plugins { id 'org.web3j' version '0.1.4' } 

Interacting with a Smart Contract 

Interacting with a Smart Contract 

Read/Write into a Smart Contract val greeter = Greeter.load(contractAddress, web3, credentials, gasLimit, gasPrice) // check contract validity toast("Greeter valid: ${greeter.isValid}") // read from contract val greeting: Future? = greeter.greet().sendAsync() val convertToString: String? = greeting?.get() toast(“Smart contract returned: $convertToString") // write to contract val transactionReceipt: Future? = greeter.changeGreeting(smartContractText.text.toString()).sendAsync() val result = "Successful transaction. Gas used: ${transactionReceipt?.get()?.blockNumber} ${transactionReceipt?.get()?.gasUsed}" toast(result) 

Questions 

Interaction with the Blockchain. Via Android THANKS! @SerjeCristian serjec