Build a Simple Java Blockchain: Mining, Transactions, and Balance Queries

To help readers grasp blockchain fundamentals, this article presents a step‑by‑step Java implementation of a simple blockchain system that supports block creation, mining new bitcoins, recording transactions, and querying wallet balances.

Block Structure

The Block class defines the essential fields of a blockchain block: an index, the block’s hash, a timestamp, a list of transactions, a nonce for proof‑of‑work, and the hash of the previous block.

public class Block {
    /** 区块索引号 */
    private int index;
    /** 当前区块的hash值,区块唯一标识 */
    private String hash;
    /** 生成区块的时间戳 */
    private long timestamp;
    /** 当前区块的交易集合 */
    private List transactions;
    /** 工作量证明，计算正确hash值的次数 */
    private int nonce;
    /** 前一个区块的hash值 */
    private String previousHash;
}

Transaction Model

A simplified transaction model treats Bitcoin owners as wallets identified by addresses. Each Transaction records a unique ID, sender address, recipient address, and amount.

public class Transaction {
    /** 交易唯一标识 */
    private String id;
    /** 交易发送方钱包地址 */
    private String sender;
    /** 交易接收方钱包地址 */
    private String recipient;
    /** 交易金额 */
    private int amount;
}

Mining Process

Mining is modeled as a miner solving a hash puzzle: Hash = SHA‑256(lastBlockHash + transactionData + nonce). The hash must start with a configurable number of zeros (e.g., four zeros) to be accepted. When a solution is found, the miner receives a reward transaction (10 bitcoins) and the new block is appended to the chain.

/**
 * 挖矿
 * @param blockchain 整个区块链
 * @param txs 需记账交易记录,包含
 * @param address 矿工钱包地址
 */
private static void mineBlock(List blockchain, List txs, String address) {
    // 加入系统奖励的交易
    Transaction sysTx = new Transaction(CryptoUtil.UUID(), "", address, 10);
    txs.add(sysTx);
    Block latestBlock = blockchain.get(blockchain.size() - 1);
    int nonce = 1;
    String hash = "";
    while (true) {
        hash = CryptoUtil.SHA256(latestBlock.getHash() + JSON.toJSONString(txs) + nonce);
        if (hash.startsWith("0000")) {
            System.out.println("=====计算结果正确，计算次数为：" + nonce + ",hash:" + hash);
            break;
        }
        nonce++;
        System.out.println("计算错误，hash:" + hash);
    }
    Block newBlock = new Block(latestBlock.getIndex() + 1, System.currentTimeMillis(), txs, nonce, latestBlock.getHash(), hash);
    blockchain.add(newBlock);
    System.out.println("挖矿后的区块链：" + JSON.toJSONString(blockchain));
}

Balance Query

To compute a wallet’s balance, the program scans all blocks, sums amounts where the address appears as a recipient, and subtracts amounts where it appears as a sender.

/**
 * 查询余额
 */
public static int getWalletBalance(List blockchain, String address) {
    int balance = 0;
    for (Block block : blockchain) {
        List transactions = block.getTransactions();
        for (Transaction transaction : transactions) {
            if (address.equals(transaction.getRecipient())) {
                balance += transaction.getAmount();
            }
            if (address.equals(transaction.getSender())) {
                balance -= transaction.getAmount();
            }
        }
    }
    return balance;
}

Result Illustration

The following diagram shows the blockchain structure, and the screenshot below displays a sample run output after mining, performing a transaction, and querying a balance.

While this implementation captures the core ideas of Bitcoin—block linking, proof‑of‑work mining, transaction recording, and balance calculation—real‑world systems add cryptographic signatures, peer‑to‑peer networking, and many other security and scalability features.