How to use blockchain technology in Java to implement decentralized applications?

public class Block {
    private String hash;
    private String previousHash;
    private String data;

    // 构造函数
    public Block(String data, String previousHash) {
        this.data = data;
        this.previousHash = previousHash;
        this.hash = calculateHash();
    }

    // 计算区块的哈希值
    public String calculateHash() {
        // 使用SHA-256算法计算哈希值
        // 省略具体实现
        return null;
    }

    // 获取区块的哈希值
    public String getHash() {
        return hash;
    }

    // 获取前一个区块的哈希值
    public String getPreviousHash() {
        return previousHash;
    }

    // 获取区块的数据
    public String getData() {
        return data;
    }
}

import java.util.ArrayList;
import java.util.List;

public class Blockchain {
    private List chain;

    // 构造函数
    public Blockchain() {
        chain = new ArrayList<>();
        // 创建初始区块
        Block genesisBlock = new Block("Genesis Block", "0");
        chain.add(genesisBlock);
    }

    // 添加一个新区块
    public void addBlock(Block newBlock) {
        newBlock.previousHash = getLatestBlock().getHash();
        newBlock.hash = newBlock.calculateHash();
        chain.add(newBlock);
    }

    // 获取链中最后一个区块
    public Block getLatestBlock() {
        return chain.get(chain.size() - 1);
    }

    // 验证区块链的完整性
    public boolean isValid() {
        for (int i = 1; i < chain.size(); i++) {
            Block currentBlock = chain.get(i);
            Block previousBlock = chain.get(i - 1);

            // 比较当前区块的哈希值和计算的哈希值是否一致
            if (!currentBlock.getHash().equals(currentBlock.calculateHash())) {
                return false;
            }

            // 比较当前区块的前一个区块的哈希值和保存的哈希值是否一致
            if (!currentBlock.getPreviousHash().equals(previousBlock.getHash())) {
                return false;
            }
        }
        return true;
    }
}

public class BlockchainTest {
    public static void main(String[] args) {
        // 创建一个新的区块链
        Blockchain blockchain = new Blockchain();

        // 添加一些新的区块
        blockchain.addBlock(new Block("Block 1", blockchain.getLatestBlock().getHash()));
        blockchain.addBlock(new Block("Block 2", blockchain.getLatestBlock().getHash()));
        blockchain.addBlock(new Block("Block 3", blockchain.getLatestBlock().getHash()));

        // 输出区块链的完整性
        System.out.println("Blockchain is valid: " + blockchain.isValid());
    }
}