给我一段java实现区块链的代码

以下是一个简单的Java实现区块链的代码：

import java.util.ArrayList;
import java.util.Date;

public class Block {

    public String hash;
    public String previousHash;
    private String data; // 我们的简单数据模拟为一个字符串
    private long timeStamp; // 当前时间戳
    private int nonce;

    public Block(String data, String previousHash) {
        this.data = data;
        this.previousHash = previousHash;
        this.timeStamp = new Date().getTime();
        this.hash = calculateHash();
    }

    public String calculateHash() {
        String calculatedHash = StringUtil.applySha256(
                previousHash +
                Long.toString(timeStamp) +
                Integer.toString(nonce) +
                data
        );
        return calculatedHash;
    }

    public void mineBlock(int difficulty) {
        String target = new String(new char[difficulty]).replace('\0', '0');
        while (!hash.substring(0, difficulty).equals(target)) {
            nonce++;
            hash = calculateHash();
        }
        System.out.println("Block Mined!!! : " + hash);
    }
}

public class StringUtil {
    public static String applySha256(String input) {
        try {
            MessageDigest digest = MessageDigest.getInstance("SHA-256");
            byte[] hash = digest.digest(input.getBytes("UTF-8"));
            StringBuffer hexString = new StringBuffer();
            for (int i = 0; i < hash.length; i++) {
                String hex = Integer.toHexString(0xff & hash[i]);
                if (hex.length() == 1) hexString.append('0');
                hexString.append(hex);
            }
            return hexString.toString();
        } catch (Exception e) {
            throw new RuntimeException(e);
        }
    }
}

public class Blockchain {

    private ArrayList<Block> blockchain = new ArrayList<Block>();
    private int difficulty = 5;

    public void addBlock(Block newBlock) {
        newBlock.mineBlock(difficulty);
        blockchain.add(newBlock);
    }

    public boolean isChainValid() {
        Block currentBlock;
        Block previousBlock;
        String hashTarget = new String(new char[difficulty]).replace('\0', '0');

        for (int i = 1; i < blockchain.size(); i++) {
            currentBlock = blockchain.get(i);
            previousBlock = blockchain.get(i - 1);

            if (!currentBlock.hash.equals(currentBlock.calculateHash())) {
                System.out.println("Current Hashes not equal");
                return false;
            }

            if (!previousBlock.hash.equals(currentBlock.previousHash)) {
                System.out.println("Previous Hashes not equal");
                return false;
            }

            if (!currentBlock.hash.substring(0, difficulty).equals(hashTarget)) {
                System.out.println("This block hasn't been mined");
                return false;
            }
        }
        return true;
    }
}

public class Main {

    public static Blockchain blockchain = new Blockchain();

    public static void main(String[] args) {

        Block genesisBlock = new Block("First Block", "0");
        blockchain.addBlock(genesisBlock);

        Block secondBlock = new Block("Second Block", genesisBlock.hash);
        blockchain.addBlock(secondBlock);

        Block thirdBlock = new Block("Third Block", secondBlock.hash);
        blockchain.addBlock(thirdBlock);

        System.out.println("Blockchain is valid: " + blockchain.isChainValid());

        // 修改一个块的数据，看看区块链是否仍然有效
        secondBlock.data = "Second Block modified";
        System.out.println("Blockchain is valid: " + blockchain.isChainValid());
    }
}

这段代码实现了一个简单的区块链，其中包含了区块的添加、挖矿、验证等功能。该代码中使用了SHA-256哈希算法来计算区块的哈希值。