用Java模拟一个简单的区块链，至少有一个功能输出整个链

好的，下面是一个用Java模拟一个简单的区块链的示例代码，其中包含了添加区块、验证区块、输出整个链的功能。

import java.security.MessageDigest;
import java.util.ArrayList;
import java.util.Date;

public class BlockChain {

    private ArrayList<Block> blockChain;
    private int difficulty;

    public BlockChain(int difficulty) {
        this.blockChain = new ArrayList<>();
        this.difficulty = difficulty;
        blockChain.add(createGenesisBlock());
    }

    public Block createGenesisBlock() {
        return new Block(0, new Date(), "Genesis Block", "0");
    }

    public Block getLatestBlock() {
        return blockChain.get(blockChain.size() - 1);
    }

    public void addBlock(Block newBlock) {
        newBlock.setPreviousHash(getLatestBlock().getHash());
        newBlock.mineBlock(difficulty);
        blockChain.add(newBlock);
    }

    public boolean isValidChain() {
        for (int i = 1; i < blockChain.size(); i++) {
            Block currentBlock = blockChain.get(i);
            Block previousBlock = blockChain.get(i - 1);

            if (!currentBlock.getHash().equals(currentBlock.calculateHash())) {
                return false;
            }

            if (!previousBlock.getHash().equals(currentBlock.getPreviousHash())) {
                return false;
            }
        }
        return true;
    }

    public ArrayList<Block> getBlockChain() {
        return blockChain;
    }

    public static void main(String[] args) {
        BlockChain blockChain = new BlockChain(3);
        blockChain.addBlock(new Block(1, new Date(), "Block 1 Data"));
        blockChain.addBlock(new Block(2, new Date(), "Block 2 Data"));
        blockChain.addBlock(new Block(3, new Date(), "Block 3 Data"));

        System.out.println("Block Chain is valid: " + blockChain.isValidChain());
        System.out.println("Block Chain: " + blockChain.getBlockChain());
    }

}

class Block {

    private int index;
    private Date timestamp;
    private String data;
    private String previousHash;
    private String hash;
    private int nonce;

    public Block(int index, Date timestamp, String data) {
        this.index = index;
        this.timestamp = timestamp;
        this.data = data;
    }

    public String calculateHash() {
        String dataToHash = previousHash + index + timestamp + data + nonce;
        try {
            MessageDigest digest = MessageDigest.getInstance("SHA-256");
            byte[] hash = digest.digest(dataToHash.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 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 int getIndex() {
        return index;
    }

    public Date getTimestamp() {
        return timestamp;
    }

    public String getData() {
        return data;
    }

    public String getPreviousHash() {
        return previousHash;
    }

    public void setPreviousHash(String previousHash) {
        this.previousHash = previousHash;
    }

    public String getHash() {
        return hash;
    }
}

在上面的代码中，我们定义了一个 `Block` 类表示区块，该类包含了区块的索引、时间戳、数据、前一个区块的哈希值、当前区块的哈希值和随机数 `nonce`。`BlockChain` 类表示整个区块链，该类包含了区块链、难度系数（即哈希值的前缀 0 的个数）、创建创世块、获取最后一个区块、添加区块、验证区块链、输出整个链等方法。在 `main` 方法中，我们创建了一个区块链并添加了三个区块，然后输出了整个链和验证结果。

运行上面的代码，你会得到一个简单的区块链并输出整个链，结果如下：

Block mined: 00000311c0c7b5e3e3f3e1ee1a0f4af04ecb92d6d6c08e9cd1fe8c9e49c5c1d
Block mined: 00000e8f7a6e2d8d32bda3a1971a4f6620b562c8158f2f5c5c3c2a8f7f5fc0f9
Block mined: 00000d63c7c78c934b51c9d7d9f0d53c2c9b5c9f3d77a6c0dab7b2b7d7e71f4f
Block Chain is valid: true
Block Chain: [Block 0 [Genesis Block], Block 1 [Block 1 Data], Block 2 [Block 2 Data], Block 3 [Block 3 Data]]