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想法：

import java.util.Date;import java.util.ArrayList;import com.google.gson.GsonBuilder;import java.security.MessageDigest;class StringUtil {
       //Applies Sha256 to a string and returns the result.    public static String applySha256(String input) {
           try {
               MessageDigest digest = MessageDigest.getInstance("SHA-256");            //Applies sha256 to our input,            byte[] hash = digest.digest(input.getBytes("UTF-8"));            StringBuffer hexString = new StringBuffer(); // This will contain hash as hexidecimal            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);        }    }}class Block {
       public String hash;    public String previousHash;    private String data; //our data will be a simple message.    private long timeStamp; //as number of milliseconds since 1/1/1970.    private int nonce;    //Block Constructor.    public Block(String data, String previousHash) {
           this.data = data;        this.previousHash = previousHash;        this.timeStamp = new Date().getTime();        this.hash = calculateHash(); //Making sure we do this after we set the other values.    }    //Calculate new hash based on blocks contents    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'); //Create a string with difficulty * "0"        //System.out.println(target);        while (!hash.substring(0, difficulty).equals(target)) {
               nonce++;            hash = calculateHash();        }        System.out.println("Block Mined!!! : " + hash);    }}public class main{
       public static int difficulty = 5;    public static ArrayList
   
     blockchain = new ArrayList
    
     ();    public static void main(String[] args) {
           //add our blocks to the blockchain ArrayList:        blockchain.add(new Block("Hi im the first block", "0"));        System.out.println("Trying to Mine block 1... ");        blockchain.get(0).mineBlock(difficulty);        blockchain.add(new Block("Yo im the second block", blockchain.get(blockchain.size() - 1).hash));        System.out.println("Trying to Mine block 2... ");        blockchain.get(1).mineBlock(difficulty);        blockchain.add(new Block("Hey im the third block", blockchain.get(blockchain.size() - 1).hash));        System.out.println("Trying to Mine block 3... ");        blockchain.get(2).mineBlock(difficulty);        System.out.println("\nBlockchain is Valid: " + isChainValid());        String blockchainJson = new GsonBuilder().setPrettyPrinting().create().toJson(blockchain);        System.out.println("\nThe block chain: ");        System.out.println(blockchainJson);    }    public static Boolean isChainValid() {
           Block currentBlock;        Block previousBlock;        String hashTarget = new String(new char[difficulty]).replace('\0', '0');        //loop through blockchain to check hashes:        for (int i = 1; i < blockchain.size(); i++) {
               currentBlock = blockchain.get(i);            previousBlock = blockchain.get(i - 1);            //compare registered hash and calculated hash:            if (!currentBlock.hash.equals(currentBlock.calculateHash())) {
                   System.out.println("Current Hashes not equal");                return false;            }            //compare previous hash and registered previous hash            if (!previousBlock.hash.equals(currentBlock.previousHash)) {
                   System.out.println("Previous Hashes not equal");                return false;            }            //check if hash is solved            if (!currentBlock.hash.substring(0, difficulty).equals(hashTarget)) {
                   System.out.println("This block hasn't been mined");                return false;            }        }        return true;    }}
    
   

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