基于Java编写第一个区块链项目

// Java implementation for creating
// a block in a Blockchain
  
import java.util.Date;
  
public class Block {
  
    // Every block contains
    // a hash, previous hash and
    // data of the transaction made
    public String hash;
    public String previousHash;
    private String data;
    private long timeStamp;
  
    // Constructor for the block
    public Block(String data,
                 String previousHash)
    {
        this.data = data;
        this.previousHash
            = previousHash;
        this.timeStamp
            = new Date().getTime();
        this.hash
            = calculateHash();
    }
  
    // Function to calculate the hash
    public String calculateHash()
    {
        // Calling the "crypt" class
        // to calculate the hash
        // by using the previous hash,
        // timestamp and the data
        String calculatedhash
            = crypt.sha256(
                previousHash
                + Long.toString(timeStamp)
                + data);
  
        return calculatedhash;
    }
}

// Java program for Generating Hashes
  
import java.security.MessageDigest;
  
public class crypt {
  
    // Function that takes the string input
    // and returns the hashed string.
    public static String sha256(String input)
    {
        try {
            MessageDigest sha
                = MessageDigest
                      .getInstance(
                          "SHA-256");
            int i = 0;
  
            byte[] hash
                = sha.digest(
                    input.getBytes("UTF-8"));
  
            // hexHash will contain
            // the Hexadecimal hash
            StringBuffer hexHash
                = new StringBuffer();
  
            while (i < hash.length) {
                String hex
                    = Integer.toHexString(
                        0xff & hash[i]);
                if (hex.length() == 1)
                    hexHash.append('0');
                hexHash.append(hex);
                i++;
            }
  
            return hexHash.toString();
        }
        catch (Exception e) {
            throw new RuntimeException(e);
        }
    }
}

// Java implementation to store
// blocks in an ArrayList
  
import java.util.ArrayList;
  
public class GFG {
  
    // ArrayList to store the blocks
    public static ArrayList<Block> blockchain
        = new ArrayList<Block>();
  
    // Driver code
    public static void main(String[] args)
    {
        // Adding the data to the ArrayList
        blockchain.add(new Block(
            "First block", "0"));
        blockchain.add(new Block(
            "Second block",
            blockchain
                .get(blockchain.size() - 1)
                .hash));
  
        blockchain.add(new Block(
            "Third block",
            blockchain
                .get(blockchain.size() - 1)
                .hash));
  
        blockchain.add(new Block(
            "Fourth block",
            blockchain
                .get(blockchain.size() - 1)
                .hash));
  
        blockchain.add(new Block(
            "Fifth block",
            blockchain
                .get(blockchain.size() - 1)
                .hash));
    }
}

// Java implementation to check
// validity of the blockchain
  
// Function to check
// validity of the blockchain
public static Boolean isChainValid()
{
    Block currentBlock;
    Block previousBlock;
  
    // Iterating through
    // all the blocks
    for (int i = 1;
         i < blockchain.size();
         i++) {
  
        // Storing the current block
        // and the previous block
        currentBlock = blockchain.get(i);
        previousBlock = blockchain.get(i - 1);
  
        // Checking if the current hash
        // is equal to the
        // calculated hash or not
        if (!currentBlock.hash
                 .equals(
                     currentBlock
                         .calculateHash())) {
            System.out.println(
                "Hashes are not equal");
            return false;
        }
  
        // Checking of the previous hash
        // is equal to the calculated
        // previous hash or not
        if (!previousBlock
                 .hash
                 .equals(
                     currentBlock
                         .previousHash)) {
            System.out.println(
                "Previous Hashes are not equal");
            return false;
        }
    }
  
    // If all the hashes are equal
    // to the calculated hashes,
    // then the blockchain is valid
    return true;
}