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Implementation of Blockchain in Java

Blockchain is the backbone Technology of Digital CryptoCurrency BitCoin

Implementation of the Blockchain: The following are the functions used in the implementation of the blockchain. 

Below is the implementation of the class block:

// 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)
    { = data;
            = previousHash;
            = new Date().getTime();
            = 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(
                + Long.toString(timeStamp)
                + data);
        return calculatedhash;

// Java program for Generating Hashes
public class crypt {
    // Function that takes the string input
    // and returns the hashed string.
    public static String sha256(String input)
        try {
            MessageDigest sha
                = MessageDigest
            int i = 0;
            byte[] hash
                = sha.digest(
            // 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)
            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",
                .get(blockchain.size() - 1)
        blockchain.add(new Block(
            "Third block",
                .get(blockchain.size() - 1)
        blockchain.add(new Block(
            "Fourth block",
                .get(blockchain.size() - 1)
        blockchain.add(new Block(
            "Fifth block",
                .get(blockchain.size() - 1)

// 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
                         .calculateHash())) {
                "Hashes are not equal");
            return false;
        // Checking of the previous hash
        // is equal to the calculated
        // previous hash or not
        if (!previousBlock
                         .previousHash)) {
                "Previous Hashes are not equal");
            return false;
    // If all the hashes are equal
    // to the calculated hashes,
    // then the blockchain is valid
    return true;

Advantages of the Blockchain: 

  1. Blockchain is a distributed network of systems. Therefore, data breaches are very difficult to be carried out.
  2. Since, Blockchain generated hashes of each block, therefore, it is very difficult to carry out malicious attacks.
  3. Data Tampering will change the hash of each block which will make the blockchain invalid

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