Creating a Simple Blockchain Application in Java

In this tutorial, we will learn how to create a simple blockchain application in Java. We will use the SHA-256 hashing algorithm to create digital signatures for each block, and we will use a linked list data structure to connect the blocks together in a chain.

Follow the steps below to create a simple blockchain application in Java:

1. Create a simple POJO class to represent the data of a block:

import java.util.Arrays;

public class Block {
    private String[] transaction;
    private long timestamp;
    private String previousBlockHash;
    private String blockHash;

    public Block(String[] transaction, long timestamp, String previousBlockHash) {
        super();
        this.transaction = transaction;
        this.timestamp = timestamp;
        this.previousBlockHash = previousBlockHash;
        this.blockHash = CryptoManager.convertToHash(previousBlockHash + timestamp + transaction);
    }

    public String[] getTransaction() {
        return transaction;
    }

    public void setTransaction(String[] transaction) {
        this.transaction = transaction;
    }

    public long getTimestamp() {
        return timestamp;
    }

    public void setTimestamp(long timestamp) {
        this.timestamp = timestamp;
    }

    public String getPreviousBlockHash() {
        return previousBlockHash;
    }

    public void setPreviousBlockHash(String previousBlockHash) {
        this.previousBlockHash = previousBlockHash;
    }

    public String getBlockHash() {
        return blockHash;
    }

    public void setBlockHash(String blockHash) {
        this.blockHash = blockHash;
    }

    @Override
    public String toString() {
        return "Block [transaction=" + Arrays.toString(transaction) + ", timestamp=" + timestamp
                + ", previousBlockHash=" + previousBlockHash + ", blockHash=" + blockHash + "]";
    }

}



2. Create a method to convert a transaction data into hash:

import java.nio.charset.StandardCharsets;
import java.security.MessageDigest;

public class CryptoManager {

    public static final String convertToHash(String data) {

        try {
            MessageDigest digest = MessageDigest.getInstance("SHA-256");
            byte[] bytes = digest.digest(data.getBytes(StandardCharsets.UTF_8));

            StringBuffer hexBuffer = new StringBuffer();
            
            //Converting each character to a hexadecimal string
            for (byte b : bytes) {
                hexBuffer.append(String.format("%02x", b));
            }
            return hexBuffer.toString();
        } catch (Exception ex) {
            throw new RuntimeException(ex);
        }
    }

}

3. To test the sample code, create a class with the main method. Next, create blocks with transaction information and store them in an ArrayList as shown in the example below:

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

public class BlockchainExample {

    public static void main(String[] args) {

        List<Block> blockChain = new ArrayList<>();

        String[] firstTransaction = {"A has $20","A gives B $5"};
        Block firstBlock = new Block(firstTransaction, new Date().getTime(), "0");
        blockChain.add(firstBlock);
        System.out.println("First block = " + firstBlock.toString());
        System.out.println("Blockchain = " + blockChain.toString());

        String[] secondTransaction = {"A has $15","A gives C $5"};
        Block secondBlock = new Block(secondTransaction, new Date().getTime(),
                firstBlock.getPreviousBlockHash());
        blockChain.add(secondBlock);
        System.out.println("Second block = " + secondBlock.toString());
        System.out.println("Blockchain = " + blockChain.toString());


        String[] thirdTransaction = {"A has $10","A gives D $5"};
        Block thirdBlock = new Block(thirdTransaction, new Date().getTime(),
                secondBlock.getPreviousBlockHash());
        System.out.println("Third block = " + thirdBlock.toString());
        System.out.println("Blockchain = " + blockChain.toString());

        String[] fourthTransaction = {"A has $5","A gives E $2"};
        Block fourthBlock = new Block(fourthTransaction, new Date().getTime(),
                thirdBlock.getPreviousBlockHash());
        System.out.println("Fourth block = " + fourthBlock.toString());
        System.out.println("Blockchain = " + blockChain.toString());

    }
}