Block.java

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

public class Block {

    public String hash;
    public String previousHash;
    public String merkleRoot;
    public ArrayList<Transaction> transactions = new ArrayList<Transaction>();
    public int difficulty;

    public  long timeStamp;
    private int nonce;

    // Block Constructor
    public Block( String previoushash){
        this.previousHash = previoushash;
        this.timeStamp = new Date().getTime();

        this.hash = calculateHash();
    }

    public String calculateHash() {
        String calculatehash = StringUtil.applySha256(
                previousHash +
                        Long.toString(timeStamp) +
                        Integer.toString(nonce) +
                       merkleRoot
        );
        return calculatehash;
    }

    public void mineBlock(int difficulty) {
        this.difficulty = 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 void addCoinbase(Transaction coinbase){
        transactions.add(coinbase);
        merkleRoot = Transaction.getMerkleRoot(transactions);
        System.out.println("Coinbase transaction added to block");

    }

    // Add transactions to this block
    public boolean addTransaction(Transaction transaction){
        // Process transaction and check if valid, unless block is genesis block then ignore.

        if(transaction == null) return false;
        if((!previousHash.equals("0"))) {

            if((!transaction.processTransaction())) {
                System.out.println("Transaction failed to process. Discarded.");
                return false;
            }
        }
        transactions.add(transaction);
        merkleRoot = Transaction.getMerkleRoot(transactions);
        System.out.println("Transaction Successfully added to Block");
        return true;
    }

}