Block.java

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

public class Block {

    public String hash, previousHash, merkleRoot;
    public ArrayList<Transaction> transactions = new ArrayList<Transaction>(); // our data will be a simple message
    private long timeStamp; // number of milliseconds since 1/1/1970
    private int nonce;

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

        this.hash = calculateHash(); // **this must be done after the other values are set
    }

    // Calculate new hash based on blocks contents
    public String calculateHash() {
        String calculatedhash = StringUtil.applySha256(
                previousHash +
                        Long.toString(timeStamp) +
                        Integer.toString(nonce) +
                        merkleRoot
                );
        return calculatedhash;
    }

    // Increases nonce value until hash target is reached.
    public void mineBlock(int difficulty) {
        merkleRoot = StringUtil.getMerkleRoot(transactions);
        String target = StringUtil.getDifficultyString(difficulty); // Create a String with difficulty * "0"
        while (!hash.substring(0, difficulty).equals(target)) {
            nonce ++;
            hash = calculateHash();
        }
        System.out.println("Block Mined!!! : " + hash);
    }

    // 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 != "0") {
            if (transaction.processTransaction() != true) {
                System.out.println("Transaction failed to process. Discarded.");
                return false;
            }
        }
        transactions.add(transaction);
        System.out.println("Transaction successfully added to Block.");

        return true;
    }


}