BlockChain example.

Block.java

import org.apache.commons.codec.digest.DigestUtils;

import java.util.Collections;
import java.util.List;

/**
 * Represents one block in the chain.
 *
 * @author Felix Klauke <info@felix-klauke.de>
 */
public class Block {

    /**
     * The delimiter of the blocks hash. Can by like $ or |.
     */
    private static final String HASH_DELIMITER = "$";

    /**
     * The timestamp when the block was created.
     */
    private final long timestamp = System.currentTimeMillis();

    /**
     * The transactions this block will verify.
     */
    private final List<Transaction> transactions;

    /**
     * The hash of the previous block in the chain.
     */
    private final String previousBlockHash;

    /**
     * The proof that some work was done for earning this block.
     */
    private final int proofOfWork;

    /**
     * Create a new block by its underlying data.
     *
     * @param previousBlockHash The hash of the previous block in the chain.
     * @param transactions The transactions this block will verify.
     * @param proofOfWork The proof that some work was done for earning this block.
     */
    public Block(String previousBlockHash, List<Transaction> transactions, int proofOfWork) {
        this.previousBlockHash = previousBlockHash;
        this.transactions = transactions;
        this.proofOfWork = proofOfWork;
    }

    public List<Transaction> getTransactions() {
        return Collections.unmodifiableList(transactions);
    }

    public long getTimestamp() {
        return timestamp;
    }

    public String getPreviousBlockHash() {
        return previousBlockHash;
    }

    public int getProofOfWork() {
        return proofOfWork;
    }

    public String getHash() {
        return DigestUtils.sha256Hex(timestamp + HASH_DELIMITER + transactions.toString() + HASH_DELIMITER + previousBlockHash + HASH_DELIMITER + proofOfWork);
    }
}

BlockChain.java

import org.apache.commons.codec.digest.DigestUtils;

import java.util.ArrayList;
import java.util.Deque;
import java.util.LinkedList;
import java.util.List;

/**
 * @author Felix Klauke <info@felix-klauke.de>
 */
public class BlockChain {

    /**
     * The prefix a proof of work has to have to be valid.
     */
    private static final String VALIDATION_PREFIX = "0000000000000000000";

    /**
     * The transactions the next created block will verify.
     */
    private final List<Transaction> transactions = new ArrayList<>();

    /**
     * All blocks in the chain.
     */
    private final Deque<Block> blocks = new LinkedList<>();

    /**
     * Start mining a new block.
     *
     * @return The block instance.
     */
    public Block mineBlock() {
        int proofOfWork = proofOfWork();

        Block block = new Block(blocks.getLast().getHash(), transactions, proofOfWork);

        transactions = new ArrayList();

        return block;
    }

    /**
     * Add a transaction that will be verified by the next block.
     *
     * @param transaction The transaction.
     */
    public void addTransaction(Transaction transaction) {
        transactions.add(transaction);
    }

    /**
     * Add a block to the chain.
     *
     * @param block The block.
     */
    public void addBlock(Block block) {
        boolean offer = blocks.offer(block);
    }

    /**
     * Generate a new proof of work.
     *
     * @return The proof of work.
     */
    public int proofOfWork() {
        return proofOfWork(blocks.getLast().getProofOfWork());
    }

    private int proofOfWork(int lastProof) {
        int proof = 0;

        while (!validateProofOfWork(lastProof, proof)) {
            proof++;
        }

        return proof;
    }

    private boolean validateProofOfWork(int lastProof, int currentProof) {
        String result = lastProof + "" + currentProof;
        return DigestUtils.sha256Hex(result).startsWith(VALIDATION_PREFIX);
    }
}

Main.java

import java.util.ArrayList;
import java.util.UUID;

/**
 * @author Felix Klauke <info@felix-klauke.de>
 */
public class Main {

    public static void main(String[] args) {
        // Genesis block
        Block block = new Block("", new ArrayList<>(), 0);

        BlockChain blockChain = new BlockChain();
        blockChain.addBlock(block);

        blockChain.addTransaction(new Transaction(UUID.randomUUID(), 10, UUID.randomUUID()));
        Block mineBlock = blockChain.mineBlock();

        System.out.println("Mined block: " + block.getHash());
        blockChain.addBlock(block);
    }
}

Transaction.java

import java.util.UUID;

/**
 * @author Felix Klauke <info@felix-klauke.de>
 */
public class Transaction {

    private final UUID source;
    private final long amount;
    private final UUID destination;

    public Transaction(UUID source, long amount, UUID destination) {
        this.source = source;
        this.amount = amount;
        this.destination = destination;
    }

    public UUID getSource() {
        return source;
    }

    public UUID getDestination() {
        return destination;
    }

    public long getAmount() {
        return amount;
    }
}