Finding a new proof of work.

ProofOfWork.java

private static final String VALIDATION_PREFIX = "00";

public int proofOfWork() {
    return proofOfWork(getLastBlock().getProofOfWork());
}

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

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

    return proof;
}

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