class MyProprietaryMessageDigest extends MessageDigest {

    @Override
    protected byte[] engineDigest() {
        // Do not use your own digest
        return null;
    }
}

class UseExistingDigest {

    protected void usingDigest {
        // instead of defining your own digest, use existing ones
        MessageDigest sha256Digest = MessageDigest.getInstance("SHA256");
        sha256Digest.update(password.getBytes());
    }
}