Chain of responsibility

/**
 * It represents a handler and has two methods: one for handling requests and other for combining handlers
 */
@FunctionalInterface
interface RequestHandler {

    // !!! write a method handle that accept request and returns new request here
    // it allows to use lambda expressions for creating handlers below

    Request handle(Request request);

    // !!! write a default method for combining this and other handler single one
    // the order of execution may be any but you need to consider it when composing handlers
    // the method may has any name
    default Request commonRequestHandler() {
        return null;
    }
}

/**
 * Accepts a request and returns new request with data wrapped in the tag <transaction>...</transaction>
 */
final static RequestHandler wrapInTransactionTag =
        (req) -> new Request(String.format("<transaction>%s</transaction>", req.getData()));

/**
 * Accepts a request and returns a new request with calculated digest inside the tag <digest>...</digest>
 */
final static RequestHandler createDigest =
        (req) -> {
            String digest = "";
            try {
                final MessageDigest md5 = MessageDigest.getInstance("MD5");
                final byte[] digestBytes = md5.digest(req.getData().getBytes("UTF-8"));
                digest = new String(Base64.getEncoder().encode(digestBytes));
            } catch (Exception ignored) { }
            return new Request(req.getData() + String.format("<digest>%s</digest>", digest));
        };

/**
 * Accepts a request and returns a new request with data wrapped in the tag <request>...</request>
 */
final static RequestHandler wrapInRequestTag =
        (req) -> new Request(String.format("<request>%s</request>", req.getData()));

/**
 * It should represents a chain of responsibility combined from another handlers.
 * The format: commonRequestHandler = handler1.setSuccessor(handler2.setSuccessor(...))
 * The combining method setSuccessor may has another name
 */
final static RequestHandler commonRequestHandler = (req) -> wrapInRequestTag.handle(createDigest.handle(wrapInTransactionTag.handle(req))); // !!! write the combining of existing handlers here

/**
 * Immutable class for representing requests.
 * If you need to change the request data then create new request.
 */
static class Request {
    private final String data;

    public Request(String requestData) {
        this.data = requestData;
    }

    public String getData() {
        return data;
    }
}