GOOS Book Distilled Part 14

A follow through of the great book Growing Object-Oriented Software, Guided by Tests with code

8 minute read

This is a series of blog posts going through the great book Growing Object Oriented Software Guided By Tests, typing in code chapter by chapter, trying to add some of my own understanding where things may not be easy to grasp in the book. I highly recommand you get a copy of the book and follow along with me. Happy coding.

This post covers Chapter 17 of the book: Teasing Apart Main.

Finding a Role

I think one of the most important skills that a programmer needs to train on, is the instinct of what and where to improve. The ability to see the flaws and have a clear vision of what he/she would like the software to work. And having the vision sometimes is more important than knowing how to achieve there. So how to have the vision in the first place? The answer is to be exposed to good stuff as much as possible. Really, this is true in developing all the skills. Once you know how you would like the final product to be, getting there is only a matter of time and practice. It’s like if you pair programming with a master programmer, you don’t need to remember every single move of his, but you do need to pay special attention to the high level vision that guides him there, like what makes he make the change? What bothers him normally and how to improve? and so on and so forth.

Enough rambling now back to our auction sniper project.

We feel that the main becomes a kitchen sink. It makes us uncomfortable and needs to be refactored. What are the things that are making us uncomfortable? This is a really good question and the author of the book anwsered it beautifully. First we should have a vision for a good Main of any program. What should Main do and what should it contain? The general idea is that Main should really only knows the top high level of the project. Initiate them, connect them and kick start the program then let go. That’s it. Having that in mind and take a look at our Main should make you frown. It has the Chat from smack library, Swing related code, SnipersTableModel, AuctionMessageTranslator to name a few. They are too low level stuff and should be hidden from main.

Extracting the Chat

The goal is to remove any reference of Chat from Main. We can move Chat into XMPPAuction, this makes sense because the XMPPAuction needs a Chat connection to do stuff anyways.

Isolating the Chat

Encapsulating the Chat

Writing a new Test

Extracting the Connection

The goal is to remove any reference of XMPPConnection from Main. We create a AuctionHouse which serves as a factory for Auction. The AuctionHouse is a singleton, it contains a XMPPConnection. To make Auctions, the only thing it needs is an itemId. In this way, we hide the XMPPConnection inside the AuctionHouse.

Source code

Extracting the SnipersTableModel

To be honest, this is where I finally lost it in the book, just too many classes and interfaces. I find it that sometimes OO design can be really confusing, because it doesn’t give a clear view of the sequence that things happen. So to clear our mind, let’s start all over again and think what is really going on beyond classes and interfaces.

The application starts. Shows the UI window. The window has an empty table view, a text field and a join button. Now let’s talk about these views. Firstly, the empty table view has to know that a new sniper is added and adds it to its rows. After adding a new sniper, it also has to be able to update that row when the sniper’s state changes. Secondly, when the join button gets clicked, we should extract the item id from text field, create a Chat with it to the server then send a JOIN message. Also we need to start listening for CLOSE and PRICE messages in the chat. When a CLOSE message is received, we need to display whether our sniper has won or not based on some conditions. When a PRICE message is received, if the price is from the current sniper, then that means our sniper is winning, otherwise, we need to send a higher bid to the server, and also showing that the sniper is bidding.

Believe it or not, this is all the sniper application has to do so far. Now let’s go through the classes and interfaces.


The XMPPAuction class implements Auction interface. Which contains only 3 methods: bid(int price), join(), and AddAuctionEventListener(AuctionEventListener listener). So we can think of its responsibilities contains:

  1. Making a bid, by sending a BID message to the chat.

  2. Joining an auction, by sending a JOIN message to the chat.

  3. Listening to chat messages sent from the server, by adding an AuctionEventListener, this listener is passed to AuctionMessageTranslator. This listener will be notified when the auction is closed or there is a price update.

public class XMPPAuction implements Auction {
    private final Announcer<AuctionEventListener> auctionEventListeners =;
    private final Chat chat;
    public XMPPAuction(XMPPConnection connection, String itemId) {
        chat = connection.getChatManager().createChat(
                auctionId(itemId, connection),
                new AuctionMessageTranslator(connection.getUser(), auctionEventListeners.announce()));

    public void bid(int price) {
        sendMessage(String.format(BID_COMMAND_FORMAT, price));

    public void join() {

    public void addAuctionEventListener(AuctionEventListener auctionEventListener) {

The XMPPAuction is a pretty low level component in the application, it handles sending XMPP messages and listening to messages sent from the server, including parsing the messages and creating a chat and etc. These are pretty low level functionalities. So we can imagine there must be higher level components that uses it as a base.


Since the application supports multiple auctions, so there must be multiple XMPPAuction instances. They all share the same XMPPConnection, actually the application needs only one XMPPConnection. The only difference is the item id. So the XMPPAuctionHouse is a factory, upon initiate it creates a connection to the server, then it uses that connection plus a given item id to create XMPPAuctions.

So the XMPPAuctionHouse maintains a XMPP connection, and create XMPPAuctions using it and the given item id.

public class XMPPAuctionHouse implements AuctionHouse {
    private final XMPPConnection connection;

    private XMPPAuctionHouse(XMPPConnection connection) {
        this.connection = connection;

    public Auction auctionFor(String itemId) {
            return new XMPPAuction(connection, itemId);

    public static XMPPAuctionHouse connect(String hostname, String username, String password) throws XMPPException {
        XMPPConnection connection = new XMPPConnection(hostname);
        connection.login(username, password, AUCTION_RESOURCE);
        return new XMPPAuctionHouse(connection);

    public void disconnect() {


We mentioned that XMPPAuction will notify to the outside when the auction is closed and the current price updates. But who will be listening to these? The answer is AuctionSniper.

public class AuctionSniper implements AuctionEventListener {

    private SniperSnapshot snapshot;
    private final Announcer<SniperListener> listeners =;
    private Auction auction;

    AuctionSniper(String itemId, Auction auction) { = auction;
        this.snapshot = SniperSnapshot.joining(itemId);

    public void addSniperListener(SniperListener listener) {

    public void auctionClosed() {
        snapshot = snapshot.closed();

    public void currentPrice(int price, int increment, PriceSource priceSource) {
        switch (priceSource) {
            case FromSniper:
                snapshot = snapshot.winning(price);
            case FromOtherBidder:
                int bid = price + increment;
                snapshot = snapshot.bidding(price, bid);

    private void notifyChange() {

    public SniperSnapshot getSnapshot() {
        return snapshot;

The AuctionSniper itself has its listeners. But it only has one method: sniperStateChanged(snapshot). What does this tell you? It is pretty clear that AuctionSniper takes what it is listening to and transforms the information into a form that better suits out application. To be more specific, it takes in the currentPrice() and auctionClosed(), and creates and keeps a SniperSnapshot and notify to outside only this snapshot.


Since we know that there will be multiple AuctionSnipers, who owns them? The anwser is SniperPortfolio. It has a list of AuctionSnipers, and when a new one is added, it notifies the outside.

public class SniperPortfolio implements SniperCollector {

    public interface PortfolioListener extends EventListener {
        void sniperAdded(AuctionSniper sniper);

    private final List<AuctionSniper> snipers = new ArrayList<>();
    private final Announcer<PortfolioListener> announcer =;

    public void addSniper(AuctionSniper sniper) {

    public void addPortfolioListener(PortfolioListener listener) {

This class is simple and has only one clear responsibility.

Now we have seen some of the key players of this application, we can sort of understand that the XMPPAuctionHouse will be creating Auction and then somehow using it to create AuctionSniper, and SniperPortfolio will be collecting the created AuctionSnipers. Now let’s move on.

Once the new AuctionSniper is add to the SniperPortfolio, it notifies outside, so who is listening to this? The SnipersTableModel is. We make it so that when the application starts, the SnipersTableModel is hooked up with SniperPortfolio to listen to this new sniper added notification. Remember in the beginning of this post we discussed that the SniperTableModel needs to respond to 2 events, one is the new auction added, the other one is that the existing auction updates. By listening to SniperPortfolio it fulfills the first responsibility. Then what about the second one? That is also easy to solve. When the SniperTableModel gets the new sniper added notification, it adds the new sniper’s snapshot into the existing table, and also registers itself as a listener into the newly created AuctionSniper to receive the snapshot updates. So that’s why the SniperTableModel ends up implements two listeners, one for new sniper added and one for existing sniper snapshot changes.

By now, you should get a better view how this complex system works. If not reread the book or this post until you everything sinks in.

This is the end of Chapter 17.

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