(Coursenotes for CSC 203 Project-based Object-oriented Programming and Design)

A class design process

In this lesson we’ll implement a simple game using an object-oriented (OO) design process. The goal is to get a tiny taste of the thought process that goes into designing an OO system with multiple interacting classes.

Game requirements

We’re going to implement the Game of Nim. Lets start by first understanding the game we’re building. The rules are simple:

  • There are two players.
  • There is a pile of sticks. We can let the players decide how many sticks should be in the pile to begin with.
  • The players take turns removing 1–3 sticks from the pile. Each turn must involve at least 1 stick being removed from the pile.
  • The player that removes the last stick from the pile loses.

We’re going to build this as a text-based game. Here is how an example game might go:

Player 1's name? Yaa
Player 2's name? Michael
How many sticks? 5
Yaa, how many sticks do you want to take? 2
Yaa takes 2 stick(s).
There are 3 left in the pile.
Michael, how many sticks do you want to take? 1
Michael takes 1 stick(s).
There are 2 left in the pile.
Yaa, how many sticks do you want to take? 1
Yaa takes 1 stick(s).
There are 1 left in the pile.
Michael, how many sticks do you want to take? 1
Michael takes 1 stick(s).
There are 0 left in the pile.
Yaa is the winner

Why follow a principled design process?

Before we proceed, I want to acknowledge that the requirements, as described above, are fairly simple. You could probably write the whole game in a couple hundred lines in a single file.

That is fine: we’re still going to use an OO process to design and implement this game. Remember, our goal is to learn to follow a principled design process when we engineer software. OO design is one such process to help us write maintainable code.

But it’s worth questioning: what do we actually mean by that? When I say we should prioritise maintainability, I’m thinking of the following:

  • Requirements change over time. Software requirements rarely stay the same. Either because the client (the person for whom we’re building the software) refines what they’re asking for, or because new features need to be added, or because we, the developers, are correcting initial misunderstandings about aspects of the requirements, change is a constant in software engineering.

  • There will be bugs. It’s likely that you’ll need to debug your program at some point. Decoupling the different modules involved in the overall game implementation will help to quickly isolate issues as well as the changes needed to resolve them.
    • This is related to the previous point. If you are making changes all over your codebase to support every change to the system, your likelihood of bugs increases significantly. The more interactions there are between the different modules involved, the larger the “search space” grows when you are trying to hunt down a bug.
  • Testing. Finally, you want to be able to test the distinct parts of your program to verify that they are working correctly, both individually and together. “Spaghetti code” is much harder to test than smaller cohesive modules, in part because it becomes difficult to even tell what a “distinct part” of the program is.

All that to say: it pays to follow a principled software design process. That is, we want to write loosely coupled, tightly cohesive modules (where “modules” can mean functions, classes, packages, etc.; I use “module” as a general term to emphasise that these principles are not unique to OOP).

  • Loose coupling: Modules can work together harmoniously, but are not dependent on each others’ implementation details. If one module needs to change how it works, that should not beget changes in other modules, or at least should minimise them.
  • Tight cohesion: The above goal can be realised by writing modules that do one thing. What that “one thing” is dependent on the problem at hand. For example, we might have a module whose job it is to manage the user interface for the game (whether it is text-based or graphical) and a module whose job it is to manage the game logic.

Designing the classes needed for the game

I find it useful to ask a series of questions to help me understand what it is I’m building.

What data do we need to keep track of to run this game? To what entities do those pieces of data belong? For what behaviours (functionality) is each entity responsible?

  • For starters, we have the Players. Based on the recorded game output at the beginning of this lesson, what data do you think a Player should hold? What behaviours do you think a Player should be able to perform? For example, the Player ought to have a name, since that is what is used in the text-based output. The Player should also be able to remove a specified number of sticks from the pile. Leading us to…
  • The pile of sticks. What are the data and behaviours here? The Pile should, at the very least, be able to keep track of how many sticks are left in it. It’s also probably a good idea to prevent the removal of too many or too few sticks.[^error-handling]
  • Finally, we have the game itself. This is sort of the “controller” of this whole thing. What data does the Game know about? It needs to know about the Players who are playing the game, and the Pile they are playing with. It also needs to manage the game logic, i.e., setting up the game, letting the players go turn-by-turn, and checking for a winner after each turn.

In the game rules, a player must remove 1–3 sticks from the pile. This means we must not allow player turns in which the player tries to remove < 1 stick or > 3 sticks. Should this check be done by the Pile, the Player, or the Game? Why?

In this text based game, the Player might enter text that cannot be parsed into an int while choosing the number of sticks they want to pick up. How should this be handled? Whose job is it to handle it?

We will discuss the above together as a class. That discussion should ideally give us an idea of what classes should exist in our system, and roughly what each of them should be responsible for.

Here is one possible implementation of the game — we may come up with something different in class, or you may come up with something different on your own. That is fine.

The Player class

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The Pile class

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The Game class

The Game class is our “controller” — it sits above and in-between the Player and Pile classes, managing and mediating those classes’ communications with each other.

That is, the Game prompts the Player to see how many sticks they want to pick up, and sends that input to the Pile. The Pile acts on this information, updating its number of sticks accordingly. The Game then inspects the Pile to see if the game is over or not (by using the Pile’s getter method).

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Supporting additional features

As we’ve said before, a central theme in software engineering is that your requirements can change. For example, as you think about designing the game as described above, think about how easy to difficult it would be to support features like the following. How much of our code would need to change to allow these features?

  • Multiple piles instead of a single pile. Traditional versions of this game can involve multiple piles of sticks, as opposed to just one. Users pick up sticks from one pile at a time, but the game ends when all piles are empty.
  • Supporting a graphical user interface. How much of our codebase needs to change to support a graphical user interface instead of (or in addition to!) a text-based interface?
  • Game history. Suppose we want to be able to show a history of games that have been played, in the system as a whole and/or for individual players. How would our classes change?

You will find that there is a balance to be achieved between two extremes. On one end, you can put in huge amounts of design effort upfront, and try to prepare your code to easily handle any updates to the requirements, i.e., by strictly adhering to principles like information hiding and creating new abstractions (e.g., classes) to support most key features. The downside is that you often don’t need all of those abstractions—that design and preparatory implementation work can go to waste1, and worse, it can make your code harder to read and understand for someone who is not already familiar with the codebase.

On the other extreme, you can eschew this upfront design work, and when changes to requirements inevitably do crop up, you can pay the piper then. Though by that time you may have accrued a fair amount of technical debt.2


  1. Many pithy acronyms for engineering principles hint at this, like YAGNI (you aren’t gonna need it) and KISS (keep it simple, stupid!) 

  2. Technical debt is a metaphor describing the situation in which developers sacrifice some software maintenance tasks (like software design, testing, documentation, refactoring) in favour of speedy implementations and deployments. Sometimes this is fine, as long as that “debt” is repaid soon. A little debt speeds development so long as it is paid back promptly with refactoring.