Roulette Solution Overview

The first section, Preliminary Survey of Classes, is a survey of the classes gleaned from the general problem statement. Refer to Problem Statement as well as the problem details in Roulette Details. This survey is drawn from a quick overview of the key nouns in these sections.

We’ll amplify this survey with some details of the class definitions in Preliminary Roulette Class Structure.

Given this preliminary of the candidate classes, A Walk-Through of Roulette is a walk-through of the possible design that will refine the definitions, and give us some assurance that we have a reasonable architecture. We will make some changes to the preliminary class list, revising and expanding on our survey.

We will also include a number of questions and answers in Roulette Solution Questions and Answers. This should help clarify the design presentation and set the stage for the various development exercises in the chapters that follow.

Preliminary Survey of Classes

To provide a starting point for the development effort, we have to identify the objects and define their responsibilities. The central principle behind the allocation of responsibility is encapsulation; we do this by attempting to isolate the information or isolate the processing into separate objects. Encapsulation assures that the methods of a class are the exclusive users of the fields of that class. It also makes each class very loosely coupled with other classes; this permits change without a destructive ripple through the application.

For example, a class for each Outcome defines objects which can contain both the name and the payout odds. Each Outcome instance can be used to compute a winning amount, and no other element of the simulation needs to share the odds information or the payout calculation.

In reading the background information and the problem statement, we noticed a number of nouns that seemed to be important parts of the game we are simulating.

  • Wheel

  • Bet

  • Bin

  • Table

  • Red

  • Black

  • Green

  • Number

  • Odds

  • Player

  • House

One common development milestone is to be able to develop a class model in the Unified Modeling Language (UML) to describe the relationships among the various nouns in the problem statement. Building (and interpreting) this model takes some experience with OO programming. In this first part, we’ll avoid doing extensive modeling. Instead we’ll try to identify some basic design features. We’ll focus in on the most important of these nouns and describe the kinds of classes that you will build.

Preliminary Roulette Class Structure

We’ll summarize some of the classes and responsibilities that we can identify from the problem statement. This is not the complete list of classes we need to build. As we work through the exercises, we’ll discover additional classes and rework some of these preliminary classes more than once.

We’ll describe each class with respect to the responsibility allocated to the class and the collaborators. Some collaborators are used by an object to get work done. We have a number of “uses-used by” collaborative relationships among our various classes.

These are the classes that seem most important:



A name for the bet and the payout odds. This isolates the calculation of the payout amount. Example: “Red”, “1:1”.


Collected by a Wheel object into the bins that reflect the bets that win; collected by a Table object into the available bets for the Player; used by a Game object to compute the amount won from the amount that was bet.



Selects the Outcome instances that win. This isolates the use of a random number generator to select Outcome instances. It encapsulates the set of winning Outcome instances that are associated with each individual number on the wheel. Example: the “1” bin has the following winning Outcome instances: “1”, “Red”, “Odd”, “Low”, “Column 1”, “Dozen 1-12”, “Split 1-2”, “Split 1-4”, “Street 1-2-3”, “Corner 1-2-4-5”, “Five Bet”, “Line 1-2-3-4-5-6”, “00-0-1-2-3”, “Dozen 1”, “Low” and “Column 1”.


Collects the Outcome instances into bins; used by the overall Game to get a next set of winning Outcome instances.



A collection of bets placed on Outcome instances by a Player. This isolates the set of possible bets and the management of the amounts currently at risk on each bet. This also serves as the interface between the Player and the other elements of the game.


Collects the Outcome instances; used by Player to place a bet amount on a specific Outcome; used by Game to compute the amount won from the amount that was bet.



Places bets on Outcome instances, updates the stake with amounts won and lost.


Uses Table to place bets on Outcome instances; used by Game to record wins and losses.



Runs the game: gets bets from Player, spins Wheel, collects losing bets, pays winning bets. This encapsulates the basic sequence of play into a single class.


Uses Wheel, Table, Outcome, Player. The overall statistical analysis will play a finite number of games and collect the final value of the Player’s stake.

The class Player has the most important responsibility in the application, since we expect to update the algorithms this class uses to place different kinds of bets. Clearly, we need to cleanly encapsulate the Player, so that changes to this class have no ripple effect in other classes of the application.

A Walk-Through of Roulette

A preliminary task is to review these responsibilities to confirm that a complete cycle of play is possible. This will help provide some design details for each class. It will also provide some insight into classes that may be missing from this overview.

One way to structure this task is to do a Class-Responsibility-Collaborators (CRC) walk-through.

As preparation, get some 5” x 8” notecards. On each card, write down the name of a class, the responsibilities and the collaborators. Leave plenty of room around the responsibilities and collaborators to write notes. We’ve only identified five classes, so far, but others always show up during the walk-through.

During the walk-through, we’ll be identifying areas of responsibility, allocating them to classes of objects, and defining any collaborating objects. An area of responsibility is a thing to do, a piece of information, or a computed result. Sometimes a complex responsibility can be decomposed into smaller pieces, and those smaller pieces assigned to other classes. There are a lot of reasons for decomposing, not all of which are apparent at first.

The basic processing outline is the responsibility of the Game class. To start, locate the Game card.

  1. Our preliminary note was that this class “Runs the game.” The responsibilities section has a summary of four steps involved in running the game.

  2. The first step is “gets bets from Player.” Find the Player card.

  3. Does a Player collaborate with a Game to place bets? If not, update the cards as necessary to include this.

  4. One of the responsibilities of a Player is to place bets. The step in the responsibility statement is merely “Places bets on Outcome instances.” Looking at the classes, we note that the Table contains the amounts placed on the Bets. Fix the collaboration information on the Player to name the Table class. Find the Table card.

  5. Does a Table collaborate with a Player to accept the bets? If not, update the cards as necessary to include this.

  6. What card has responsibility for the amount of the bet? It looks like Table. We note one small problem: the Table contains the collection of amounts bet on Outcome instances.

    What class contains the individual “amount bet on an Outcome?” This class appears to be missing. We’ll call this new class Bet and start a new card. We know one responsibility is to hold the amount bet on a particular Outcome.

    We know three collaborators: the amount is paired with an Outcome, all of the Bet s are collected by a Table, and the Bet s are created by a Player. We’ll update all of the existing cards to name their collaboration with Bet.

  7. What card has responsibility for keeping all of the Bet instances? Does Table list that as a responsibility? We should update these cards to clarify this collaboration.

You should continue this tour, working your way through spinning the Wheel to get a list of winning Outcome instances. From there, the Game can get all of the Bet instances from the Table and see which are based on winning Outcome instances and which are based on losing Outcome instances. The Game can notify the Player of each losing Bet, and notify the Player of each winning Bet, using the Outcome to compute the winning amount.

This walk-through can provide an overview of some of the interactions among the objects in the working application. You may uncover additional design ideas. The most important outcome of the walk-through is a sense of the responsibilities and the collaborations required to create the necessary application behavior.

Roulette Solution Questions and Answers

Why does the Game class run the sequence of steps? Isn’t that the responsibility of some “main program?”

Coffee Shop Answer. We haven’t finished designing the entire application, so we need to reflect our own ignorance of how the final application will be assembled from the various parts. Rather than allocate too many responsibilities to Game, and possibly finding conflicts or complication, we’d rather allocate too few responsibilities until we know more.

From another point of view, designing the main program is premature because we haven’t finished designing the entire application. We anticipate a Game object being invoked from some statistical data gathering object to run one game. The data gathering object will then get the final stake from the player and record this. Game object’s responsibilities are focused on playing the game itself. We’ll need to add a responsibility to Game to collaborate with the data gathering class to run a number of games as a “session”.

Technical Answer. In procedural programming (especially in languages like COBOL), the “main program” is allocated almost all of the responsibilities. These procedural main programs usually contain a number of elements, all of which are very tightly coupled. This is a bad design, since the responsibilities aren’t allocated as narrowly as possible. One small change in one place breaks the whole program.

In OO languages, we can reduce the main program to a short list of object constructors, with the real work delegated to the objects. This level of coupling assures us that a small change to one class has no impact on other classes or the program as a whole.

Why is Outcome a separate class? Each object that is an instance of Outcome only has two attributes; why not use an array of Strings for the names, and a parallel array of integers for the odds?

Representation. We prefer not to decompose an object into separate data elements. If we do decompose this object, we will have to ask which class would own these two arrays? If Wheel keeps these, then Table becomes very tightly coupled to these two arrays that should be Wheel object’s responsibility. If Table keeps these, then Wheel is privileged to know details of how Table is implemented. If we need to change these arrays to another storage structure, two classes would change instead of one.

Having the name and odds in a single Outcome object allows us to change the representation of an Outcome. For example, we might replace the String as the identification of the outcome, with a collection of the individual numbers that comprise this outcome. This would identify a straight bet by the single winning number; an even money bet would be identified by an array of the 18 winning numbers.

Responsibility. The principle of isolating responsibility would be broken by this “two parallel arrays” design because now the Game class would need to know how to compute odds. In more complex games, there would be the added complication of figuring the rake. Consider a game where the Player object’s strategy depends on the potential payout. Now the Game and the Player both have copies of the algorithm for computing the payout. A change to one must be paired with a change to the other.

The alternative we have chosen is to encapsulate the payout algorithm along with the relevant data items in a single bundle.

If Outcome encapsulates the function to compute the amount won, isn’t it just a glorified subroutine?

If you’re background is BASIC or FORTRAN, this can seem to be true. A class can be thought of as a glorified subroutine library that captures and isolates data elements along with their associated functions.

A class is more powerful than a subroutine library with private data. For example, classes introduce inheritance as a way to create a family of closely-related definitions.

We discourage trying to mapping OO concepts back to other non-OO languages.

What is the distinction between an Outcome and a Bet?

We need to describe the propositions on the table on which you can place bets. The propositions are distinct from an actual amount of money wagered on a proposition. There are a lot of terms to choose from, including bet, wager, proposition, place, location, or outcome. We opted for using Outcome because it seemed to express the open-ended nature of a potential outcome, different from an amount bet on a potential outcome. We’re considering the Outcome as an abstract possibility, and the Bet as a concrete action taken by a player.

Also, as we expand this simulation to cover other games, we will find that the randomized outcome is not something we can directly bet on. In Roulette, however, all outcomes are something we can be bet on, as well as a great many combinations of outcomes. We will revisit this design decision as we move on to other games.

Why are the classes so small?

First-time designers of OO applications are sometimes uncomfortable with the notion of emergent behavior. In procedural programming languages, the application’s features are always embodied in a few key procedures. Sometimes a single procedure, named main().

A good OO design partitions responsibility. In many cases, this subdivision of the application’s features means that the overall behavior is not captured in one central place. Rather, it emerges from the interactions of a number of objects.

We have found that smaller elements, with very finely divided responsibilities, are more flexible and permit change. If a change will only alter a portion of a large class, it can make that portion incompatible with other portions of the same class. A symptom of this is a bewildering nest of if-statements to sort out the various alternatives. When the design is decomposed down more finely, a change can be more easily isolated to a single class. A sequence of if-statements can be focused on selecting the proper class, which can then carry out the desired functions.

Looking Forward

Now that we’ve considered the overall structure of the solution, we can start to look at the class definitions.

We’ll take a “bottom-up” approach to the classes. The idea is to define the small things first, and then combine those things into larger and more sophisticated components.

The simplest thing we’ve seen is an individual outcome. For example, a bin on the wheel contains “Red” as an actual outcome. A player will make a bet on “Red” as an expected outcome. We’ll start there in the next chapter.