Class identification

The class identification process is extremely important in object-oriented design. However, it is not a focus in algorithm focusing courses like COP3014 and COP3530 so you may decide how much effort to invest.

Introduction

If you have done some search on class identification in object-oriented programming, you will know that there are many methods to achieve this purpose. It takes experiments to find one method that works best for you. The method described here is only my personal understanding of what works well for students in teaching.

My Recommended Method

1. Tell the story in detail

   • Also known as “user story”, similar to the concept of “specification” and “requirement”

   • Be very detailed from many perspective

   • It is a simpler way compare to “use case” method and is usually good enough for a small project

2. Extract meaningful keywords (concepts)

   • nouns, verbs, and sometimes adjectives

3. Check keywords against rules to categorize them into the group of objects, attributes and behaviors;

   • A good object is an aggregation of attributes and behaviors;

   • Attributes and behaviors in one object should be related (e.g. attributes being the subject or object of the behaviors, a.k.a. coherence);

   • An attribute may also be an object as an object can own another object;

   • Make sure the keywords are within the boundary of the software; Sometimes you will need to discard keywords that are actually actors from outside of your software;

4. Identify the relationships among all keywords (concepts)

5. Generalize objects to classes (for class-based OOP languages)

   • Objects that share same attributes and behaviors can be described as a class;

   • A class is a blue print of a group of objects and an object is an instance of a class;

6. Review and confirm, repeat step 1-5 if necessary.

   • The identification process is purely problem dependent. One keyword may be a class in this system but an attribute in another system; Therefore, check carefully;

   • Check if you have missed any part of the user story;

How to learn

Just practice. Imagine realistic problems to solve and practice class identifications on these problems.

Notations

Part of the UML (unified modeling language) class diagram notations are used in class for an efficient communication.

• A box with three sections to represent a class

  • class name

  • list of data members (mostly instance variables)

  • list of methods

• Arrows with various arrowhead style to represent the relationship between classes

• + to represent public class members

• - to represent public class members

• name: type to indicate an instance variable with a name and a type

• methodName(parameterName: parameterType, ...): returnType to represent a method methodName with a returnType. Return type can be omitted for void method or constructors.

Example

1. Story

   Develop a software to manage a restaurant. When a customer visit the restaurant, he review the menu first and place an order. The order will be delivered first and then paid. Menu holds menu items with prices listed. The customer can order multiple servings of a same item. The manager can add, remove, modify items in the menu.

   Some assumption: Restaurant has a unique name. Only one menu is used. The items in an order is delivered together.

2. Keywords/Concepts

   • nouns: restaurant, menu, customer, order, food(item), price, quantity

   • verbs: visit, read(show), place, deliver, pay

3. Categorize

   • out of boundary: customer, visit

   • classes: Restaurant, Order, Menu, MenuItem, Item

   • attributes: name(restaurant), name(item), price, quantity

   • behaviors: show (menu), place (order), deliver (food)

4. Relationships

   • A restaurant manages a name, a menu, many orders; manages orders

   • A menu owns menu items, shows itself, adds/removes/modifies menu items

   • A menu item owns name, and price

   • An order owns order items, adds/removes/modify quantities of items

   • An order can be placed, delivered and paid

   • An order item owns reference to the menu item and quantity

5. Generalize to class

   • Restaurant class

   • Menu class

   • Order class

   • Item class

   • MenuItem class

6. Review and improve