1 TCSS 360, Spring 2005 Lecture Notes Design Guidelines 2 Relevant Reading: Object-Oriented Design Heuristics Arthur Riel

2 Action vs. object-oriented What is the difference between an "action- oriented" and "object-oriented" application? action-oriented: procedural: functional decomposition easy to find data dependencies by looking at function headers not easy to find functional dependencies by looking at pieces of data (see fig 3.3, p31) object-oriented: data decomposition, decentralized functionality data in front of coder's mind, functionality in back

3 When action-oriented works When does it work better to use an action- oriented style for application building? when they put the data in the same file with the functions on it; essentially makes a class so why not just make it a class? forces programmer to use good conventions forces coders to stick with good design

4 "God class" problem What are "god classes?" What is wrong with having a "god class" in your system? god class: a class that hoards too much of the data or functionality of a system. (problem: not modular!) Heuristic 3.1: Distribute system intelligence horizontally as uniformly as possible, that is, the top-level classes in a design should share the work uniformly. Heuristic 3.2: Do not create god classes/objects in your system. Be very suspicious of a class whose name contains Driver, Manager, System, or Subsystem. Heuristic 3.4: Beware of classes that have too much non- communicating behavior, that is, methods that operate on a proper subset of the data members of a class. God classes often exhibit much non-communicating behavior.

5 Forms of god classes behavioral god class: holds too much of system's logic often has a name ending with "System", "Manager" data god class: holds too much of system's data often has a bunch of access / search methods

6 Related data and behavior Heuristic 3.3: Beware of classes that have many accessor methods defined in their public interface. Having many implies that related data and behavior are not being kept in one place. location of "policy" behavior should be in the place where that policy is enforced / enacted What's wrong with the 2 following designs? each has policy behavior in a class that shouldn't have it

7 "Controller" classes What is a "controller" class? Is it good or bad? controller class: a class that only contains behavior (might contain some fields, but they are superfluous) usually grabs state from other classes and acts on it often has an "-er" or "-or" verb phrase as its name, such as DataLoader, PasswordChecker similar to a behavioral God class on a smaller scale design on right below has a controller class (checking of prereqs should be done by course offering)

8 Controllers, cont'd. problems with controller classes data (entity) and behavior (control) should be together, not in separate classes controller classes are basically the action-oriented paradigm it becomes difficult to ask a piece of data, "what functionality depends on you?" in the real world, people dislike controllers... microwave, radio, car have data and behavior together VCR / tape has control separate because controller is expensive What about a TV? It has remote control separate; why? physical convenience, price, dependency; you never want one without the other

9 Model and view model: classes in your system that are related to the internal representation of the state of the system often part of the model is connected to file(s) or database(s) examples (card game): Card, Deck, Player examples (bank system): Account, User, UserList view: classes in your system that display the state of the model to the user generally, this is your GUI (could also be a text UI) should not contain crucial application data Different views can represent the same data in different ways Example: Bar chart vs. pie chart examples: PokerPanel, BankApplet

10 Model-view-controller model-view-controller (MVC): common design paradigm for graphical systems controller: classes that connect model and view defines how user interface reacts to user input (events) receives messages from view (where events come from) sends messages to model (tells what data to display) sometimes part of view (see left) Model Controller View data for rendering events updates Model View Component Controller

11 Advantages of MVC decreases coupling simplifies complex user interface code multiple controllers may be defined based on desired behavior changes to part can affect others without requiring the changed object to know details of others increases cohesion only the view is concerned with pixels and screen-based data improved flexibility new views can be added without changing model change the feel (controller) without changing the look (view) increases reuse one model can be represented in several ways

12 Model-view separation Heuristic 3.5: In applications that consist of an object-oriented model interacting with a user interface, the model should never be dependent on the interface. The interface should be dependent on the model. (bad) example: Making a system with lots of classes that know how to render themselves; the GUI just calls card.draw(), deck.draw(), player.draw(). GUI should render them, not delegate! doesn't this violate Heuristic 2.9: Keep related data and behavior in one place? Shouldn't things know how to draw themselves? the behavior of drawing things is related to the GUI, because it is part of the GUI's appearance. So it belongs in the GUI classes. Having things draw themselves locks them in to one representation.

13 Representing the real world Heuristic 3.6: Model the real world whenever possible. Example of room heat: fig , p37-38 model the room as a way to decide if heat is needed, and to encapsulate sensors

14 Proliferation of classes What is the "proliferation of classes" problem? proliferation of classes: When object-oriented design leads us to design a system that has too many classes that are too small in size and scope, making the system hard to use, debug, and maintain ("ravioli code") Heuristic 3.7: Eliminate irrelevant classes from your design. irrelevant classes often have only get/set methods Heuristic 3.8: Eliminate classes that are outside the system. don't model behavior of a blender just because you sell blenders; don't model a user just because they show up with a ticket Heuristic 3.9: Do not turn an operation into a class. Be suspicious of any class whose name is a verb, especially those that have only one piece of meaningful behavior. Ask if that piece of behavior needs to be migrated to some existing or undiscovered class.

15 Agent classes What is an "agent class?" How does it differ from a controller? Is it good or bad to have them in your system? agent class: a class that acts as a middle-man to help two or more other classes communicate with each other. different from controller because it doesn't contain heavy logic example: Farmer class to link Cow and Milk classes example: Librarian class to link Book and Shelf classes Heuristic 3.10: Agent classes are often placed in the analysis model of an application. During design time, many agents are found to be irrelevant and should be removed. what defines whether an agent is relevant? a relevant agent must have some other behavior beyond simply being a middle-man; it must have some useful purpose of its own

16 Is the controller relevant? in the model-view-controller paradigm, the controller acts largely as an agent Should it be there? View model representation Model business logic Controller user interaction UpdateEvent User Actions Change View SetState Get State

17 In-class exercise In your project groups, define a set of classes and their major attributes and operations, for a calendar management system. Multiple users should be able to use the system to create appointments (either individual appointments or group meetings), see an overview of their calendars at various levels of granularity (day, week, month), edit or delete existing appointments, and set reminders to go off for an existing appointment. Appt. data should be persistent (save and load). What classes would you use in this system? What model would you use? What data would it contain? What views of the model(s) would you want? After some time to discuss, we'll do a quick discussion of each group's design and critique it.

18 Heuristics 2 quick reference Heuristic 2.1: All data should be hidden within its class. Heuristic 2.2: Users of a class must be dependent on its public interface, but a class should not be dependent on its users. Heuristic 2.3: Minimize the number of messages in the protocol of a class. Heuristic 2.4: Implement a minimal public interface that all classes understand. Heuristic 2.5: Do not put implementation details such as common-code private functions into the public interface of a class. Heuristic 2.6: Do not clutter the public interface of a class with items that users of that class are not able to use or are not interested in using. Heuristic 2.7: Classes should only exhibit nil or export coupling with other classes, that is, a class should only use operations in the public interface of another class or have nothing to do with that class. Heuristic 2.8: A class should capture one and only one key abstraction. Heuristic 2.9: Keep related data and behavior in one place. Heuristic 2.10: Spin off nonrelated behavior into another class (i.e., noncommunicating behavior). Heuristic 2.11: Be sure the abstractions that you model are classes and not simply the roles objects play.

19 Heuristics 3 quick reference Heuristic 3.1: Distribute system intelligence horizontally as uniformly as possible, that is, the top-level classes in a design should share the work uniformly. Heuristic 3.2: Do not create god classes/objects in your system. Be very suspicious of a class whose name contains Driver, Manager, System, or Subsystem. Heuristic 3.3: Beware of classes that have many accessor methods defined in their public interface. Heuristic 3.4: Beware of classes that have too much noncommunicating behavior. Heuristic 3.5: In applications that consist of an object-oriented model interacting with a user interface, the model should never be dependent on the interface. Heuristic 3.6: Model the real world whenever possible. Heuristic 3.7: Eliminate irrelevant classes from your design. Heuristic 3.8: Eliminate classes that are outside the system. Heuristic 3.9: Do not turn an operation into a class. Be suspicious of any class whose name is a verb or is derived from a verb, especially those that have only one piece of meaningful behavior (don't count set, get, print). Heuristic 3.10: Agent classes are often placed in the analysis model of an application. During design time, many agents are found to be irrelevant and should be removed.