Object-Oriented (OO) Design CSE 5236: Mobile Application Development Course Coordinator: Dr. Rajiv Ramnath Instructor: Adam C. Champion, Ph.D. Reading: Applying UML and Patterns, Chaps. 1, 6 (OO ref.); Big Nerd Ranch Guide, Chap. 2 (Android/MVC)

Elements of Good OO Design Idea: Capture complexity of real-world problems, solutions via objects Classes and responsibilities Polymorphism helps represent the real-world Achieve system goals through collaboration Principles: Loose coupling, high cohesion Abstraction Encapsulation, information hiding Methodology: Scenario-Driven Design Implement objects needed for vertical slices of system Technique: CRC-Card-Based Design

You should know what each term means! Terminology Check Class Object Method Function Class method Subclass (implementation) Subtype (interface) Interface Abstract class Virtual method You should know what each term means!

OO Design Process Capture narratives of the environment, application (app) Via observations, talking with domain experts, stakeholders, end users Identify specific “circumstances” where an app would be used Capture using text, storyboards, sketches Identify domain classes, assign responsibilities; collaborators Evaluate using OO Checklist Develop the application flow: use cases, screen flows Map domain model to Android framework model Map app flow to framework model; connect UI to domain model; identify collaborators, update model/flow Add contracts Evaluate using OO Checklist (again)

Identify Objects and Classes Examine nouns, noun phrases as candidates Adjectives: candidate attributes or subtypes Group into categories: potential abstract classes, superclasses Write down the purpose of each class Eliminate redundant or non-domain classes

Identify, Assign Responsibilities Start with verbs and verb phrases (i.e. actions) Assign to the appropriate classes Distribute evenly: don’t give one class too much work Don’t break the class definition Locate responsibility with information Locate related information together

Map Domain Model to Framework Patterns Usually a variation of MVC

Identify Collaborations Examine narratives, storyboards, use cases Create scenarios Walkthrough scenarios Identify interactions between classes These are the collaborations

Evaluate Using Design Checklist Each class must have: Clear name Cohesive description of responsibility Long-lived state Collaborators

Record on CRC Cards (1) Source: K. Beck and W. Cunningham, “A Laboratory For Teaching Object-Oriented Thinking,” Proc. ACM OOPSLA, 1989. http://c2.com/doc/oopsla89/paper.html

Record on CRC Cards (2) Sources: K. Beck and W. Cunningham, “A Laboratory For Teaching Object-Oriented Thinking,” Proc. ACM OOPSLA, 1989; C. Larman, Applying UML and Patterns, 3rd ed., Addison-Wesley, 2004.

Exercise: Tic-Tac-Toe Tic-tac-toe, also spelled tick-tack-toe, or noughts and crosses, as it is known in the UK, Ireland, Australia, New Zealand, is a pencil-and-paper game for two players, who take turns marking the spaces in a 3×3 grid with the symbols X and O respectively. The X player usually goes first. The player who succeeds in placing three respective marks in a horizontal, vertical, or diagonal row wins the game. Extend this by adding a narrative about playing the game on an Android device: Tic-tac-toe for Android will implement the Tic-tac-toe paper game as an Android app. In it, human users will be able to play Tic-tac-toe against the computer. Multiple games may be played in each session, with either the computer playing first or the human playing first on an electronic board that will be displayed on the device’s touch screen. Scores for each session will be accumulated. If the user quits the session, scores will be reset.

Nouns and Verbs Nouns: pencil, paper, game, nought, cross, player, X, O, space, symbol, grid, mark, vertical row, horizontal row, diagonal row, human user, human, computer, session, board, touchscreen, score. (Candidate objects and classes) Verbs: take turn, mark, goes, place, win, implement, play, playing first, display, accumulate, quit, reset. (Candidate responsibilities)

Consolidate: Nouns Remove pencil, paper, touchscreen – physical objects Symbol and mark identical – retain symbol. User vs. player – retain player Remove one of board and grid Remove touchscreen – physical Row is a component Session is an instance of game Remove “pencil” and “paper” as physical things not relevant to an Android-based game. Observe that “symbol” and “mark” are identical and retain “symbol.” Observe that “nought” and “O” and “cross” and “X” are identical and, being from the U.S. of A, remove the hated British monstrosities, and leave “O” and “X”. Further note that “O” and “X” appear to be either instances or sub-classes of Symbol. Compare “User” and “Player”. Retain “player” as the player in the game (and note that the “user” is the person using the Android device). Note that “human user” and “human” might be identical concepts, and further that they as well as “computer” are instances or sub-classes of “player.” Realize that “board” and “grid” close enough in meaning that one of them could be removed. We decide to keep “grid,” and, in fact rename it to “game grid.” What about “touch screen?” Since it appears to refer to a physical component of the phone, a first inclination might be to remove it. On the other hand, something needs to handle the visual display of the board. It could be the board itself, or we could separate the data structure that represents the board from its visual manifestation. So that is what we decide to do. But we rename it to Board so as to address our first issue with it. Consider “row” as a component of “game grid,” and “vertical row,” “diagonal row” and “horizontal row” as essentially being different sub-classes or instances of “row”; we don’t know which yet. Consider “session” as being an instance of “game,” with “score” being an attribute of either the “game” or the two players.

Consolidate: Verbs Take turn, goes, play – retain play Mark vs. place vs. …? Use place symbol Remove implement – irrelevant to game Retain display, accumulate, exit and reset Remove “take turn” and “goes” as being close enough to “play.” Retain “play.” For now, retain “playing first” and the missing “playing second” as potentially refinements of “play.” The final design will show that they are not needed. Remove “mark” when used as a verb as being close enough to “place.” Retain “place,” but rename it “place symbol.” Remove “implement” as not being a responsibility relevant to the game, but rather to the process of building it. Retain “display,” “accumulate,” “exit,” and “reset” as valid responsibilities.

Candidate Classes, Responsibilities Classes: Symbol, Player, Human, Computer, Board, Row, and Game (with attribute Score) Instances: O, X of the class Symbol Responsibilities: play, place, display, accumulate (scores), quit, and reset.

Allocate Responsibilities to Classes Class Game is allocated the responsibilities: play, accumulateScores, quit, and reset. Class Board has Display responsibilities. Class GameGrid has Place. Symbol, Player, Human, Computer, and Row have no responsibilities yet. Keep?

Map Domain Model to Framework Patterns Controller classes map to Activities, e.g. GameSession Visual elements (if any, remember we’re doing domain object design) map to views Pure domain objects map to “plain old Java object” (POJO) hierarchies

General Scenario Start a new game. Determine who plays first: the human or the computer. Assign the X symbol to the first player; assign the O symbol to the second player. The first player places his symbol at an empty location on the board. The second player does likewise. Repeat until one player has three of his symbols in a row, column, or diagonal, or no more squares are in play, in which case the game ends in a draw. Accumulate scores for the players. The winning player’s score increments by 1; the losing player’s score does not change. In a draw, both players’ scores remain the same. If the user wishes, start a new game; else, quit.

Screens and Screen Flows in Tic-Tac-Toe

Scenario Walkthrough – Verification, Identifying Collaborators No class to respond to starting new game. Create one: GameController? GameController and Game collaborate Symbol creation, placement? Symbol and Board. placeSymbol invokes Play? Game needs checkResult? Board, GameGrid and Game are collaborators. Etc.

Final Classes, Responsibilities Game: Represents a single Tic-Tac-Toe game. Responsibilities: play, checkResult Collaborators: GameSession, GameView, Grid. GameView: Represents the visual display of a Tic-Tac-Toe game. Responsibilities: placeSymbol, showScores Collaborators: Game GameGrid: Represents 3×3 Tic-Tac-Toe grid. Responsibilities: placeSymbol, getEmptySquares GameSession: Represents Tic-Tac-Toe play session (multiple games) Responsibilities: playNewGame, quit, decidePlayers, accumulateScores Collaborators: Game, GameView Symbol – represents a Tic-Tac-Toe symbol (i.e., an X or an O) Responsibilities: None

Contracts Game: GameView: play(Grid, Symbol, x, y) returns Success, Failure checkResultAndSetState(Grid) returns nothing isActive() returns true or false isWon() returns true or false isDrawn() returns true or false GameView: placeSymbol(Symbol, X, Y) returns Success, Failure showScores(PlayerOneScore, PlayerTwoScore) returns nothing.

Questions and comments? Thank You Questions and comments?