CS 303E Lecture 19: Designing Classes

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CS 303E Lecture 19Designing Classes
It's supposed to be hard! If it wasn't hard,
everyone would do it. The hard... is what makes
it great! - Jimmy Dugan, A League of their Own
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How are Complex Programs Designed?

• A problem analysis provides requirements of the
  program.
• In an object oriented design the object is King!
• The purpose of the design phase is to discover
  the classes necessary to meet the requirements.
• Important methods in a class are also discovered
  during design.

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Design Classes for a Connect Four Program

• Connect four is a game with two players.
• Each player takes turns dropping a checker into
  one of 7 slots.
• The checkers fall down to the bottom most
  available row in that column. (6 rows per
  column)
• The first player to get 4 checkers in a row,
  vertically, horizontally, or diagonally wins.
• thinks.com/java/connect4/connect4.htm

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Requirements

• Create a program that allows two people to play
  Connect Four on the computer.
• The player playing black always goes first.
• The computer determines a winner or if the game
  has resulted in a tie and announces this
• Player may replay after each game
• Players may concede a game

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Starting the Design

• Look for nouns in the requirements.
• nouns normally are used to create classes?
• Look for verbs
• verbs normally are used to create methods in a
  class
• What nouns / classes are there in connect four?
• What verbs are there in connect four?

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Who Does What???

• Initial design results in many classes and
  methods
• It may not be clear which class is responsible
  for each method
• A CRC card is created for each class.
• classes, responsibility, and collaborators card
• An index card with the name of the class at the
  top.
• index card so as to limit the number of methods
  in each class. Avoid large classes!

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CRC Card
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More on CRC Cards

• CRC cards are a simple and informal tool used to
  design a program.
• A collaborator is any class that helps accomplish
  the task or method
• Several people can participate in design
• each represents a few classes
• role playing of classes to determine
  responsibilities and collaborators

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Connect Four Classes

• Player
• Checker
• Board
• Display
• And possibly others

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Methods for Connect Four

• Move / drop checker
• Display board
• Clear / reset board or game
• Check for win
• possibly others

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Design Another Program

• Design a program to play solitaire
• Nouns / Classes
• Verbs / Methods?