Good Class Design - Introduction to Programming in Java - Lecture Slides, Slides of Java Programming

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• Objectives
  • introduce coupling, cohesion, responsibility-driven design, and refactoring

7. Good Class Design

Topics

• 1. Why Class Design?
• 2. World of Zuul
• 3. Code Design Concepts
• 4. Code Duplication
• 5. Poor RDD
• 6. Ease of Modification
• 7. Implicit Coupling

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1. Why Class Design?

• This part concentrates on how to create well-designed classes - getting code to work isn't enough
• Benefits of good design:
  • simplifies debugging, modification, maintenance
  • increases the chances of reusing code

2. World of Zuul

• We look at two versions of a simple adventure game called "World of Zuul" (Zuul for short).
• Both versions have the same features, but the first is full of bad design choices, which I'll correct, leading to an improved second version.

Zuul Concepts

• Zuul is quite basic:
  • the user can move between a series of rooms, get help, and quit
• A real adventure game would allow multiple users, include hidden treasure, secret passwords, death traps, and more.

Zuul Map

pub outside theatre

lab office

rooms

exits/ doors

The user starts in the The exits are to the "outside" room. North, South, East, and West

Class Descriptions

• ZuulGame
  • creates the rooms, the parser, and starts the game. It evaluates and executes the commands that the parser returns.
• Parser
  • repeatedly reads a line from the terminal and interprets it as a two word command which it returns as a Command object continued Docsity.com

• CommandWords
  • holds an array of all the command words in the game, which is used to recognise commands - "go", "quit", "help"
• Room
  • represents a room in the game, connected to other rooms via exits. The exits are labelled "north", "east", "south", and "west".

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The First Adventure Game

• Colossal Cave Adventure (1976) was the first computer adventure game.
• It was designed by Will Crowther, a real-life cave explorer, who based the game's layout on part of an actual US cave system.

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• More info:
  • http://www.rickadams.org/adventure/
  • http://en.wikipedia.org/wiki/ Colossal_Cave_Adventure

3.1. Coupling

• Coupling refers to the links between separate units (classes) in an application.
• If two classes depend closely on many details of each other, they are tightly coupled. Usually bad.
• Good design aims for loose coupling. Good.

Loose Coupling

• In class diagrams, loose coupling means less association lines
• Loose coupling makes it possible to:
  • understand one class without reading others
  • change one class without affecting others
• Loose coupling make debugging, maintenance, and modification easier.

• High cohesion makes it easier to:
  • understand a class or method
  • use descriptive names
  • reuse classes or methods in other applications

3.3. Responsibility-driven Design (RDD)

• Each class should be responsible for manipulating and protecting its own data - e.g. don't use public fields
• RDD leads to low coupling, where code changes are localized - i.e. they only affect the class/method that is being modified