Designing Classes October 11, 2007 1 Design Principles High cohesion - everything in a class is related Low coupling - a class has limited dependencies on other classes Abstraction - a class can be used easily without knowing how it is implemented Encapsulation - a class hides design decisions, making them easy to change 2 Thursday, October 11, 2007
Identifying Classes Examine problem domain. Model real world objects with classes: Physical things: dice, thermometer Organizations: team, college People: player, student, customer 3 Identifying Classes (2) Examine solution domain Agents: objects specialized in a particular activity, like StringTokenizer, BufferedReader Events: mouse actions, window actions External software/hardware: files, sensors, databases 4 Thursday, October 11, 2007
Unified Modeling Language Collection of graphical notations used to document designs Class diagrams - show classes and static connections between them Sequence diagrams - show dynamic behavior of objects carrying out an action State diagrams - show “lifecycle” of an object 5 Class Diagrams Class 6 Thursday, October 11, 2007
UML Class Icon Name Attributes Methods 7 This Sounds Silly But... Start identifying classes by carefully reading an English description of the system Nouns -> classes, or maybe attributes of classes Verbs -> methods 8 Thursday, October 11, 2007
Questions When is a noun a class, when is it an attribute and when is it neither? If a verb is a method, which class does it belong to? 9 Identify Relationships between Classes Association: “has” Example: a car has an engine Will turn into an instance variable Dependency: “uses” Example: an event handler depends on the event Often parameters to methods 10 Inheritance / implements Thursday, October 11, 2007
Class Diagrams Class Dependence 0 or more Association Inheritance 11 Thursday, October 11, 2007
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