GRASP

GRASP GRASP: an acronym that stands for General Responsibility Assignment Software Patterns I think a RAP is a better acronym. Do the words general & software pull their weight? Referring to the 9 heuristics as “patterns” is a stretch.

The 9 GRASP Information Expert Creator Information expert: the class that has the information necessary to fulfill the responsibility. Assign a responsibility to the information expert. Creator Who creates instances of class A? Class B when: B has A[s] B is an information expert for A (has the constructor arguments).

The 9 GRASP … Controller Low coupling High cohesion Who handles a system event? The system’s façade [controller] class. Low coupling Minimize inter-class coupling High cohesion Maximize intra-class cohesion

The 9 GRASP: Polymorphism To vary a general behavior by class, extend a base class. Example: Command pattern. The Lectures page links to the java files below These classes illustrate bad as well as good style. Cell.java CellImpl.java ActionItem.java ExpressItem.java APTSpawn.java

The 9 GRASP: Pure Fabrication Assign a highly cohesive set of responsibilities to a class that is: Not in the application domain Created to support OOD: High cohesion Low coupling Examples Façade, adaptor, & bridge patterns are pure fabrications that lower coupling. PersistentStorage class (book’s example).

The 9 GRASP: Indirection Mediate: Acting through an intervening agent. To interpose between parties, esp. for the purpose of effecting an agreement; as, to mediate between nations. A client/service mediator class. Lowers coupling Examples: Façade, Adaptor, Bridge (Sound familiar?  ) Client Mediator Service

The 9 GRASP: Protected Variations Define a stable interface to hide an area of instability. Yes: List list = new ArrayList(); // java.util.List not java.awt.List No: ArrayList list = new ArrayList(); for ( Iterator i = c.iterator(); i.hasNext(); ) { doSomething( i.next() ); } Java List, Map interfaces. Create protected variations only around an area of instability.