Short note: GRASP

GRASP stands for General Responsibility Assignment Software Patterns (Principles), described by Craig Larman. They are a set of guidelines for deciding which class should be responsible for what in an object-oriented design. Responsibilities are of two kinds: doing (compute, create, control others) and knowing (private data, related objects).

The nine GRASP patterns

1. Information Expert - give the responsibility to the class that holds the needed information.
2. Creator - class B creates A if B contains, aggregates, records or closely uses A.
3. Controller - first object beyond the UI that receives and coordinates a system event (a use case or session controller).
4. Low Coupling - keep dependency between classes low.
5. High Cohesion - keep the responsibilities of a class strongly related and focused.
6. Polymorphism - use polymorphic operations for behaviour that varies by type, not conditionals.
7. Pure Fabrication - create an artificial class (not from the domain) to achieve low coupling and high cohesion, e.g. a database storage class.
8. Indirection - add an intermediate object to decouple two classes.
9. Protected Variations - protect elements from the variations in other elements using stable interfaces.

Example: in a POS, Sale.getTotal() follows Information Expert, Sale creates SalesLineItem (Creator), and ProcessSaleController handles enterItem() (Controller).

GRASP gives the reasoning behind good object design and is applied while drawing interaction diagrams.