4.2 Design Objects with Patterns

Object design is sometimes described as some variation of the following:

After identifying your requirements and creating a domain model, then add methods to the software classes, and define the messaging between the objects to fulfill the requirements.

Experienced object-oriented developers (and other software developers) build up a repertoire of both general principles and idiomatic solutions that guide them in the creation of software. These principles and idioms, if codified in a structured format describing the problem and solution, and given a name, may be called patterns.

For example, here is a sample pattern:

Pattern Name: Information Expert

Solution: Assign a responsibility to the class that has the information needed to fulfill it.

Problem It Solves: What is a basic principle by which to assign responsibilities to objects?

Most simply, a pattern is a named problem/solution pair that can be applied in new context, with advice on how to apply it in novel situations and discussion of its trade-offs.

4.2.1    GRASP

General Responsibility Assignment Software Patterns (or Principles), abbreviated GRASP, consists of guidelines for assigning responsibility to classes and objects in object-oriented design.

·         guides in assigning responsibilities to collaborating objects.

The different patterns and principles used in GRASP are:

1.      Creator

2.      Information expert

3.      Low Coupling

4.      Controller

5.      High Cohesion

Responsibility of GRASP:

Ø  ●Responsibility can be:–accomplished by a single object.–or a group of object collaboratively accomplish a responsibility.

Ø  ●GRASP helps us in deciding which responsibility should be assigned to which object/class.

Ø  ●Identify the objects and responsibilities from the problem domain, and also identify how objects interact with each other.

Ø  ●Define blue print for those objects – i.e. class with methods implementing those responsibilities.

1.         Creator:

●Who creates an Object? Or who should create a new instance of some class?

●“Container” object creates “contained” objects.

●Decide who can be creator based on the objects association and their interaction.

Example for Creator

●Consider VideoStore and Video in that store.

●VideoStore has an aggregation association with Video. I.e, VideoStore is the container and the Video is the contained object.

●So, we can instantiate video object in VideoStore class

2.   Information Expert:

●Given an object o, which responsibilities can be assigned to o?

●Expert principle says – assign those responsibilities to o for which o has the information to fulfill that responsibility.

●They have all the information needed to perform operations, or in some cases they collaborate with others to fulfill their responsibilities.

Example for Expert

●Assume we need to get all the videos of a VideoStore.

●Since VideoStore knows about all the videos, we can assign this responsibility of giving all the videos can be assigned to VideoStore class.

●VideoStore is the information expert.

3.   Low Coupling

●How strongly the objects are connected to each other?

●Coupling – object depending on other object.

●When depended upon element changes, it affects the dependant also.

●Low Coupling – How can we reduce the impact of change in depended upon elements on dependant elements.

●Prefer low coupling – assign responsibilities so that coupling remain low.

●Minimizes the dependency hence making system maintainable, efficient and code reusable

Two elements are coupled, if

–One element has aggregation/composition association with another element.

–One element implements/extends other element.

here class Rent knows about both VideoStore and Video objects. Rent is depending on both the classes.

Example for low coupling

●VideoStore and Video class are coupled, and Rent is coupled with VideoStore. Thus providing low coupling.

4.         Controller

●Deals with how to delegate the request from the UI layer objects to domain layer objects.

●when a request comes from UI layer object, Controller pattern helps us in determining what is that first object that receive the message from the UI layer objects.

●This object is called controller object which receives request from UI layer object and then controls/coordinates with other object of the domain layer to fulfill the request.

●It delegates the work to other class and coordinates the overall activity.

●We can make an object as Controller, if

–Object represents the overall system (facade controller)

–Object represent a use case, handling a sequence of operations (session controller).

Benefits

–can reuse this controller class.

–Can use to maintain the state of the use case.

–Can control the sequence of the activities

Bloated Controllers

●Controller class is called bloated, if

–The class is overloaded with too many responsibilities.

Solution – Add more controllers

–Controller class also performing many tasks instead of delegating to other class.

Solution – controller class has to delegate things to others

4.       High Cohesion

●How are the operations of any element are functionally related?

●Related responsibilities in to one manageable unit.

●Prefer high cohesion

●Clearly defines the purpose of the element

Benefits

–Easily understandable and maintainable.

–Code reuse

–Low coupling