NJIT

More GRASP Patterns

Chapter 22

Applying UML and Patterns

Craig Larman

Prepared By: Krishnendu Banerjee

Objectives

Learn to apply the following GRASP patterns: Polymorphism Pure Fabrication Indirection Protected Variations

Introduction

GRASP stands for General Responsibility Assignment Software Patterns. It is a learning aid of fundamental principles

by which responsibilities are assigned to objects and objects are designed.

We already learned about the first five GRASP patterns: Information Expert, Creator, High Cohesion,

Low Coupling, Controller

Introduction

In this presentation we will look at four new GRASP patterns: Polymorphism Indirection Pure Fabrication Protected Variations

Polymorphism

Problem Who is responsible when behaviors vary by

type? How to handle alternatives based on type? How to create pluggable software

components?

Polymorphism

Solution Polymorphism is a fundamental principle in

designing how a system is organized to handle similar variations.

When related alternatives or behaviors vary by type (class), assign responsibility for the behavior using polymorphic operations to the types for which the behavior varies.

Polymorphism

UML Notation for Interfaces and Return types

Polymorphism: Example

NextGen POS application supporting multiple Tax Calculators is a polymorphic operation.

Polymorphism: Example

Polymorphism: Contraindications

Developers may not critically evaluate true likelihood of variability before investing in increased flexibility.

Developers may indulge in speculative “future-proofing” against unknown possible variations.

Polymorphism: Benefits

Extensions required for new variations are easy to add.

New implementation can be introduced without affecting clients.

Pure Fabrication

Problem What object should have the responsibility,

when you do not want to violate High Cohesion and Low Coupling, or other goals, but solutions offered by Information Expert are not appropriate?

Pure Fabrication

Solution Assign a highly cohesive set of responsibilties

to an artificial class that does not represent a problem domain concept - something made up, in order to support high cohesion and low coupling and reuse.

Pure Fabrication: Example

Suppose there is a requirement to save the Sale instances in a relational database.

Per Information Expert, this responsibility would be assigned to the Sale class itself, because the sale has the data that needs to be saved.

Pure Fabrication: Example

But the above solution leads to: Low Cohesion: Including tasks not related to

the concept of sale-ness. High Coupling: The Sale class is coupled to

the relational database interface. Low Reusability: The general task of saving to

the database may be needed by other classes. Assigning it to Sale class causes poor reuse and lots of duplication.

Pure Fabrication: Example

A reasonable solution is to create a new class that is solely responsible for saving objects. This class is a Pure Fabrication.

Pure Fabrication: Contraindications

Behavioral decomposition into pure fabrication to object design and more familiar with decomposing or organizing objects is sometimes over used by those new software in terms of functions.

Pure Fabrication: Benefits

High Cohesion is supported because that only focuses on a very specific responsibility are factored into a fine grained class set of related tasks.

Reuse potential may be increased because of the presence of fine grained pure fabrication classes.

Indirection

Problem Where to assign a responsilibity, to avoid direct

coupling between two or more things? How to de-couple objects so that low coupling is

supported and reuse potential remains higher? Solution

Assign the responsibilty to an intermediate object to mediate between other components so that they are not directly coupled.

Indirection: Example

Tax Calculator Adapter These objects act as intermediaries to the

external tax calculators. By adding a level of indirection and adding polymorphism, the adapter objects protect the inner design against variations in the external interfaces.

Indirection: Example

Indirection: Benefits

Low Coupling between components.

Protected Variations

Problem How to design objects, subsystems and

systems so that the variations or instability in these elements does not have an undesirable impact on other elements?

Solution Identify points of predicted variation or

instability; assign responsibilities to create a stable interface around them.

Protected Variations

PV is the root principle motivating most of the mechanism and patterns in programming and design to provide flexibility and protection from variation.

It is essentially the same as David Parnas’s information hiding and Bertrand Meyer’s Open-Close Principle.

Protected Variations: Mechanisms

Core Protected Variation Data encapsulation, interfaces,

polymorphism, indirection.

Data Driven Design Reading codes, class file paths, class name.

Service Lookup JNDI, LDAP, Java’s Jini, UDDI.

Protected Variations: Mechanisms

Interpreter Driven Design Rule interpreters, virtual machines, neural

network engines, logic engines. Reflective of Meta-level Design

Java introspector Uniform Access

Language supported constructs that do not change with change in underlying implementation.

Protected Variations: Mechanisms

The Liskov Substitution Principle (LSP) Software methods that work with a class T

should work with all subclasses of T.

Structure-Hiding Design Places constraints on what object you should

send messages to within a method.

Protected Variations: Example

In the external tax calculator problem, the point of instability is the different interfaces to the external tax calculators.

By adding a level of indirection, an interface, and using polymorphism with various adaptors, protection within the system from variations in external calculator APIs is achieved.

Protected Variations: Contraindications

The cost of speculative “future-proofing” at evolution point may outweigh the cost incurred by a simple design that is reworked as necessary. Evolution point is a speculative point of variation that may arise in future, but not specified in current requirement.

Protected Variations: Benefits

Extensions required for new variations are easy to add.

New implementations can be introduced without affecting clients.

Coupling is lowered. The impact or cost of changes be lowered.