More GRASP Patterns

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More GRASP Patterns

• Chapter 22
• Applying UML and Patterns
• Craig Larman

Prepared By Krishnendu Banerjee
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Objectives

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

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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

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Introduction

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

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Polymorphism

• Problem
• Who is responsible when behaviors vary by type?
• How to handle alternatives based on type?
• How to create pluggable software components?

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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.

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Polymorphism

• UML Notation for Interfaces and Return types

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Polymorphism Example

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

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Polymorphism Example
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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.

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Polymorphism Benefits

• Extensions required for new variations are easy
  to add.
• New implementation can be introduced without
  affecting clients.

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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?

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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Indirection Example
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Indirection Benefits

• Low Coupling between components.

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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.

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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 Parnass
  information hiding and Bertrand Meyers
  Open-Close Principle.

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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, Javas Jini, UDDI.

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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.

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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.

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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.

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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.

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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.