Design Pattern (1)

Hierachy of Pattern Kenowledge

• Design Pattern
  - Strategy pattern : defines a family of algorithms, encapsulates each one, and makes them interchangeeable. Strategy lets the algorithm vary independetly from clients using it.
• OP Principles
  - Enacapsulate what varies
  - Favor composition over inheirtance
  - Program to interface, not implementations
  
• OO Basics
  - Abstraction, Encapsulation, Polymorphism, Inheritance

 

Abstraction

• Hiding details
  - for the purpose of ismplifying and managing the design of complex software.
• Allows to separate categories and concepts related to problems frm specifi c instances of implementation

 

Information hiding and Encapsulation

• Design principles for hiding internal design decisions related to the selected algorithm and data structures of a module from the outside world.
  - the internal design decisions are most likely to change
  - Thus, reduce the side effects of any future maintenance or modification of the design and hence minimizing the effect on the other modules in the design.
• Information hiding : Encapsulation = Principle : Technique

  

  Encapsulation in Template Method Pattern

 

 

Separation of Concerns (SoC)

• Design principle for separating a computer program into distinct sections, such that each esction addresses a separate concern.
• A concern is a set of information that affects the code of a computer program.
• Can be traced back to "divide and conquer"
• A program that embodies SoC well is called modular.
• Modularity, and hence separation of concerns, is achived by encapsulating information inside a section of code that has a well-defined interface.

 

Interfaces

• Interfaces are the points of accesses through which modules or systems communicate.
• Each abstraction should possess a well defined interface clearly describing the expected inputs to and outputs from the abstraction.
  
• If the objects are fully encapsulated then the interface will describe the only way in which obejcts may be accessed by other objects.

 

Modularity

• Goal of design
  - partition the system into moudles and assign reponsibility among the components.
• Modularity reduces the toal complexity a programmer has to deal with at any one time.

 

 

Measuring Modularity

• Coupling is a measure of the dependencies between modules.
• Cohesion is ameasure of how strongly the elements in a module are  related.
• Strong  cohesion will reduce relations between modules.