1. COP 3331 Object Oriented Analysis and Design Chapter 7 – Design by Abastraction Jean Muhammad

2. Slides by David Gaitros Overview Introduction to Patterns Generic Components Abstract Coupling

3. Slides by David Gaitros Introduction to Patterns Design reusable components Template method Strategy Factory Iterator

4. Slides by David Gaitros Introduction to Patterns Design pattern was originally articulated by Christopher Alexander and his colleagues to describe architectural designs. The architectural design of any building can be captured in one of 253 “patterns”. Each pattern describes a problem that occurs over and over gain in an environment, and then describes the core solution.

5. Slides by David Gaitros Introduction to Patterns Software design patterns are classified into three categories: – Creational Patterns – Structural Patterns – Behavioral Patterns

6. Slides by David Gaitros Introduction to Patterns Describing design patterns Pattern Name: Category: creational, structural, or behavioral Intent: Also Known As: Applicability: Structure: Participants:

7. Slides by David Gaitros Generic Components Refactoring: Refactoring is the process of identifying recurring code and organizing/modifying the code such that it can be used more then once.

8. Slides by David Gaitros Generic Components Refactoring consists of: – Identifying code segments in a program that implement the same logic. – Capturing this logic in a generic component that is defined once. – Restructuring the program so that every occurence of the code segment is replaced with a reference to the generic component.

9. Slides by David Gaitros Generic Components Refactoring by Inheritance class Common{ void computeAll (... ){ computeStep1(); computeStep2(); computeStep3(); } class ComputationA class ComputationB extends Common{ extends Common { void method1(...){ void method2(...) { computeAll() computeAll()

10. Slides by David Gaitros Generic Components Refactoring by Delegation To refactor the recurring code sequence in the previous example using delegation, we introduce a helper class instead of inheritance.

11. Slides by David Gaitros Generic Components class Helper{ void computeAll (... ){ computeStep1(); computeStep2(); computeStep3(); } class ComputationA class ComputationB void compute(...){ void compute(...) { helper.computeAll() helper.computeAll() } } Helper helper; Helper helper;

12. Slides by David Gaitros Template Method Use of an abstract class that serves as a template for classes with shared functionality. – An abstract class contains behavior common to all of its subclasses. – The common behavior is captured in non-abstract methods. – Using abstract classes ensures that all subclasses will inherit the same behavior.

13. Slides by David Gaitros Strategy Pattern The strategy pattern can be considered as a variation of the Template method – The hood method and template method reside in different classes. – The abstract methods declared in the Strategy class are the hood methods. – The methods in the Context class that call the hook methods are the template methods.

14. Slides by David Gaitros Strategy Pattern Strategy Pattern should be used when: – Many related classes differ only in their behavior – Different variations of an algorithm are needed. – An algorithm uses data that clients should not know about. – A class defines many behaviors, which appear as multiple conditional statements in a method.

15. Slides by David Gaitros Abstract Coupling The strategy pattern is an example of abstract coupling. A client accesss a service through an interface or an abstract class without knowing the actual concrete class that provided the service.

16. Slides by David Gaitros Abstract Coupling Customer Service Provider A Service Provider B Service Provider C Example of Direct Coupling – No Abstraction

17. Slides by David Gaitros Abstract Coupling Service Provider A Service Provider B Service Provider C Customer Standard Service Protocol Abstract Coupling – Any customer calls the Standard Service Protocol and the correct interface is automatically used.