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Question 5: Dependency Injection. Description: Allows the injection of objects into a class, rather than relying on the class to create the object itself

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question 5 dependency injection

Question 5: Dependency Injection

Description:

Allows the injection of objects into a class, rather than relying on the class to create the object itself

It is a pattern in which responsibility for object creation and object linking is removed from the objects themselves and transferred to a factory

The pattern seeks to establish a level of abstraction via a public interface, and to remove dependency on components by (for example) supplying a plug-in architecture

Martin Fowler described three main styles of dependency injection.

The Constructor Injection: A constructor is used to decide how to inject ‘A’ implementation into the ‘B’ class. For this to work, B class needs to declare a constructor that includes everything it needs injected.

The Setter Injection: define setter method that will accept the injection

The Interface Injection:by defining an interface that I'll use to perform the injection through

question 7 model view controller
Question 7: Model View controller
  • The problems:
  • User interface tends to change more frequently than business logic: Therefore if presentation code and logic codes are combined in a single object, one will have to modify an object containing business logic every time there is a change in the user interface.
  • Application displays the same data in different ways. If the user changes data in one view, the system must be able to update all other views of the data automatically
  • User interface code tend to be more device – dependent than business logic (migrating application from browser-based application to support PDAs or web enabled cell phone, the user interface codes will need much changes while the business logic might remain the same)
  • The MVC pattern separates the modelling of the domain, the presentation, and the actions based on user input into three separate classes (separate the business logic from the graphical user interfaces).
  • Participants
  • Model: The model has the components that use and rely on the underlying data.
  • View: The view houses the user interface components. It manages the display of information.
  • Controller: The controller contains the components that control which view is being displayed.
  • Implementation : use Observer pattern to keep the views and the associated model synchronized.
question 12 extract super class

Student

getName() : string

getId() : int

getDept() : string

listcourses(): void

Institution

getName() : string

getId() : int

getDept() : string

Employee

getName() : string

getId() : int

getDept() : string

computeSalary():double

Student

listcourses(): void

Employee

computeSalary():double

Question 12: Extract Super Class

Motivation:

  • Duplicate code is a bad ways of implementing system
  • There are two classes with similar features
  • Each class does similar implementation in the same way or in different way
  • This promotes code repetition and duplication.
  • The choice of solving the problem is extract class. The choice is between inheritance and delegation. It the two class shares interface as well as behaviour then inheritance is best suited.

Sample:

The classes student and employee in IT University consist of similar methods

  • Mechanism:
  • Create blank superclass, and make the original classes to be subclasses to the superclass
  • Move the common feature (apply pull up method), move common fields, and constructor
  • bodies to move common elements to the superclass
  • Compile and test after each pull
  • Check each client of the superclass.
  • If they only share the same interface then change the required type to superclass.
  • If subclass inherits from other class, then use delegate in any language that does not support multiple inheritance (e.g C#, java)
question 16 code generation
Question 16: Code Generation
  • Problem on merging hand written code and generate code:
  • It is complicated to maintain. That is, it is hard to trace hand written code in generated code, which thence leads to complexity and difficulty in maintaining.

To address the problems:

  • Use comments or regions to separate hand written from generated code
  • Put hand written code in subclass (relies on naming convention, NOT transparent)
  • Merging (by treating generated code as coming from A and hand written coming from B.
  • Then use the CVS to merge the two pieces together.
  • Using partial classes (program dependent only C#) by spanning the classes into several files
  • Use of CodeDom versus simple source text generation using visitor
  • The text based code generation:
  • Language dependent i.e it can only generate code for one specific language
  • Most cases generate syntax error (e.g misplacement of “;”, )
  • It is hard to see what is produced
  • CodeDOM
  • Language independent i.e. it can generate code for different languages.
  • Syntax error free: the source is represented by an abstract syntax tree