Q1. Give an example of how design problems help you choose the suitable design pattern?

Transcription

1 Tutorial #3

2 Part #1

3 Q1. Give an example of how design problems help you choose the suitable design pattern?

4 To use patterns in your design, you need to recognize that any design problem you are facing may have an associated pattern that can be applied. For example: Tell several objects that the state of some other object has changed (Observer pattern). Tidy up the interfaces to a number of related objects that have often been developed incrementally (Façade pattern). Provide a standard way of accessing the elements in a collection, irrespective of how that collection is implemented (Iterator pattern). Allow for the possibility of extending the functionality of an existing class at run-time (Decorator pattern).

5 Q2. Explain the reuse levels using examples.

6 1- The abstraction level At this level, you don t reuse software directly but use knowledge of successful abstractions in the design of your software. Design pattern, Architectural pattern 2- The object level At this level, you directly reuse objects from a library rather than writing the code yourself. Chart library in VB 3- Component level Components are collections of objects and object classes that you reuse in application systems. Chart Editor, Text Editor, some Widgets 4- The system level At this level, you reuse entire application systems. product lines-cots

7 Q3. Give examples about factors that affect the reuse cost?

8 Search time: searching for a reusable component may take long time and assessing whether or not it meets your needs. The costs of buying the reusable software: For large off-the-shelf systems, these costs can be very high. The costs of adapting and configuring the reusable software components or systems to reflect the requirements of the system that you are developing. The costs of integrating reusable software elements with each other (if you are using software from different sources) and with the new code that you have developed.

9 Q4. Give an example about an open source application that you use every day. What is its license? Explain this license.

10 The GNU General Public License (GPL). This is a so-called reciprocal license that means that if you use open source software that is licensed under the GPL license, then you must make that software open source.

11 Part #2

12 Q1-What is the best design pattern for the following:

13 A-Stocks monitoring application that tracks many stock prices throughout the day notifying users with updated prices and stock market status. Answer: Observer pattern Within this application we have a Stock class that models various stocks that are traded. This class contains an instance variable which represents the current ask price, which fluctuates throughout the day. In order to display this information to the user, the application uses a StockDisplay class that writes to stdout (standard output). Within this application, an instance of the Stock class acts as the subject and an instance of the StockDisplay class as the observer.

14 Stock StockDisplay

15 B-Newspaper publisher website, which publishes a daily newspaper and a monthly magazine. Customers will have the ability to subscribe to the newspaper and magazine, and notifications of new released publications with updated information will be sent to the subscribers. Answer: Observer pattern The paper object needs to have a property subscribers that is an array storing all subscribers. The act of subscription is merely adding to this array. When an event occurs, paper loops through the list of subscribers and notifies them. The notification means calling a method of the subscriber object. Therefore, when subscribing, the subscriber provides one of its methods to paper s subscribe() method.

16 C-Dresses online store, receives orders from customer and produce an invoice for each payment. Answer: Facade pattern The client has placed an order without having knowledge of how internal classes are functioning. Once the order is placed the façade class layer calls the methods of the subsystems like Inventory for inventory check and Payment for processing of the payment. After processing it returns the control to the client class with the confirmation about the order being

17 D-Order system designed for Coffee shop that offers different kinds of beverages. The customer can ask for several condiments like steamed milk, soy, and Chocolate, and have it all topped off with whipped milk. The Coffee shop has to provide prices for each combination of beverages and condiments.

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19 E. On modern television sets, next and previous buttons are used to change channels. When the viewer selects the next button, the next tuned channel will be displayed. When surfing through channels, the channel number is not important, but the programming is. If the programming on one channel is not of interest, the viewer can request the next channel, without knowing its number. If you need to implement this case. Which design pattern you are going to use?

20 The Iterator design pattern provides ways to access elements of an aggregate object sequentially without exposing the underlying structure of the object. And in this case, user can access any channel in the list without affecting the structure of the object.