To Lecture Notes

IT 313 -- Mar 2, 2020

Review Exercise

1. What is a software design pattern?
   Ans: It is is a general, reusable solution to a commonly occurring problem in software design.
2. What are the Factory and Singleton patterns?
   Ans: Instead of using a constructor, the Factory pattern uses a static method, traditionally called getInstance, that returns an object.
3. When do you want to use the Factory pattern?
   Ans: The Factory pattern is used in two situations: (a) when the getInstance method might return objects from an inheritance hierarchy of classes, (b) when objects are reused; when an object is needed, the getInstance method first checks if an object is available to reuse.  If not, a new object is created using a private constructor.
4. Why is the Singleton pattern controversial?
   Ans: Because only one instance of the class is created, every user wanting to use that object is given a reference to it. This makes the object global; global objects are notoriously difficult to test because it is hard to know who is using the object.
5. What is the difference between lazy and eager instantiation?
   Ans: Lazy instantiation means wait until the object is used to instantiate it; eager instantiation means that the object is created immediately when the software using it starts up.

Observer Pattern

• The Observer Pattern is facilitated by these two items: the Observable class and the Observer interface.
• The Observable class and the Observer interface are now depreciated in the Java Class Library. This is because they are not considered to be "industrial strength."
• See the messageboard1 module in the Observer Example.
• The MessageBoard object is the observable.
• Here is a UML diagram showing the relationship between the MessageBoard and Student classes with the Observable class and the Observer interface.
• The Student objects are the observers.
• The MessageBoard object
  
  1. must extend the Observable class to qualify as an observable.
  2. contains a posted message that may be useful to the observers.
  3. may add one or more observers (Student objects) with the addObserver method.
  4. may delete observers with the deleteObservers method.
  5. calls the setChange method to show that the message has been changed.
  6. calls the notifyObservers method whenever the message is changed to notify all the observers. Calling notifyObservers automatically calls its clearChange after all observers have been notified.
  7. the notifyObservers method automatically calls the update method of each registered observer.
• A Student object
  
  1. must implement the Observer interface to qualify as observer.
  2. supplies the required update method (which will be automatically called by notifyObservers in Item 7 of the MessageBoard object).
  3. in addition, each observer can go back to the observable to obtain additional information.
• Look at the messageBoard2 module in the Observer Example.
• Look at the casino module in the Observer Example.
• The casino module has a SlotMachine simulator with three reels. Each reel contains these 20 symbols:
  

  7  $  ?  +  apple  bar  bell  cash
  cent  cherries  diamond  grapes  
  heart  horseshoe  lemon  orange
  plum  pound  watermelon  win
  

• Here is a stock photo that shows some of these symbols:
  
• When a slot machine is played, each of the three reals shows one symbol. A jackpot is obtained if three 7s are obtained.
• The probability that a jackpot occurs in one play is 1/8000.
• The probability that a jackpot occurs at least once in 10,000 tries is:
       1 - (1 - 1/8000)^10,000 = 0.714.
• In the casino example, a slot machine is an observable.
• The casino manager is an observer that records, for each jackpot, the ID number of the machine and the time at which the jackpot occurred.

Project 7

• Look at the Project 7 Description

State Pattern

• The state pattern represents each state in a state diagram as a separate class.
• The State Example is based on this state diagram for a gumball machine. (This state diagram was drawn with Violet.)
• See the state module in the Patterns2 Example.

Strategy Pattern

• The Strategy pattern encapsulates the strategy of an algorithm in an object. The programmer can use this Strategy object without understanding the details of the algorithm.
• See the sorting1 and sorting2 modules in the Patterns2 Example.