Hannah Whaley

1
Intro to OOP

• UK Developers Workshop 2006
• Hannah Whaley

2
What well cover

• What is OOP?
• Benefits of OOP
• OOP and BB
• Examples
• Main Principles
• Designing for OOP

3
What is Object Oriented Programming?

• Paradigm for programming, replacing procedural as
  industry standard
• Bridges the gap between concepts that humans know
  and concepts computers know
• Uses the idea of real world objects in
  programming

• OOP lets you model a collection of objects that
  communicate to achieve a complex task

4
Benefits of OOP

• Code is broken into small pieces
• Reusable
• Easily maintainable
• Only change something in one place
• Can find it and read it easier
• Reduces complex structures into simple ones
• More natural, readable structure
• It scales well
• Lots of libraries use other peoples pieces
• Collectively these small pieces become complex

5
Classes and objects

• Class
• Description of a group of items with
• common properties (variables)
• common behaviours (methods)
• Like a blueprint
• Object
• Instance of a class
• what it knows (variables)
• what it can do (methods)

6
Example

• Cat class
• Colour, breed, age, number of paws, tail
• Instance of a cat (specific state)
• Black, Bombay, 13 weeks
• Another instance of a cat (different state)
• Ginger, Tabby, 3 years

All have access to behaviours such as meowing,
scratching, hunting, eating etc.
7
Blackboard and OOP

• Blackboard is written in an object oriented style
• We can declare instances of classes that they
  have created and use them in our code
• Can get and set variables, and call the methods
  contained in the class

8
Blackboard Example

• Blackboard have created the User Class
• We can declare an instance of a User
• Can access get and set methods
• Can set the variables to be the current users
  details
• Can get the variables and print them to screen

User localUser ctx.getUser()
String localFirstname localUser.getGivenName()
out.println(Hi localFirstname) Hi Hannah
9
More Concepts of OOP

• Encapsulation
• Ensure all related code and data are kept
  together
• Not directly visible or accessible to outside
  world/other classes
• Access to data and code has to be explicitly
  allowed
• Like its in a capsule o)

10
More Concepts of OOP

• Inheritance
• Can derive new classes from existing ones
• Can extend existing program code
• Form class hierarchies
• Class Animal
• Class Cat
• Class Dog
• Both extend Animal Class, but will define
  additional states and behaviours
• A good example in Blackboard is the
  authentication, where it is not uncommon to write
  a Custom Authentication module that extends
  Blackboards original one

11
More Concepts of OOP

• Polymorphism
• A variable declared to be of a certain class can
  refer to objects of that class or any subclass
• A more abstract level of design
• Could design an algorithm to fill a list with
  objects and print each of their details
• List could contain different objects, so long as
  they all had a print method

12
Coupling and Cohesion

• Coupling
• How closely two classes depend on each other
• The more dependant, the more tightly coupled they
  are
• Low coupling is best!
• Cohesion
• The number and type of responsibilities a class
  has
• The more varied the responsibilities, the lower
  the cohesion
• High cohesion is best!

13
Designing for OOP

• If you are writing your own classes you want to
  design them well
• Remember encapsulation
• Remember coupling and cohesion
• There are ways to decide on what will become
  classes and what fields and methods they should
  have
• Take a requirements document and highlight all
  the nouns
• These make a list of candidate classes
• Accept or reject them
• Irrelevant
• Repeated
• Variable
• Write descriptions for the accepted classes
• Look for verbs for methods

14
Candidate Class Example

• Requirements for alarm system
• We plan to use an alarm system to end
  presentations on time
• The end time will be set on a clock displayed in
  minutes and hours
• The system will check the clock time and ring a
  bell when the clock gets to that time

15
Candidate Class Example

• Requirements for alarm system
• We plan to use an alarm system to end
  presentations on time
• The end time will be set on a clock displayed in
  minutes and hours
• The system will check the clock time and ring a
  bell when the clock gets to that time
• Possible classes
• Alarm Accept
• System Reject, irrelevant
• Time Reject, attribute of clock
• Clock Accept
• Minutes Reject, attribute of clock
• Hours Reject, attribute of clock
• System Reject, repeated
• Bell Reject, attribute of Alarm
• Clock Reject, repeated
• Time Reject, repeated

16
Candidate Class Example

• Alarm
• Bell
• RingBell()
• Clock
• Hours
• Minutes
• Time
• SetTime()
• GetTime()

Times UP!!!!!!!!
17
The End

• That has hopefully been either a quick intro or a
  quick refresher to OOP
• Well see code examples for BB that demonstrate
  this through the day
• Any questions?