Object Oriented Programming Language

• Encapsulation (via the notion of class)
• Inheritence: method for extending existing classes by adding methods and fields
• Polymorphism: Behaviour varies depending on the actual type of an object

References

• A Reference holds the address of an instance
• A reference is NOT the instance
• Speed Dial analogy (no Inheritence)
• The VALUE of a reference is the address it holds
• a=b calculated by computing the VALUE of the right side and placing it in the left
• Java and Python are pass by VALUE
• Java and Python occasionally garbage collect instances that have no references to them.

Using Objects

• c=Circle() does the following
  • Create a new instance of circle
  • Call the initialization code __init__ for a new Circle (constructor)
  • Create a new reference called c
  • Assign the address of the newly created/initialized Circle to c

Our goals...

• Problem solve: Understand a given problem (there is a LOT to this!!) Determine if a solution takes the form of a computer program (and if so)...
• Create Conceptual Solution: Describe the system to be modelled. That is, give an english description of the system
• Concepts mapped to OO Paradigm:
  • Think OO:Analyze the problem/solution, view it as a collection of interacting objects, then identify the
    • classes/objects. These are the concepts that need to be captured. Sometimes noun or noun phrases in the above description.
    • methods (RESPONDS-TO): for each class. These are the things that the class can do. Verbs usually identify these in the problem description.
    • attributes (HAS-A): for each class. These are things that each instance has, they define the state of the instance.
    • inheritence (IS-A): classes that this class extends
  • Think Algorithmically:Write algorithms for each of the methods
• Python/Java : Translate the above to something the computer can understand (Python/Java etc.).

Exercise: Balloon

Exercise: JugPuzzle

• Problem Statement: There are three water jugs, which can hold 8, 5 and 3 gallons of water respectively. The first jug is full, and the other two are empty. It is necessary to divide the water into two equal portions, that is, to put exactly four gallons into both the 8 and 5 gallon jugs. The only operation allowed is to pour some or all of the contents of one jug into another.

  Can the water be divided into two equal portions? How?

• For the Jug class, identify
  • IS-A: inheritance (extends)
  • HAS-A: attributes
  • RESPONDS-TO: methods (including toString(), the method that allows a user to fetch a String representation of an instance).
  • constructors: special initialization code used when creating a new Jug()

Introduction to UML

• In the above diagram... each rectangle is a class. The class has 3 sections, from top to bottom... class name variables (captures HAS-A) methods (captures RESPONDS-TO) Class variables are underlined. Instance variables are not. Public and private is also shown (private with a lock). Similarly for methods. There are two tyes of arrows shown. The arrow between Pizza and Shape captures HAS-A. In this case, we need a single instance variable to implement this arrow. The arrows between Circle and Shape AND between Square and Shape captures IS-A (inheritance).
• Exercise Write the solution to the Jug problem using UML
• Mapping UML to Python

Exercises

• Translate your conceptual Jug solution to Python
• to test your class. Create a few Jugs, and spill water between them.
• Create class JugPuzzle (Conceptual then UML then Python). Test the JugPuzzle class.
• For a solution, see the week 01 lab.

Inheritence and Polymorphism

• Inheritence (IS-A): method for extending existing classes by adding methods and fields
• Subclass and superclass, what it means Subclass inherits all methods and attributes of superclass. (ie Shapes UML Diagram)
• object: is the superclass of all classes.
• extending classes in python to define subclass/superclass relation between two classes ... class Z: # same as class Z(object) ''' methods go here ''' pass class A(object): # A is a subclass of object ''' methods go here ''' pass class B(A): # B is a subclass of A ''' methods go here ''' pass class C(A): # C is a subclass of A ''' methods go here ''' pass class D(B): # D is a subclass of B ''' methods go here ''' pass What does it mean? class D(B) == D 'IS-A' B which means... D 'inherits' all 'HAS-A' from B Each instance of class D has all instance variables that class B has. D 'inherits' all 'RESPONDS-TO' from B An instance of class D has all of the methods that an instance of class B does. When you call a method on an instance of class D, the search goes from D methods to B methods to A methods to object methods.
• Method overriding: Method in subclass has same name as method in superclass. In Java and Python, method search proceedes from class of instance back towards object.
• Polymorphism: behaviour varies depending on the actual type of an object.
• Speed Dial example (extended)

Examples

• Balloon.py