Computing Insights for Teachers Summer 2006

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 is pass by VALUE
• Java occasionally garbage collects instances that have no references to them.

Using Objects

• Circle c=new Circle(); does the following
  • Create a new instance of circle
  • Call the initialization code for a new Circle (constructor)
  • Create a new reference to Circle called c
  • Assign the address of the newly created/initialized circle to c
• Exercise: Using the Shapes code below (and drJava) create a face (circles for eyes, square for nose, rectangle for mouth).
  • Remember to import java.awt.*; so you can use Color.red etc.
  • Shapes Doc
  • Shapes.zip
• Example: Create a few references to circle, change which reference refers to which circle and call methods on them. Explain.

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
• Java: Translate the above to something the computer can understand (Java).

Jug Exercise

• 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

• Shapes UML Diagram
• Exercise Write the solution to the Jug problem using UML
• Mapping UML to Java

Exercises

• Translate your conceptual Jug solution to Java (create Jug.java)
• Use the Jug public static void main(String [] args) to test your class. Create a few Jugs, and spill water between them.
• Create class JugPuzzle (Conceptual then UML then Java). Use the JugPuzzle main to test the class.
• Create class PlayJugPuzzle which allows a user at the console to play an instance of JugPuzzle.
• PlayJugPuzzle.java

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.
• extends: used to define subclass/superclass relation between two classes.
• Method overriding: Method in subclass has same name as method in superclass. In Java, 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)

Exercises

• Create a Shapes reference and use it to point to different Shapes and move them.
• Create a method which swaps any pair of shapes. SwapPosition.java
• Create an array of different Shapes and move them all together.
• Extend Circle so that each Circle knows its area and perimeter.

Design Patterns

• Design Patterns (Composite and Visitor)

Interfaces: Like RCA plugs at the back of your sound system

• Make Circles Comparable (see the Java api) by their area
• Sort an array of Circles (see Arrays.sort(Object []))

Exercise:

• How would you modify the original Shapes Hierarchy so that all Shapes have an area, perimeter and are Comparable by area?
• Carry out the above suggestion, then create an array of Shapes and sort them.