Advanced Java at the ACSE Summer Institute 2007

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).

Introduction to UML

• Shapes UML Diagram
• Mapping UML to 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.