1. CSC 205 – Java Programming II
Lecture 1
Jan 9, 2002

2. Why OO? The Big Picture Software is inherently complex
Complexity of the problem domain
Difficulty of managing the development process
Flexibility possible through software
Problems of characterizing the behavior of discrete systems

3. Examples of Complex Systems
Mercer
A university … HR
Academics
Athletics
… …
CLA
Engr.
CSC
… …
Math
People
… …
Student
Employee …
Graduate …
UnderGrad
Admin.
Faculty

4. Attributes of Complex Systems
Hierarchy: a collection of interrelated subsystems.
Dividable until some lest level is reached
Arbitrary criterion for choosing subsystems
Cohesive and loosely coupled subsystems
One component is in charge of certain tasks
Intra-component communications are less frequent
Common patterns among subsystems
Evolved from existing simpler systems

5. Bring Order to Chaos The role of decomposition
The technique of mastering complexity has been
known since ancient time:
divide et impera (divide and rule) -- Dijkstra
OO v.s. procedural decomposition
procedural: top-down structure design with each component denote a major step in some overall process
OO: viewing the world as a set of autonomous agents that collaborate to perform some higher level behavior
Which is better?

6. OO vs Procedural -- Procedural

7. OO vs Procedural -- OO

8. Bring Order to Chaos – cont’d
The role of abstraction
An individual can comprehend only about seven (7) chunks of information at one time!
Ignore inessential details, deal with the generalized, idealized model of the object
The role of hierarchy
Object structure: illustrates how different objects collaborate with each other
Class structure: highlights common structure and behavior within a system

9. Elements of the Object Model
Abstraction: model the essential attributes
Encapsulation: hide implementation details
Modularity: (think package in Java)
Hierarchy: ranking/ordering of objects
Typing*: enforcement of the class
Concurrency*: distinguishes active object
Persistence*: transcends time and/or space

10. Abstraction Idealized structure model of objects
Recognize similarities
Ignore differences (for the time being)
For a particular purpose
Provide crisply defined conceptual bounders
Distinguish an object from other kinds of objects

11. Abstraction

12. Abstraction (2) Characterize the behavior of an object
by considering the services it provides to other objects (or its responsibilities)
(client-server) contract model
from the outside view
Works together with encapsulation
Expose what an object can do and how to request for services (which constitute the behavior of an object)
Hide implementation details

13. Encapsulation

14. Responsibility Responsibilities of an object include
State: information to store
Implemented with variables in Java
Operations:
Implemented with methods in Java
Preconditions: conditions assumed by operations
Postconditions: conditions satisfied by operations
Invoke operations by message passing

15. Message Passing Three components that comprise a message:
The object to which the message is addressed (YourBicycle)
The name of the method to perform (changeGears)
Any parameters needed by the method (lowerGear)

16. Modularity

17. Hierarchy Hierarchy is a ranking or ordering of abstractions
Two most important hierarchies
The “part-of” relationship, or aggregation
The “is-a” relationship , or inheritance
Inheritance, also known as subclassing
Single inheritance: the only legal way in Java
Multiple inheritance: supported in C++

18. Subclassing in Java A subclass SavingsAccount:
public class SavingsAccount extends Account {
private double interestRate;
public double getInterestRate() {
return interestRate; }
public void setInterestRate(double rate) {
interestRate = rate;

19. The Base Class: Account
public class Account {
// Instance variables
private double balance;
// Constructors
public Account(double initialBalance) {
balance = initialBalance; }
public Account() {
balance = 0.0;

20. Writing a Subclass A SavingsAccount object will contain two variables:
balance, and
interestRate

21. Writing a Subclass
Methods that can be applied to SavingsAccount objects:
getInterestRate
setInterestRate
deposit (inherited)
withdraw (inherited)
getBalance (inherited)
close (inherited)

22. Hierarchy: “part-of” relationship

23. Hierarchy: “is-a” relationship

24. Typing

25. Typing Supports Polymorphism
Shape
+draw()
Oval
Rectangle
+draw()
+draw() //
Shape s = ShapeBuilder.getNextShape();
s.draw();

26. Concurrency

27. Persistence