CSC 205 – Java Programming II

Lecture 1Jan 9, 2002

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

Examples of Complex Systems

• A universityAcademics

CLA Engr.… …

CSC Math… … People

Student Employee… …

Graduate UnderGrad Admin. Faculty……

Mercer

HR… Athletics

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

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?

OO vs Procedural -- Procedural

OO vs Procedural -- OO

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

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

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

Abstraction

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

Encapsulation

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

Message Passing

• Three components that comprise a message: 1. The object to which the message is addressed (YourBicycle) 2. The name of the method to perform (changeGears) 3. Any parameters needed by the method (lowerGear)

Modularity

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++

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; }}

The Base Class: Account

public class Account { // Instance variables private double balance;

// Constructors public Account(double initialBalance) { balance = initialBalance; }

public Account() { balance = 0.0; }}

Writing a Subclass

• A SavingsAccount object will contain two variables:• balance, and • interestRate

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

Hierarchy: “part-of” relationship

Hierarchy: “is-a” relationship

Typing

Typing Supports Polymorphism

//Shape s = ShapeBuilder.getNextShape();s.draw();//

Shape

+draw()

Oval

+draw()

Rectangle

+draw()

Concurrency

Persistence