compsci230s2c 2012 software design and · compsci230s2c 2012 software design and construction...

Object-Oriented Programming – Key Concepts

COMPSCI230S2C 2012Software Design and Construction

Agenda & Reading

COMPSCI 230Handout 022

Topics: Object-Oriented Programming Abstraction and information hiding Division into parts Inheritance and polymorphism Composition Vs Inheritance

Reading Java 2, The Complete Reference, Chapter 6, “Introducing Classes” The Java Tutorial

http://java.sun.com/docs/books/tutorial/java/javaOO/classes.html http://java.sun.com/docs/books/tutorial/java/javaOO/objects.html http://java.sun.com/docs/books/tutorial/java/concepts/inheritance.html

Exercise: 02

Object-oriented Approach


The object-oriented approach to software development aims to: Manage the complexity of large non-trivial systems Produce change-resilient software Promote reuse of existing code

Key skills of object-oriented developers are: Creativity Intelligence An ability to use abstractions

3. Object-oriented Programming


Abstraction An abstraction is a view or representation of an entity that includes only

the attributes of significance in a particular context Abstraction is essential when working with complex systems Without abstraction, the programmer faces an overwhelming level of

detail Examples:

ComplexObject

IdealisedModel

IgnoreInessential Details

Instance variables:Names : StringDateOfBirth : Date

Methods:getSurname(): StringgetFirstname (): StringgetAge(): IntegersetSurname(name: String): void

Customer

Instance Variables:item: Vector

Methods:push(object:Object): voidpop(): Object

Stack

Instance Variables:topLeftCorner: PointWidth: IntegerHeight: Integer

Methods:paint(): voidRectangle



Information Hiding Hides unnecessary implementation details from the object user Internal data should not be directly accessible from an object instance

Use Protected or Private access modifiers Use Accessor or Mutator methods to access instance variables (get or set)

Ball b1

xPos:10

yPos: 20

Color: Red



Inheritance Ability to create new classes from existing classes Allows reuse of implementation Studies the type-of relationship between derived classes and base class

Classical Inheritance: “is-a” relationship Example: fruits and types of fruit (an apple is a type of fruit)

Polymorphism Different objects can respond differently to the same message Polymorphism is a very powerful mechanism, and it is at the very heart of

inheritance



Steps Identify the problem OO Analysis: Define all the types of objects and relationships between

them How?

Building whole systems from parts Identifying commonalities among different entities May use Composition/aggregation or Inheritance

4. Composition/Aggregation Combine simple objects or data types

into more complex ones Whole/part relationship or “has-a”

relationship Many books use terms interchangeably Composition:

When an object is destroyed, other objects belonging to it should be destroyed as well

Examples: Car – engine (Every car has an engine) Vector – elementData[]

Aggregation: Aggregation differs from ordinary

composition in that it does not imply ownership

Car – passengers (Cars sometimes have passengers)


Example: Car.java & Engine.java

Inheritance


Generalisation Look for conceptual commonalities in the abstractions

Common attributes/state Common methods or behaviour

“is-a” relationship Subclass “is-a” kind of Superclass

Superclass (base class) defines the general state and behaviour Subclass (derived) class : Created from an existing class, by extending the existing class, rather than

rewriting it Examples Person

Employee & Customer (Customer “is-a” Person)

Example: Person.java & Employee.java

Composition Vs Inheritance


Means of transportation

Wheeled vehicle Packhorse

Bicycle Automobile

Component

Button Label TextComponent

TextField TextArea

Car

Engine

Cylinders Crankshaft

Body Wheels

Human body

Bone Muscles Nervous system Skin

Composition Vs Inheritance


Suppose we want to create a class to represent circles: Composition or Inheritance?

Circle is a point with a radius? Circle has a point with a radius?

General guideline Inheritance : Is-a Composition : has-a

But there is no rulebook - the objective is clean, understandable and maintainable code that can be implemented in reasonable time

Some more guidelines: Inheritance provides a means for constructing highly reusable components, but

needs to be used very carefully Choose composition first when creating new classes from existing classes. You

should only used inheritance if it is required by your design. If you use inheritance where composition will work, your designs will become needlessly complicated

Composition


import java.awt.Point;public class Circle {

private Point p;private int radius;public Circle (int x, int y, int radius) {p = new Point (x, y);this.radius = radius;

}public double getX() {return p.getX ();

}public double getY() {return p.getY ();

}public int getRadius () {return radius;

}// additional code

}

Circle c1 = new Circle();System.out.println("x=" + c1.getX() + ", y=" + c1.getY());System.out.println("radius=" + c1.getRadius());

Example: Circle_a/Circle.java

Inheritance We could reuse Point class

since it already has code for representing a point position


Circle c1 = new Circle();System.out.println("x=" + c1.getX() + ", y=" + c1.getY());System.out.println("radius=" + c1.getRadius());

import java.awt.Point;public class Circle extends Point {

private int radius;

public Circle() {}public int getRadius () {return radius;

}// additional code

}

Inherited from Point class

Instance variable: radius

Example: Circle_i/Circle.java

Review


Object Oriented Programming Define all the types of objects that are part of the user’s work environment = OO Analysis Define all the additional types of objects involving the user interface and operating environment = OO

Design Write statements that define all types of objects, including their attributes and behaviors = OO

Programming Abstraction

The ability of a language to take a concept and create an abstract representation of that concept within a program

Information Hiding How well does this language hide an object’s internal implementation?

Inheritance How does this language promote code reuse? Is-a relation

Polymorphism How does this language let us treat related objects in a similar fashion?

Composition Has-a relation

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