March 31, 2000CS102-01Lecture 1.3 Introduction to Object-Oriented Programming CS 102-02 Lecture 1-3

March 31, 2000 CS102-01 Lecture 1.3

Introduction to Object-Oriented Programming

CS 102-02

Lecture 1-3


Before You Write a Program...

• Decide on data– What input does your program need?– What data will it manipulate?– What information will it produce?

• Actions– Things your program does


Types of Data

• Related data– Automobiles– Whole numbers– Fractions– Sentences


How Do Actions and Data Relate?

• Actions can act on data– Square root procedures act on numbers– Problem: Need different actions for different

data

• Actions and data can coexist– Objects combine both data and actions into one

package


Driving a Car

• When you step on the gas in a car, do you think:– “I’m calling the accelerate procedure and

passing it a pink Chevy Malibu and a pedal position.”

– Or, “Chevy Malibus ‘know’ how to accelerate already and I’m just asking the car to do its thing.”?


The World is Full of Objects I

• Some objects in the world, and what they can do:

Object Actions

Chevy Malibus Stop, go, turn

Microscopes Focus, insert slide, remove slide

Hotels Make up rooms, check in guests


The World is Full of Objects II

• Some objects in the world, and what they know:

Object Data

Chevy Malibus Gas level, coolant level, top speed

Microscopes Slide light on?

Hotels Number of conference rooms


Object Relationships

• Objects can contain other objects– Composition– “Has a” (or “hath” if you’re an English author)

relationship

• Objects are more specific versions of other objects– Inheritance– “Is a” relationship


Inheritance

Planes Trains

Ford Chrysler

Malibu

Chevrolet

GM

Automobiles

Vehicles


Same Car, Different View

Planes Trains

Sports Car Truck

Chevy Malibu

Inexpensive

Lexus LS400

Expensive

Sedan

Automobiles

Vehicles


The Welcome Applet// A first program in Java

// import Applet class

import java.applet.Applet;

// import Graphics class

import java.awt.Graphics;

public class Welcome extends Applet {

public void paint( Graphics g ) {

g.drawString( "Welcome to Java Programming!", 25, 25 );

}

}

A “Welcome” is a kind of “Applet”


Reuse, Reuse, Reuse

• O-O concepts make it easy to reuse– Inheritance: Someone else creates the general,

and you add specifics– Composition: Put the puzzle pieces together

“Writing good software is hard, so avoid it whenever possible.”

- Kurt Fenstermacher


Building an Airport

• What actions do airports know how to perform?

• What attributes do airports have?

• Are airports a kind of something?

• What kinds of airports are there?

• Do airports have logical subparts?


“Just Like Summer Vacation, -- No Class”

• A class is a specification of :– Structure (the data, a.k.a. instance variables)

– Actions (methods)

– Inheritance (parents, or derived structure and actions)

for objects.


Examples of Classes

• Related groups of things constitute a class

• Share the same structure, actions (behavior) and similarly derived

– Aardvarks– Airports– Applets


Classes in Java

If you’ll need a group of related objects, create a class:

class Point { int x, y; }

Define a class with:

class classname {

Class definition (some data and/or some actions)

}


Classes Have Data• Airport class

– Gates– Runways– Airlines

• Class data goes inside the class definition, usually at the very beginning:public class Time1 { private int hour; // 0 - 23 private int minute; // 0 - 59 private int second; // 0 - 59


Classes Know Actions

• Classes aren’t just data, but actions too– At the airport

• Delivering baggage

• Preparing for plane’s arrival

– Class actions are called methods


Types

• Type is similar to class: a collection of data and actions

• Usually, we’ll consider type and class to be the same thing– In Java there are interfaces and classes


Abstract Data Types

• ADTs (from HTP 6.16) are implemented in Java with classes– An airport class represents the abstract notion

of a class– The Platonic “form”


Objects are Instances

• Classes are the overarching concepts– Concept “airport” is an abstract notion

• Objects are instances of those classes– O’Hare, LAX and Heathrow are concrete

instances of airports

Airport : O’Hare :: Class : Object


Creating an Object

Use new:

Airport peotone = new Airport(“Peotone, IL”);

Give the variable a name

What type of variable is it?

Some airport specification

You want a new what?


Java Object Magic

• Creating objects is easy in Java– Forget about memory ‘cuz Java’s simple

• Want another airport, just call new again!


Creating Instances from Classes

• Real-world– Spend money to hire construction crews– Lay asphalt– Build roads

• In Java, build airports with constructors– Special methods defined in a class which set up

new objects– Same name as class


Building Timepublic class Time1 { private int hour; // 0 - 23 private int minute; // 0 - 59 private int second; // 0 - 59

// Time1 constructor initializes each // instance variable to zero. Ensures // that each Time1 object starts in a // consistent state. public Time1() { setTime( 0, 0, 0 ); }

Anybody can create a new Time1 object

Constructors have the same name as the class


Hiding (a.k.a. Encapsulating) Data• Airport operations

– Do you know:• Outer marker?

• NDB?

• ATIS Frequency for O’Hare?

• Use the airport because you only need to know a little– Parking lot, ticket counter, baggage claim, ...


Why Hide Data?• Can’t break it

– What if you could change the tower frequency?– Double-check data

• Easier for you– What if you couldn’t get on a plane without

knowing how to operate a jetway?

• Inner workings can change– Change the guts of the airport, but don’t change

the ticket counter, baggage claim, ...


Hiding Data in Javapublic class Time1 { private int hour; // 0 - 23 private int minute; // 0 - 59 private int second; // 0 - 59

// Time1 constructor initializes each instance variable // to zero. Ensures that each Time1 object starts in a // consistent state.

public Time1() { setTime( 0, 0, 0 ); }

Nobody can mess with hour, minute or second

Nobody can set hour = “ABC” or minute = “456.45”


Hiding Timepublic class Time1 { private int hour; // 0 - 23 private int minute; // 0 - 59 private int second; // 0 - 59

// Time1 constructor initializes each // instance variable to zero. Ensures // that each Time1 object starts in a // consistent state. public Time1() { setTime( 0, 0, 0 ); }

Anybody can create a new Time1 object

One of our reasons for data hiding


Object-Oriented Means… I• Objects: Combining data and actions under

one roof

• Hierarchies: An ranking of abstractions– Inheritance: The “is a” hierarchy– Composition: The “part of” hierarchy

• Abstraction: What distinguishes an object from other kinds objects, given a particular perspective


Object Oriented Means… II

• Hiding data: Only the essentials are visible to the outside world

• Modularity: Group related abstractions together

Documents

March 31, 2000CS102-01Lecture 1.3 Introduction to Object-Oriented Programming CS 102-02 Lecture 1-3