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Introduction to Object-Oriented

Programming

April 3, 1998

CS 102-02

Lecture 1-3

CS102-02 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

April 3, 1998 CS102-02 Lecture 1-3

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

Chevy Malibus

Microscopes

Hotels

Actions

Stop, go, turn

Focus, insert slide, remove slide

Make up rooms, check in guests


The World is Full of Objects II

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

Object

Chevy Malibus

Microscopes

Hotels

Data

Gas level, coolant level, top speed

Slide light on?

Number of conference rooms


How are Objects Related?

So, Nat'ralists observe, a Flea

Hath smaller Fleas that on them prey,

And these have smaller Fleas to bite 'em,

And so proceed ad infinitum.

-J. Swift


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


April 3, 1998

Planes

Inheritance

Vehicles

Trains Automobiles

Ford Chrysler GM

Chevrolet

Malibu


April 3, 1998

Same Car, Different View

Vehicles

Planes Trains

Sports Car

Automobiles

Truck Sedan

Inexpensive Expensive

Chevy Malibu Lexus LS400


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

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

Me

• O-O concepts make it easy to reuse

– Inheritance: Someone else creates the general, and you add specifics

– Composition: Put the puzzle pieces together


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

Airport : O’Hare :: Class : Object

• 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


Creating an Object

Give the variable a name

Some airport specification

Use new :

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

What type of variable is it?

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 Time public class Time1 { private int hour; // 0 - 23 private int minute; // 0 - 59 private int second; // 0 - 59

Constructors have the

// Time1 constructor initializes each

// instance variable to zero. Ensures

// that each Time1 object starts in a

// consistent state.

public Time1() { setTime( 0, 0, 0 ); } same name as the class

Anybody can create a new Time1 object


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 Java public 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


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

Nobody can mess with hour , minute or second

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


Hiding Time public 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


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

Anybody can create a new Time1 object

April 3, 1998

One of our reasons for data hiding


Airport

Modules

Modular Design

Passenger

Terminal

Maintenance

& Storage

• View related abstractions together

• Modules are useful only if they’re loosely connected

– Passenger Terminal and Maintenance, not a

Door and its hinges


Modularity in Java

• Java has two ways to separate modules:

– Classes

• Use class to define

• Separate classes with access modifiers

– Packages

• Group related classes together

• Optionally, declare a package using package in the first line of the file package transportation.airport;

• Without package , code is part of the default package


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


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