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

March 31, 2000CS102-01Lecture 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

March 31, 2000CS102-01Lecture 1.3 Types of Data Related data –Automobiles –Whole numbers –Fractions –Sentences

March 31, 2000CS102-01Lecture 1.3 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

March 31, 2000CS102-01Lecture 1.3 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.”?

March 31, 2000CS102-01Lecture 1.3 The World is Full of Objects I Some objects in the world, and what they can do: ObjectActions Chevy MalibusStop, go, turn MicroscopesFocus, insert slide, remove slide HotelsMake up rooms, check in guests

March 31, 2000CS102-01Lecture 1.3 The World is Full of Objects II Some objects in the world, and what they know: ObjectData Chevy MalibusGas level, coolant level, top speed MicroscopesSlide light on? HotelsNumber of conference rooms

March 31, 2000CS102-01Lecture 1.3 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

March 31, 2000CS102-01Lecture 1.3 Inheritance

March 31, 2000CS102-01Lecture 1.3 Same Car, Different View

March 31, 2000CS102-01Lecture 1.3 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”

March 31, 2000CS102-01Lecture 1.3 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

March 31, 2000CS102-01Lecture 1.3 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?

March 31, 2000CS102-01Lecture 1.3 “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.

March 31, 2000CS102-01Lecture 1.3 Examples of Classes Related groups of things constitute a class Share the same structure, actions (behavior) and similarly derived –Aardvarks –Airports –Applets

March 31, 2000CS102-01Lecture 1.3 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) }

March 31, 2000CS102-01Lecture 1.3 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; // private int minute; // private int second; //

March 31, 2000CS102-01Lecture 1.3 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

March 31, 2000CS102-01Lecture 1.3 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

March 31, 2000CS102-01Lecture 1.3 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”

March 31, 2000CS102-01Lecture 1.3 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

March 31, 2000CS102-01Lecture 1.3 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?

March 31, 2000CS102-01Lecture 1.3 Java Object Magic Creating objects is easy in Java –Forget about memory ‘cuz Java’s simple Want another airport, just call new again!

March 31, 2000CS102-01Lecture 1.3 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

March 31, 2000CS102-01Lecture 1.3 Building Time public class Time1 { private int hour; // private int minute; // private int second; // // 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

March 31, 2000CS102-01Lecture 1.3 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,...

March 31, 2000CS102-01Lecture 1.3 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,...

March 31, 2000CS102-01Lecture 1.3 Hiding Data in Java public class Time1 { private int hour; // private int minute; // private int second; // // 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”

March 31, 2000CS102-01Lecture 1.3 Hiding Time public class Time1 { private int hour; // private int minute; // private int second; // // 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

March 31, 2000CS102-01Lecture 1.3 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

March 31, 2000CS102-01Lecture 1.3 Object Oriented Means… II Hiding data: Only the essentials are visible to the outside world Modularity: Group related abstractions together