Introduction to Object-Oriented Programming

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Introduction to Object-Oriented Programming

• CS 102-02
• Lecture 1-3

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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

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Types of Data

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

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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

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Driving a Car

• When you step on the gas in a car, do you think
• Im calling the accelerate procedure and passing
  it a pink Chevy Malibu and a pedal position.
• Or, Chevy Malibus know how to accelerate
  already and Im just asking the car to do its
  thing.?

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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
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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
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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
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Object Relationships

• Objects can contain other objects
• Composition
• Has a (or hath if youre an English author)
  relationship
• Objects are more specific versions of other
  objects
• Inheritance
• Is a relationship

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Inheritance
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Same Car, Different View
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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
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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

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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?

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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.

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Examples of Classes

• Related groups of things constitute a class
• Share the same structure, actions (behavior) and
  similarly derived
• Aardvarks
• Airports
• Applets

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Classes in Java
If youll 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)
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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
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Classes Know Actions

• Classes arent just data, but actions too
• At the airport
• Delivering baggage
• Preparing for planes arrival
• Class actions are called methods

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Types

• Type is similar to class a collection of data
  and actions
• Usually, well consider type and class to be the
  same thing
• In Java there are interfaces and classes

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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

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Objects are Instances
Airport OHare Class Object

• Classes are the overarching concepts
• Concept airport is an abstract notion
• Objects are instances of those classes
• OHare, LAX and Heathrow are concrete instances
  of airports

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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?
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Java Object Magic

• Creating objects is easy in Java
• Forget about memory cuz Javas simple
• Want another airport, just call new again!

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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

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Building 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 // consistent state.
public Time1() setTime( 0, 0, 0 )
Constructors have the same name as the class
Anybody can create a new Time1 object
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Hiding (a.k.a. Encapsulating) Data

• Airport operations
• Do you know
• Outer marker?
• NDB?
• ATIS Frequency for OHare?
• Use the airport because you only need to know a
  little
• Parking lot, ticket counter, baggage claim, ...

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Why Hide Data?

• Cant break it
• What if you could change the tower frequency?
• Double-check data
• Easier for you
• What if you couldnt get on a plane without
  knowing how to operate a jetway?
• Inner workings can change
• Change the guts of the airport, but dont change
  the ticket counter, baggage claim, ...

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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 // 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
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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 // consistent state.
public Time1() setTime( 0, 0, 0 )
Anybody can create a new Time1 object
One of our reasons for data hiding
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Modular Design
Airport Modules
Passenger Terminal
Maintenance Storage

• View related abstractions together
• Modules are useful only if theyre loosely
  connected
• Passenger Terminal and Maintenance, not a Door
  and its hinges

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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

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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

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Object Oriented Means II

• Hiding data Only the essentials are visible to
  the outside world
• Modularity Group related abstractions together