IAT 265
Objects
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SCHOOL OF INTERACTIVE ARTS + TECHNOLOGY [SIAT] | WWW.SIAT.SFU.CA
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Outline

Object-oriented programming
– Object components
– Rocket
– Primitive types and Object References
 Objects,
another metaphor
 Why objects?
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Classes vs Objects
A
Class is a blueprint for a bicycle
 An Object is a bicycle
 Many
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bicycles, one blueprint
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Parts of a class

Classes define fields, constructors and methods

Fields are the variables that will appear inside every
instance of the class
– Each instance has its own values


Constructors are special methods that define how to
build instances (generally, how to set the initial values
of fields)
Methods are how you do things to instances
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Defining the rocket class
class Rocket
{
// fields
float rotation = 0;
float xPos;
float yPos;
final int halfWidth = 10;
final int halfHeight= 10;
void draw() {
pushMatrix();
translate(xPos, yPos);
rotate(rotation);
triangle(0, -halfHeight, -halfWidt
// constructor
Rocket( int initialX,
int initialY,
float initialRot )
{
xPos = initialX;
yPos = initialY;
rotation = initialRot;
}
rectMode(CORNERS);
rect(-halfWidth + 5, halfHeight, rect(halfWidth - 8, halfHeight, ha
popMatrix();
}
}
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Using the class to create instances



Classes define a type
You can now declare variables of this type and initialize them using
the constructor
Like arrays, the keyword new is used to tell Java to create a new
object
Rocket r1, r2 ;
void setup() {
r1 = new Rocket(75, 10, 0);
r2 = new Rocket(50, 50, PI/2);
}
void draw() {
r1.draw();
r2.draw();
}
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Primitive types
 Primitive
types are determined by machine
architecture
byte:
short:
int:
long:
float:
double:
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8bits
16bits
32bits
64bits
32bits
64bits
reference: (JVM Dependent)
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Reference
 Like
a remote control
 a reference is a primitive
thing that points at objects
 the new keyword causes
the reference to point at
a new instance of the
object
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Arrays
 int[]
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nums = new int[7] ;
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Array of objects
 Dog[]
pets = new Dog[7];
 It starts as an array of null references
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Array of objects
Dog[] pets = new Dog[7] ;
pets[0] = new Dog();
pets[1] = new Dog();
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Objects
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Real Objects
 Real-world
objects have
– State
– Behavior
 Bicycle
– State
• selected gear, current pedal cadence, speed
– Behavior
• Change Gear, Set Cadence, Apply Brakes
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Software Object

State
int gear ;
float speed ;
float cadence ;

Behavior
ChangeGears(int g);
Brake( float level );
ChangeCadence( float c );
int GetGear();
float GetSpeed();
…
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Java directly supports Objects

Java has direct syntactic and semantic support for Objects
Syntax:
class Bicycle
{
private int cadence = 0;
private int speed = 0;
private int gear = 1;
void changeCadence(int newValue) {
cadence = newValue;
}
void changeGear(int newValue) {
gear = newValue;
}
}
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Java directly supports Objects

Java has direct syntactic and semantic support for Objects
Semantics:
class Bicycle
{
Only these methods
private int cadence = 0;
can read or write
Bicycle private data
private int speed = 0;
private int gear = 1;
void changeCadence(int newValue) {
cadence = newValue;
}
void changeGear(int newValue) {
gear = newValue;
}
}
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Java Semantic support
 Programming
objects:
usually takes place with
ClockThing clock = new ClockThing();
clock.setSecond( 12 );
clock.setMinute( 18 );
clock.setHour( 3 );
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Even Arrays are objects
int[] bob = new int[10] ;
bob[4] = 123 ;
println( bob.size() );
Bicycle[] bikes = new Bicycle[10] ;
bikes[0] = new Bicycle();
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Sets and Gets
 what
can you do with private data?
– to set it:
– to get it:
setVarName( varType newValue)
varType getVarName()
 Why?
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Temperature object
class temp
// constructor not shown
{
private float kelvin ;
void setCelsius( float C );
{
if( C < -273.15 )
return ; // perhaps an error message would be in order
else
kelvin = C + 273.15 ;
}
float getCelsius()
{
return( kelvin - 273.15 );
}
float setKelvin( float k )
{ if( k < 0 )
return ;
else
kelvin = k ;
}
}
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Temperature object
 Controls
access
 Ensures correctness
– can only run a setXYZ() to change temp
– can only do getXYZ() to get the value in the
desired scale
 Who
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cares?
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Who cares?

When you want to:
– Solve the problem once and forget it
– Reuse the solution elsewhere
– Establish rules for use and change of data

The principle:
– Information hiding
– By interacting only with an object's methods, the
details of its internal implementation remain hidden
from the outside world.
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Principle: Code re-use
 If
an object already exists, you can use
that object in your program.
 Specialists build, you use
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Principle: Define the Interface
 Define
the interface:
– The list of methods with Defined Operation
 The
interface is the thing that other people
use
 If you have the same interface with the
same meaning
– You can plug in a better implementation!
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Define the Interface
 If
you have the same interface with the
same meaning
– You can plug in a better implementation!
– You can plug in a More Interesting
implementation!
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Summary of principles
 Hide
unnecessary details
 Clearly define the interface
 Allow and support code re-use
 Build
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on the work of others
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