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SCHOOL OF INTERACTIVE ARTS + TECHNOLOGY [SIAT] | WWW.SIAT.SFU.CA

IAT 265OO programming - Inheritance

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Outlineg Object-oriented programming

– objects– classes

• sets (mutators) and gets (accessors)• object methods

– Inheritance: Subclasses• Rocket, ArmedRocket

g Collections– ArrayList

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Inheritance

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Classesg Types

– Primitives: int, float, char, boolean …– Objects: array, string, class …

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Objectsg We’ve worked with some objects

before, like Arrays.g We can make our own objects, to

keep related data together, with methods to control that data.

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Classesg Classes are the blueprints for our

new objects.g To declare a new Class (a new type

of object):

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class MyToy { // fields (class variables) // methods (class functions)}

Fields and Methods

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class MySquare { int xPos, yPos; MySquare(x, y) {

xPos = x;yPos = y;

}

void drawMe() {rect(xPos, yPos, 50, 50);

}}

x y

drawMe()

fields

constructor

methods

(one kind of method)

Fields and Methods

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class MySquare { int xPos, yPos;

MySquare(x, y) {xPos = x;yPos = y;

}

void drawMe() {rect(xPos, yPos, 50, 50);

}}

x y

drawMe()

MySquare square1 = new MySquare(10, 10);MySquare square2 = new MySquare(20, 90);

10 10

drawMe()

20 90

drawMe()

square1 square2

Fields and Methods

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class MySquare { int xPos, yPos;

MySquare(int x, int y) {xPos = x;yPos = y;

}

void drawMe() {rect(xPos, yPos, 50, 50);

}}

MySquare square1 = new MySquare(10, 10);MySquare square2 = new MySquare(20, 90);

x y

drawMe()

10 10

drawMe()

20 90

drawMe()

square1 square2

square1.drawMe();square2.drawMe();

Arrays of Objects?g Let’s make a bunch of squares!

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MySquare[] squares = new MySquare [10] ;

// initialize all of our squares.for (int i = 0; i < 10; i ++) { squares[i] = new MySquare(i*10, i*10);}

squares[4].drawMe(); // draw the 4th square.

Recap: Rocketg Earlier, we created the Rocket class

– Constructor: Rocket(int initialX, int initialY, float initialRot )

– Methodsdraw()rotateClockwise()rotateCounterClockwise()fireThrusters()

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Asteroidsg Let’s adapt this to make an array of

Asteroids for our Rocket

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class Asteroid { //fields float rotation = 0; float xPos, yPos; float velocityX, velocityY; long lastDrawMillis = 0; …}

Asteroidsg When we create an asteroid, let’s

have it start in a random position, and move in a random direction.

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class Asteroid { … // constructor Asteroid() {

xPos = random(0, 400);yPos = random(0, 400);rotation = random(0, TWO_PI);velocityX = sin(rotation)*10;velocityY = -cos(rotation)*10;

}

Asteroids

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class Asteroid { … // draw method void draw () {

Revisit our exampleg So far we have a rocket that flies around

in a field of asteroids

g What if we want our rocket to be able to fire – But we don’t want to get rid of our non-firing

rocket

g Create a subclass!Jun 13, 2014, 2014 IAT 265

Inheritanceg Subclasses inherit fields and

methods from parent

class ArmedRocket extends Rocket {…

}

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Our subclass needs a constructorg Our empty ArmedRocket example creates an error

– Processing doesn’t know how to construct an ArmedRocket

g We want the ArmedRocket constructor to do the same work as the Rocket constructor

ArmedRocket(int initialX, int initialY, float initialRot) {

super(initialX, initialY, initialRot); }

The keyword super means to refer to the parent class.

In this case, to call the Parent Class ConstructorJun 13, 2014, 2014 IAT 265

Now we have ArmedRocketg We can use an ArmedRocket now in

our example

g But, it’s basically just a copy of Rocket

g The only reason to define an ArmedRocket is to add new capabilities or to override old ones

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Add a fire() method g We want our fire method to draw a

missile that shoots out of the rocket

g We could have the fire method draw the missile…– Is there a problem with this?

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Missiles should also be objectsg The object oriented solution is to make the

missile an object as well– All the different types of “things” in our domain

should have a corresponding class

g Like asteroids and rockets, the missile class should know how to draw itself– A Missile is similar to a rocket (position, rotation,

draw method, etc.)

g Now our ArmedRocket.fire() method can just create and return a missile

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The fire() methodMissile fire() { Missile m = new Missile(xPos, yPos, rotation);

return m;}

g Now add code in loop to draw missilesJun 13, 2014, 2014 IAT 265

Missiles destroy asteroidsg So far we have a rocket that flies around

in a field of asteroids and fires

g Now we want our missiles to blow up asteroids– This means we need a variable number of

asteroids. – How do we do this with an array?– Use an ArrayList!– Also need to figure out when we have a

collisionJun 13, 2014, 2014 IAT 265 22

The Java SDK g Java comes with thousands of

classes in the Java Platform API

g Documentation is available on Sun’s website

g Let’s look at ArrayList

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ArrayList g It’s a resizeable list

– Can add and delete things without worrying about declaring the size

g The main methods we care about are add(), get(), and remove(), and size()

g Steps in using ArrayList– Declare a variable of type ArrayList– Create a new ArrayList and assign it to the variable– Call add(), get() and remove() and size() on ArrayList as you

need them

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Parents and childreng Remember that we declared a child class ArmedRocket whose

parent was Rocketg Remember that classes are types

– So ArmedRocket is a type and Rocket is a type

g So, here are some legal assignments– ArmedRocket r1 = new ArmedRocket(50, 60, 0);– Rocket r2 = new Rocket(50, 60, 0); – Rocket r3 = new ArmedRocket(50, 60, 0);

g But this is illegal– ArmedRocket r4 = new Rocket(50, 60, 0);

g Same goes for method arguments as well…Jun 13, 2014, 2014 IAT 265 25

Rocket Inheritance

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Rocket:xPos, YPos, velocityX, velocityY, rotation

Rocket(x,y,rotation) draw()

ArmedRocket extends RocketxPos, YPos, velocityX, velocityY, rotationArmedRocket(x,y,rotation) draw() fire()

Inherits from

Using ArrayList.add()g The argument type of the add method is

Object– Object is the parent class of all classes– With an object argument type, you can pass in

an object of any class

g So, to initialize our asteroids… ArrayList asteroids = new ArrayList(); for(int i = 0; i < numAsteroids; i++) asteroids.add(new Asteroid());

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Getting things out of an ArrayListg ArrayList.get(int i) – returns the

ith object (starting with 0)

g But this doesn’t work!asteroids.get(i).draw();Why?

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Need to cast back from Objectg Since things are put in an ArrayList as

Object, they come back out as Object– It’s like they forget their more detailed type– So, when using ArrayList (or any container

class), need to cast back to the more detailed type

Asteroid asteroid = (Asteroid)asteroids.get(i);

if (!asteroid.collision(r1)) asteroid.draw();Jun 13, 2014, 2014 IAT 265 29

Pushing collision detection into the Asteroid

g In the current code, detecting collision takes place in loop()g But it is cleaner (more object-oriented) if Asteroid itself

knows how to detect collision– Detecting collision depends on knowing the boundaries of the

asteroid, which properly belongs in the asteroid class

boolean collision(Rocket r) { if ((r.xPos >= xPos - 26 && r.xPos <= xPos + 22) && (r.yPos >= yPos - 24 && r.yPos <= yPos + 26)) return true; else return false; }

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Destroying asteroidsg When a missile hits an Asteroid, we need to

destroy it– This was the whole reason for using ArrayList– Big asteroids turn into two small asteroids– Small asteroids disappear

void destroy(ArrayList asteroids) { asteroids.remove(this); if (large) { asteroids.add(new Asteroid(false, xPos, yPos, lastDrawMillis)); asteroids.add(new Asteroid(false, xPos, yPos, lastDrawMillis)); } }

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Super and thisg this is a special variable that always refers to

the current instance (object)– Useful in methods to refer to yourself– this.method() – calls a method on yourself (but normally

you just directly call the method)– this() – calls a constructor on yourself (useful for one

version of a constructor to call another)

g super is a special variable that always refers to the superclass portion of an object (the object cast into it’s superclass)– super.method() – calls the superclass’s method– super() – calls the superclass’s constructor

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Summaryg ArrayList, a Java Platform collection class

g Learned about super and subclasses as types– Any instance of a subclass is an instance of the

superclass, but not visa-versa– Can cast more abstract classes (parents) into more

concrete classes (children)

g The Java keywords super and this – Special variables that can be used within a method

to refer to yourself (the superclass portion of yourself and all of yourself)

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