1. ______________________________________________________________________________________ SCHOOL OF INTERACTIVE ARTS + TECHNOLOGY [SIAT] | WWW.SIAT.SFU.CA IAT 265 OO programming - Inheritance IAT 2651

2. Outline  Object-oriented programming –objects –classes sets (mutators) and gets (accessors) object methods –Inheritance: Subclasses Rocket, ArmedRocket  Collections –ArrayList Jun 13, 2014, 2014IAT 2652

3. Inheritance Jun 13, 2014, 2014IAT 2653

4. Classes  Types –Primitives: int, float, char, boolean … –Objects: array, string, class … Jun 13, 2014, 2014IAT 265

5. Objects  We’ve worked with some objects before, like Arrays.  We can make our own objects, to keep related data together, with methods to control that data. Jun 13, 2014, 2014IAT 265

6. Classes  Classes are the blueprints for our new objects.  To declare a new Class (a new type of object): Jun 13, 2014, 2014IAT 265 class MyToy { // fields (class variables) // methods (class functions) }

7. Fields and Methods Jun 13, 2014, 2014IAT 265 class MySquare { int xPos, yPos; MySquare(x, y) { xPos = x; yPos = y; } void drawMe() { rect(xPos, yPos, 50, 50); } xy drawMe() fields constructor methods (one kind of method)

8. Fields and Methods Jun 13, 2014, 2014IAT 265 class MySquare { int xPos, yPos; MySquare(x, y) { xPos = x; yPos = y; } void drawMe() { rect(xPos, yPos, 50, 50); } xy drawMe() MySquare square1 = new MySquare(10, 10); MySquare square2 = new MySquare(20, 90); 10 drawMe() 2090 drawMe() square1square2

9. Fields and Methods Jun 13, 2014, 2014IAT 265 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); xy drawMe() 10 drawMe() 2090 drawMe() square1 square2 square1.drawMe(); square2.drawMe();

10. Arrays of Objects?  Let’s make a bunch of squares! Jun 13, 2014, 2014IAT 265 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 4 th square.

11. Recap: Rocket  Earlier, we created the Rocket class –Constructor: Rocket(int initialX, int initialY, float initialRot ) –Methods draw() rotateClockwise() rotateCounterClockwise() fireThrusters() Jun 13, 2014, 2014IAT 265

12. Asteroids  Let’s adapt this to make an array of Asteroids for our Rocket Jun 13, 2014, 2014IAT 265 class Asteroid { //fields float rotation = 0; float xPos, yPos; float velocityX, velocityY; long lastDrawMillis = 0; … }

13. Asteroids  When we create an asteroid, let’s have it start in a random position, and move in a random direction. Jun 13, 2014, 2014IAT 265 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; }

14. Asteroids Jun 13, 2014, 2014IAT 265 class Asteroid { … // draw method void draw () {

15. Revisit our example  So far we have a rocket that flies around in a field of asteroids  What if we want our rocket to be able to fire –But we don’t want to get rid of our non-firing rocket  Create a subclass! Jun 13, 2014, 2014IAT 265

16. Inheritance  Subclasses inherit fields and methods from parent class ArmedRocket extends Rocket { … } Jun 13, 2014, 2014IAT 265

17. Our subclass needs a constructor  Our empty ArmedRocket example creates an error –Processing doesn’t know how to construct an ArmedRocket  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 Constructor Jun 13, 2014, 2014IAT 265

18. Now we have ArmedRocket  We can use an ArmedRocket now in our example  But, it’s basically just a copy of Rocket  The only reason to define an ArmedRocket is to add new capabilities or to override old ones Jun 13, 2014, 2014IAT 265

19. Add a fire() method  We want our fire method to draw a missile that shoots out of the rocket  We could have the fire method draw the missile… –Is there a problem with this? Jun 13, 2014, 2014IAT 265

20. Missiles should also be objects  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  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.)  Now our ArmedRocket.fire() method can just create and return a missile Jun 13, 2014, 2014IAT 265

21. The fire() method Missile fire() { Missile m = new Missile(xPos, yPos, rotation); return m; }  Now add code in loop to draw missiles Jun 13, 2014, 2014IAT 265

22. Missiles destroy asteroids  So far we have a rocket that flies around in a field of asteroids and fires  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 collision Jun 13, 2014, 2014IAT 26522

23. The Java SDK  Java comes with thousands of classes in the Java Platform API  Documentation is available on Sun’s website  Let’s look at ArrayList Jun 13, 2014, 2014IAT 26523

24. ArrayList  It’s a resizeable list –Can add and delete things without worrying about declaring the size  The main methods we care about are add(), get(), and remove(), and size()  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 Jun 13, 2014, 2014IAT 26524

25. Parents and children  Remember that we declared a child class ArmedRocket whose parent was Rocket  Remember that classes are types –So ArmedRocket is a type and Rocket is a type  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);  But this is illegal –ArmedRocket r4 = new Rocket(50, 60, 0);  Same goes for method arguments as well… Jun 13, 2014, 2014IAT 26525

26. Rocket Inheritance Jun 13, 2014, 2014IAT 26526 Rocket: xPos, YPos, velocityX, velocityY, rotation Rocket(x,y,rotation) draw() ArmedRocket extends Rocket xPos, YPos, velocityX, velocityY, rotation ArmedRocket(x,y,rotation) draw() fire() Inherits from

27. Using ArrayList.add()  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  So, to initialize our asteroids… ArrayList asteroids = new ArrayList(); for(int i = 0; i < numAsteroids; i++) asteroids.add(new Asteroid()); Jun 13, 2014, 2014IAT 26527

28. Getting things out of an ArrayList  ArrayList.get(int i) – returns the ith object (starting with 0)  But this doesn’t work! asteroids.get(i).draw(); Why? Jun 13, 2014, 2014IAT 26528

29. Need to cast back from Object  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, 2014IAT 26529

30. Pushing collision detection into the Asteroid  In the current code, detecting collision takes place in loop()  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; } Jun 13, 2014, 2014IAT 26530

31. Destroying asteroids  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)); } Jun 13, 2014, 2014IAT 26531

32. Super and this  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)  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 Jun 13, 2014, 2014IAT 26532

33. Summary  ArrayList, a Java Platform collection class  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)  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) Jun 13, 2014, 2014IAT 26533