1. ______________________________________________________________________________________ SCHOOL OF INTERACTIVE ARTS + TECHNOLOGY [SIAT] | WWW.SIAT.SFU.CA IAT 334 OO programming IAT 3341

2. Outline  Object-oriented programming –objects –classes sets (mutators) and gets (accessors) object methods –Inheritance: Subclasses Rocket, ArmedRocket  Collections –ArrayList June 22, 2010IAT 3342

3. June 22, 2010IAT 3343 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

4. June 22, 2010IAT 3344 Defining the rocket class class Rocket { // fields float rotation = 0; float xPos; float yPos; final int halfWidth = 10; final int halfHeight= 10; // constructor Rocket( int initialX, int initialY, float initialRot ) { xPos = initialX; yPos = initialY; rotation = initialRot; }

5. June 22, 2010IAT 3345 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); }  Nice, but my rockets don’t do anything (need methods!)

6. June 22, 2010IAT 3346 Adding a draw routine to our Rocket void draw() { pushMatrix(); translate(xPos, yPos); rotate(rotation); triangle(0, -halfHeight, -halfWidth, halfHeight, halfWidth, halfHeight); rectMode(CORNERS); rect(-halfWidth + 5, halfHeight, -halfWidth + 8, halfHeight + 3); rect(halfWidth - 8, halfHeight, halfWidth - 5, halfHeight + 3); popMatrix(); } Don’t need arguments because we use the fields But we could define additional arguments if we wanted to No Arguments!

7. June 22, 2010IAT 3347 Calling methods on objects  You call methods on instances  Think of the method as something you are asking the object to do  For example, we can now ask the rockets to draw themselves r1.draw();  In general, to call a method, take the name of the variable holding the object + “.” + the method name myObject.myMethod();

8. Objects June 22, 2010IAT 3348

9. Real Objects  Real-world objects have –State –Behavior  Bicycle –State selected gear, current pedal cadence, speed –Behavior Change Gear, Set Cadence, Apply Brakes June 22, 2010IAT 3349

10. Software Object  State int gear ; float speed ; float cadence ;  Behavior ChangeGears(int g); Brake( float level ); ChangeCadence( float c ); int GetGear(); float GetSpeed(); … June 22, 2010IAT 33410

11. 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; } IAT 33411

12. Java directly supports Objects  Java has direct syntactic and semantic support for Objects Semantics: 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; } IAT 33412 Only these methods can read or write Bicycle private data

13. Java Semantic support  Programming usually takes place with objects: ClockThing clock = new ClockThing(); clock.setSecond( 12 ); clock.setMinute( 18 ); clock.setHour( 3 ); June 22, 2010IAT 33413

14. Even Arrays are objects int[] bob = new int[10] ; bob[4] = 123 ; println( bob.size() ); Bicycle[]bikes = new Bicycle[10] ; bikes[0] = new Bicycle(); June 22, 2010IAT 33414

15. Sets and Gets  what can you do with private data? –to set it: setVarName( varType newValue) –to get it: varType getVarName()  Why? June 22, 2010IAT 33415

16. Temperature object class temp { private floatkelvin ; 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 ; } IAT 33416

17. 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 cares? June 22, 2010IAT 33417

18. 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. June 22, 2010IAT 33418

19. Principle: Code re-use  If an object already exists, you can use that object in your program.  Specialists build, you use June 22, 2010IAT 33419

20. 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! June 22, 2010IAT 33420

21. 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! June 22, 2010IAT 33421

22. Summary of principles  Hide unnecessary details  Clearly define the interface  Allow and support code re-use  Build on the work of others June 22, 2010IAT 33422

23. Inheritance June 22, 2010IAT 33423

24. Classes  Types –Primitives: int, float, char, boolean … –Objects: array, string, class … June 22, 2010IAT 334

25. 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. June 22, 2010IAT 334

26. Classes  Classes are the blueprints for our new objects.  To declare a new Class (a new type of object): June 22, 2010IAT 334 class MyToy { // fields (class variables) // methods (class functions) }

27. Fields and Methods June 22, 2010IAT 334 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)

28. Fields and Methods June 22, 2010IAT 334 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

29. Fields and Methods June 22, 2010IAT 334 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();

30. Arrays of Objects?  Let’s make a bunch of squares! June 22, 2010IAT 334 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.

31. Recap: Rocket  In Lab 2, we created the Rocket class –Constructor: Rocket(int initialX, int initialY, float initialRot ) –Methods draw() rotateClockwise() rotateCounterClockwise() fireThrusters() June 22, 2010IAT 334

32. Asteroids  Let’s adapt this to make an array of Asteroids for our rocket from Lab 2 June 22, 2010IAT 334 class Asteroid { //fields float rotation = 0; float xPos, yPos; float velocityX, velocityY; long lastDrawMillis = 0; … }

33. Asteroids  When we create an asteroid, let’s have it start in a random position, and move in a random direction. June 22, 2010IAT 334 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; }

34. Asteroids June 22, 2010IAT 334 class Asteroid { … // draw method void draw () {

35. 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! June 22, 2010IAT 334

36. Inheritance  Subclasses inherit fields and methods from parent class ArmedRocket extends Rocket { … } June 22, 2010IAT 334

37. 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 June 22, 2010IAT 334

38. 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 June 22, 2010IAT 334

39. 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? June 22, 2010IAT 334

40. 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 June 22, 2010IAT 334

41. The fire() method Missile fire() { Missile m = new Missile(xPos, yPos, rotation); return m; }  Now add code in loop to draw missiles June 22, 2010IAT 334

42. 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 June 22, 2010IAT 33442

43. 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 June 22, 2010IAT 33443

44. 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 June 22, 2010IAT 33444

45. 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… June 22, 2010IAT 33445

46. Rocket Inheritance June 22, 2010IAT 33446 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

47. 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()); June 22, 2010IAT 33447

48. 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? June 22, 2010IAT 33448

49. 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(); June 22, 2010IAT 33449

50. 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; } June 22, 2010IAT 33450

51. 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)); } June 22, 2010IAT 33451

52. 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 June 22, 2010IAT 33452

53. 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) June 22, 2010IAT 33453