sep 26, fall 2006iat 800 lecture 6 methods and classes
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Sep 26, Fall 2006 IAT 800
Today
A few good programming practices Methods Classes
– Rockets, asteroids
– Subclasses – Methods that return values– BImage
Sep 26, Fall 2006 IAT 800
Good Practices
Comments are your friend!– Put a block of commenting at the top of each
program that explains what it does– Comment in code what a section will do– Comment when you appropriate code!!
Auto-format is also your friend!! Your programs will have at least three
sections • Variable declaration• setup()• draw()
Sep 26, Fall 2006 IAT 800
Methods
Methods, also known as – Functions (methods which return
something)– Procedures (methods which don’t return
stuff) Method declaration
Method call
void vendingMachine( int coinCents ){ println("You inserted "+coinCents+" cents."); }
int quarter = 25;vendingMachine(quarter);
Sep 26, Fall 2006 IAT 800
Methods
Method declaration
• The method declaration is like a blueprint• Doesn't matter if declaration is before or
after call!• Parameter name (e.g. coinCents) is just the
name given to data passed into the method…
void vendingMachine( int coinCents ){ println("You inserted "+coinCents+" cents."); }
void means doesn't return anything
parameter
Sep 26, Fall 2006 IAT 800
Methods
Method call
• vendingMachine(quarter) == vendingMachine(25)
• Method call must occur in either setup(), draw(), or another method…
• You can call it as many times as you like!– vendingMachine()s for everyone!!
int quarter = 25;vendingMachine(quarter);
argument
Sep 26, Fall 2006 IAT 800
Classes
Types– Primitives: int, float, char, boolean …– Objects: array, string, class …
Sep 26, Fall 2006 IAT 800
Objects
We’ve worked with some objects before, like Arrays.
We can make our own objects, to keep related data together, and methods to control that data.
Sep 26, Fall 2006 IAT 800
Classes
Classes are the blueprints for our new objects.
To declare a new Class (a new type of object):
class MyToy { // fields (class variables) // methods (class functions)}
Sep 26, Fall 2006 IAT 800
Fields and Methods
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)
Sep 26, Fall 2006 IAT 800
Fields and Methods
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
Sep 26, Fall 2006 IAT 800
Fields and Methods
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();
Sep 26, Fall 2006 IAT 800
Arrays of Objects?
Let’s make a bunch of squares!
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.
Sep 26, Fall 2006 IAT 800
Asteroids
Let’s adapt this to make an array of Asteroids for our rocket from lecture.
class Asteroid { //fields float rotation = 0; float xPos, yPos; float velocityX, velocityY; long lastDrawMillis = 0; …}
Sep 26, Fall 2006 IAT 800
Asteroids
When we create an asteroid, let’s have it start in a random position, and move in a random direction.
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;
}
Sep 26, Fall 2006 IAT 800
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!
Sep 26, Fall 2006 IAT 800
Inheritance
Subclasses inherit fields and methods from parent
Class ArmedRocket extends Rocket {…
}
Sep 26, Fall 2006 IAT 800
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 call the method in the parent with the same name
Sep 26, Fall 2006 IAT 800
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
Sep 26, Fall 2006 IAT 800
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?
Sep 26, Fall 2006 IAT 800
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
Sep 26, Fall 2006 IAT 800
The fire() method
Missile fire() { Missile m = new Missile(xPos, yPos,
rotation); return m; }
Now add code in loop to draw missiles
Sep 26, Fall 2006 IAT 800
Using the keyPressed() method
keyPressed() is a built in Processing method that is called when a key is pressed
Useful because you can tie events to keystrokes instead of to loop()– If we fire missiles in loop(), notice that we fire
many times even if we push the key quickly– This is because loop() is called many times a
second