LCC 6310 The Computer as an Expressive Medium Lecture 7Overview Programming concepts Subclasses Processing, pp.320-322 Methods that return values Talk about Assignment 2 Project 1 - discussionRevisit 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 missiles? But we don’t want to get rid of our non-firing rocket We can create a subclass!Inheritance Subclasses inherit fields and methods from parent We create our subclass as follows: class ArmedRocket extends Rocket { … } Let's try to add this to our Rocket example in Processing…Our subclass needs a constructor Our empty ArmedRocket example causes 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. Back to Processing…Now we have ArmedRocket We can now use an ArmedRocket in our example Instead of creating an instance of Rocket, we can create an instance of Armed Rocket in Processing… But at the moment it’s basically just a copy of Rocket The only reasons to define an ArmedRocket are: To add new capabilities, or To override old onesSo add a fire() method An ArmedRocked should be able to fire missiles So we want a 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?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.) But its initial position will be the position of the ArmedRocket that fired it Also, missiles aren't going to wrap around the display. Instead, they should travel on a straight path and disappear off-screen. Let's create a missile class...The missile class class Missile { final float velocity = 150; float xPos, yPos; float velocityX, velocityY; long lastDrawMillis; Missile(float initialX, float initialY, float rotation) { xPos = initialX; yPos = initialY; // get the missile's velocity in x and y directions based on its orientation velocityX = sin(rotation) * velocity; velocityY = -cos(rotation) * velocity; lastDrawMillis = millis(); } // Also needed: drawMe() and isVisible() methods... }Missile drawMe() void drawMe() { // update the current and last draw times long currentMillis = millis(); float timeSinceLastDraw = ((float)currentMillis -(float)lastDrawMillis)/1000; lastDrawMillis = currentMillis; // update the position based on the velocity and the time // elapsed since the last time the asteroid was drawn xPos = xPos + velocityX * timeSinceLastDraw; yPos = yPos + velocityY * timeSinceLastDraw; pushMatrix(); translate(xPos, yPos); ellipse(0, 0, 5, 5); popMatrix(); }When to draw the missile? Missiles travel on a straight path, so we only want to draw a missile when it's within the bounds of the display So we'll need a method that checks if the missile is within bounds It returns a boolean: true if in bounds, false otherwise boolean isVisible() { // missiles don't wrap around so they are only visible when // they are within the bounds of the display window if ((xPos > XSIZE) || (xPos < 0) || (yPos > YSIZE) || (yPos < 0)) return false; else return true; }The fire() method Now that we have a missile object, our ArmedRocket.fire() method can just create and return a missile… Missile fire() { Missile m = new Missile(xPos, yPos, rotation); return m; } Now we need to add code in draw() to draw missiles…But first, a place to store missiles First we'll need some variables to store the missiles We can use an array like we did for Asteroids We'll also need a size for the array. This will be the maximum number of missiles that can be displayed at a time static final int MAX_MISSILES = 5; Missile[] m; We also need to create an array instance in our setup() method m = new Missile[MAX_MISSILES]; Now for the drawing part…Drawing missiles In our draw() method, we want to draw all the existing missiles that have been fired by our ArmedRocket for(int i = 0; i < MAX_MISSILES; i++) { if (m[i] != null) { m[i].drawMe(); if (!m[i].isVisible()) // delete missile when it goes off-screen m[i] = null; } }That's all great, but how do we fire?! Let's use the keystroke 'm' to fire a missile if (key == 'm') { // Only add a missile if there are less than MAX_MISSILES on screen println("Pressing the missile button"); for(int i = 0; i < MAX_MISSILES; i++) { if (m[i] == null) { m[i] = r1.fire(); break; // java keyword to break out of a loop - no reason to keep // looking for blank missile slots once we've found one } } } Where do we add this code? What happens if we put it where we currently check for other keystrokes?Using the keyPressed() method So far we've been checking for key presses in the draw() loop If we fire missiles in the draw() loop, notice that we fire many times even if we push the key quickly This is because draw() is called many times a second Instead we can use the keyPressed() method keyPressed() is a built in Processing method that is called when a key is pressed (similar to the mousePressed() method we've seen already) Useful because you can tie events to keystrokes instead of to draw()keyPressed() method void keyPressed() { // With keyPressed(), we only check to see if we should fire a missile if (key == 'm') { // Only add a missile if there are less than MAX_MISSILES on screen // Runs through the missile array checking for any empty slots (null) for(int i = 0; i < MAX_MISSILES; i++) { if (m[i] == null) { m[i] = r1.fire(); break; // java keyword to break out of a loop - no reason to keep // looking for blank missile slots once we've found one } } } } Much better!! Now our ArmedRocket works as desired.Assignment 2 Posted online, due 5pm Friday A2-01: Using beginShape() and endShape(), create a composition with five or more vertices. A2-02: Using beginShape() and endShape(), create a composition with ten or
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