Mehran Sahami Handout #23 CS 106A October 17, 2007 UFO Game Example Based on a handout by Patrick Young. /* * File: UfoGame.java * ------------------ * This program plays a game where the user tries to * shoot a UFO before the UFO "lands". */ import acm.program.*; import acm.graphics.*; import java.awt.*; import java.awt.event.*; public class UfoGame extends GraphicsProgram { /** Size and speed of UFO */ private static final int UFO_WIDTH = 40; private static final int UFO_HEIGHT = UFO_WIDTH / 2; private static final int UFO_SPEED = 5; /** Size and speed of bullets */ private static final int BULLET_SPEED = 10; private static final int BULLET_DIAM = 5; /** Animation cycle delay */ private static final int DELAY = 10; public void run() { setup(); while (!gameOver()) { moveUFO(); moveBullet(); checkForCollisions(); pause(DELAY); } } /** setup UFO and add mouse listeners */ private void setup() { ufo = new GRect(UFO_WIDTH, UFO_HEIGHT); ufo.setFilled(true); add(ufo, getWidth(), 0); // UFO starts at top right ufoToLeft = true; addMouseListeners(); } /** determines if game is over -- true if either * the UFO is destroyed or if the UFO lands */ private boolean gameOver() { return (ufo == null) || (ufo.getY() >= getHeight() - UFO_HEIGHT); } – 2 – /** when mouse is clicked create bullet, unless a bullet * already exists. */ public void mouseClicked(MouseEvent e) { if (bullet == null) { bullet = new GOval(BULLET_DIAM, BULLET_DIAM); bullet.setFilled(true); bullet.setColor(Color.RED); add(bullet, (getWidth() - BULLET_DIAM) / 2, getHeight() - BULLET_DIAM); } } /** moves UFO, if UFO has moved to edge of screen, moves * UFO down and changes UFO direction. */ private void moveUFO() { if (ufoToLeft) { ufo.move(-UFO_SPEED, 0); if (ufo.getX() <= 0) { ufoToLeft = false; ufo.move(0, UFO_HEIGHT); } } else { ufo.move(UFO_SPEED, 0); if (ufo.getX() >= getWidth() - UFO_WIDTH) { ufoToLeft = true; ufo.move(0, UFO_HEIGHT); } } } /** moves bullet */ private void moveBullet() { if (bullet != null) { bullet.move(0, -BULLET_SPEED); } } /** checks for bullet interaction with the world * (either colliding with the UFO or moving offscreen */ private void checkForCollisions() { collideWithUFO(); moveOffScreen(); } – 3 – /** checks to see if UFO and bullet collide, if so * bullet and UFO are removed and both variables are * set to null. */ private void collideWithUFO() { if (bullet != null) { GObject collObj = getElementAt(bullet.getX(), bullet.getY()); if (collObj == ufo) { remove(ufo); remove(bullet); ufo = null; bullet = null; } } } /** determines if bullet has moved of screen, * if it has, removes bullet, sets variable to null */ private void moveOffScreen() { if (bullet != null) { if (bullet.getY() <= -BULLET_DIAM) { remove(bullet); bullet = null; } } } /* private instance variables */ private GRect ufo; private Mehran Sahami Handout #23 CS 106A October 17, 2007 UFO Game

Mehran Sahami Handout #23 CS 106A October 17, 2007 UFO Game Example Based on a handout by Patrick Young. /* * File: UfoGame.java * ------------------ * This program plays a game where the user tries to * shoot a UFO before the UFO "lands". */ import acm.program.*; import acm.graphics.*; import java.awt.*; import java.awt.event.*; public class UfoGame extends GraphicsProgram { /** Size and speed of UFO */ private static final int UFO_WIDTH = 40; private static final int UFO_HEIGHT = UFO_WIDTH / 2; private static final int UFO_SPEED = 5; /** Size and speed of bullets */ private static final int BULLET_SPEED = 10; private static final int BULLET_DIAM = 5; /** Animation cycle delay */ private static final int DELAY = 10; public void run() { setup(); while (!gameOver()) { moveUFO(); moveBullet(); checkForCollisions(); pause(DELAY); } } /** setup UFO and add mouse listeners */ private void setup() { ufo = new GRect(UFO_WIDTH, UFO_HEIGHT); ufo.setFilled(true); add(ufo, getWidth(), 0); // UFO starts at top right ufoToLeft = true; addMouseListeners(); } /** determines if game is over -- true if either * the UFO is destroyed or if the UFO lands */ private boolean gameOver() { return (ufo == null) || (ufo.getY() >= getHeight() - UFO_HEIGHT); }
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/** when mouse is clicked create bullet, unless a bullet * already exists. */ public void mouseClicked(MouseEvent e) { if (bullet == null) { bullet = new GOval(BULLET_DIAM, BULLET_DIAM); bullet.setFilled(true); bullet.setColor(Color.RED); add(bullet, (getWidth() - BULLET_DIAM) / 2, getHeight() - BULLET_DIAM); } } /** moves UFO, if UFO has moved to edge of screen, moves * UFO down and changes UFO direction. */ private void moveUFO() { if (ufoToLeft) { ufo.move(-UFO_SPEED, 0); if (ufo.getX() <= 0) { ufoToLeft = false; ufo.move(0, UFO_HEIGHT); } } else { ufo.move(UFO_SPEED, 0); if (ufo.getX() >= getWidth() - UFO_WIDTH) { ufoToLeft = true; ufo.move(0, UFO_HEIGHT); } } } /** moves bullet */ private void moveBullet() { if (bullet != null) { bullet.move(0, -BULLET_SPEED); } } /** checks for bullet interaction with the world * (either colliding with the UFO or moving offscreen */ private void checkForCollisions() { collideWithUFO(); moveOffScreen(); }
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/** checks to see if UFO and bullet collide, if so * bullet and UFO are removed and both variables are * set to null. */ private void collideWithUFO() { if (bullet != null) { GObject collObj = getElementAt(bullet.getX(), bullet.getY()); if (collObj == ufo) { remove(ufo); remove(bullet); ufo = null; bullet = null; } } } /** determines if bullet has moved of screen, * if it has, removes bullet, sets variable to null */ private void moveOffScreen() { if (bullet != null) { if (bullet.getY() <= -BULLET_DIAM) { remove(bullet); bullet = null; } } } /* private instance variables */ private GRect ufo;
private GOval bullet;
private boolean ufoToLeft; // when true, UFO is moving to left
}