Tutorial - Writing a Space Invaders game in Java

So far the monsters have beared with us slaughtering them without retaliation. In this step we add a firing behaviour to the monsters, so they can hit back, too.

We will define a new actor called "Laser" that will represent the thingies that the critters will shoot at us. We will use the following graphics for the laser, called disparo0.gif, disparo1.gif and disparo2.gif:

The Laser class is very similar to the Missile class. The only differences are the fact that the Laser has several frames, and a frame speed, and that it goes downwards and that it's slower than the missile:


1     /**
2      * Curso B?sico de desarrollo de Juegos en Java - Invaders
3      * 
4      * (c) 2004 Planetalia S.L. - Todos los derechos reservados. Prohibida su reproducci?n
5      * 
6      * http://www.planetalia.com
7      * 
8      */
9     package version24;
10    
11    
12    public class Laser extends Actor {
13      protected static final int BULLET_SPEED=3;
14      
15      public Laser(Stage stage) {
16        super(stage);
17        setSpriteNames( new String[] {"disparo0.gif","disparo1.gif","disparo2.gif"});
18        setFrameSpeed(10);
19      }
20      
21      public void act() {
22        super.act();
23        y+=BULLET_SPEED;
24        if (y > Stage.PLAY_HEIGHT)
25          remove();
26      }
27    }
28

Now we must make the monsters shoot randomly. We - rather arbitrarily - decide that in each turn, a monster has a 1% probability of firing. Considering that there are more than a hundred turns per second, this gives a rather decen firing rate. Actually, any frequency greater than that results in a laser shower. The process of firing is very similar to that of the player - a new actor (a Laser) is created and placed just below the monster that is firing:


1     /**
2      * Curso B?sico de desarrollo de Juegos en Java - Invaders
3      * 
4      * (c) 2004 Planetalia S.L. - Todos los derechos reservados. Prohibida su reproducci?n
5      * 
6      * http://www.planetalia.com
7      * 
8      */
9     package version24;
10    
11    public class Monster extends Actor {
12      protected int vx;

13      protected static final double FIRING_FREQUENCY = 0.01;

14      
15      public Monster(Stage stage) {
16        super(stage);
17        setSpriteNames( new String[] {"bicho0.gif","bicho1.gif"});
18        setFrameSpeed(35);
19      }
20      
21      public void act() {
22        super.act();
23        x+=vx;
24        if (x < 0 || x > Stage.WIDTH)
25          vx = -vx;

26        if (Math.random()<FIRING_FREQUENCY)
27          fire();

28      }
29    
30      public int getVx() { return vx; }
31      public void setVx(int i) {vx = i; }
32      
33      public void collision(Actor a) {
34        if (a instanceof Bullet || a instanceof Bomb) {
35          remove();
36          stage.getPlayer().addScore(20);
37        }
38      }
39      

40      public void fire() {
41        Laser m = new Laser(stage);
42        m.setX(x+getWidth()/2);
43        m.setY(y + getHeight());
44        stage.addActor(m);
45      }

46    }
47

And finally we must make the player react to collisions with lasers. For each hit, the shield will decrease 10 units:


1     /**
2      * Curso B?sico de desarrollo de Juegos en Java - Invaders
3      * 
4      * (c) 2004 Planetalia S.L. - Todos los derechos reservados. Prohibida su reproducci?n
5      * 
6      * http://www.planetalia.com
7      * 
8      */
9     package version24;
10    
11    import java.awt.event.KeyEvent;
12    
13    public class Player extends Actor {
14      public static final int MAX_SHIELDS = 200;
15      public static final int MAX_BOMBS = 5;
16      protected static final int PLAYER_SPEED = 4;
17      protected int vx;
18      protected int vy;
19      private boolean up,down,left,right;
20      private int clusterBombs; 
21      private int score;
22      private int shields;
23        
24      
25      public Player(Stage stage) {
26        super(stage);
27        setSpriteNames( new String[] {"nave.gif"});
28        clusterBombs = MAX_BOMBS;
29        shields = MAX_SHIELDS;
30        score = 0;
31      }
32      
33      public void act() {
34        super.act();
35        x+=vx;
36        y+=vy;
37        if (x < 0 ) 
38          x = 0;
39        if (x > Stage.WIDTH - getWidth())
40          x = Stage.WIDTH - getWidth();
41        if (y < 0 )
42          y = 0;
43        if ( y > Stage.PLAY_HEIGHT-getHeight())
44          y = Stage.PLAY_HEIGHT - getHeight();
45      }
46    
47      public int getVx() { return vx; }
48      public void setVx(int i) {vx = i; }
49      public int getVy() { return vy; }
50      public void setVy(int i) {vy = i; }
51      
52      
53      protected void updateSpeed() {
54        vx=0;vy=0;
55        if (down) vy = PLAYER_SPEED;
56        if (up) vy = -PLAYER_SPEED;
57        if (left) vx = -PLAYER_SPEED;
58        if (right) vx = PLAYER_SPEED;
59      }
60      
61      public void keyReleased(KeyEvent e) {
62        switch (e.getKeyCode()) {
63          case KeyEvent.VK_DOWN : down = false;break;
64          case KeyEvent.VK_UP : up = false; break;
65          case KeyEvent.VK_LEFT : left = false; break; 
66          case KeyEvent.VK_RIGHT : right = false;break;
67        }
68        updateSpeed();
69      }
70      
71      public void keyPressed(KeyEvent e) {
72        switch (e.getKeyCode()) {
73          case KeyEvent.VK_UP : up = true; break;
74          case KeyEvent.VK_LEFT : left = true; break;
75          case KeyEvent.VK_RIGHT : right = true; break;
76          case KeyEvent.VK_DOWN : down = true;break;
77          case KeyEvent.VK_SPACE : fire(); break;
78          case KeyEvent.VK_B : fireCluster(); break;
79        }
80        updateSpeed();
81      }
82      
83      public void fire() {
84        Bullet b = new Bullet(stage);
85        b.setX(x);
86        b.setY(y - b.getHeight());
87        stage.addActor(b);
88      }
89      
90      public void fireCluster() {
91        if (clusterBombs == 0)
92          return;
93          
94        clusterBombs--;
95        stage.addActor( new Bomb(stage, Bomb.UP_LEFT, x,y));
96        stage.addActor( new Bomb(stage, Bomb.UP,x,y));
97        stage.addActor( new Bomb(stage, Bomb.UP_RIGHT,x,y));
98        stage.addActor( new Bomb(stage, Bomb.LEFT,x,y));
99        stage.addActor( new Bomb(stage, Bomb.RIGHT,x,y));
100       stage.addActor( new Bomb(stage, Bomb.DOWN_LEFT,x,y));
101       stage.addActor( new Bomb(stage, Bomb.DOWN,x,y));
102       stage.addActor( new Bomb(stage, Bomb.DOWN_RIGHT,x,y));
103     }
104     
105     public int getScore() {   return score; }
106     public void setScore(int i) { score = i;  }
107     public void addScore(int i) { score += i;  }
108   
109     public int getShields() { return shields; }
110     public void setShields(int i) { shields = i;  }
111     public void addShields(int i) {
112       shields += i;
113       if (shields > MAX_SHIELDS) shields = MAX_SHIELDS;
114     }
115     
116     public void collision(Actor a) {
117       if (a instanceof Monster ) {
118         a.remove();
119         addScore(40);
120         addShields(-40);
121       }

122       if (a instanceof Laser ) {
123         a.remove();
124         addShields(-10);

125       }
126       if (getShields() < 0)
127         stage.gameOver();
128     } 
129   
130     public int getClusterBombs() {  return clusterBombs;  }
131     public void setClusterBombs(int i) {  clusterBombs = i; }
132   
133   }
134

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Actor.java	Bomb.java	Bullet.java	Invaders.java
Laser.java	Monster.java	Player.java	SpriteCache.java
Stage.java

bicho.gif	bicho0.gif	bicho1.gif	bicho2.gif
bombD.gif	bombDL.gif	bombDR.gif	bombL.gif
bombR.gif	bombU.gif	bombUL.gif	bombUR.gif
disparo.gif	disparo0.gif	disparo1.gif	disparo2.gif
explosion.wav	misil.gif	missile.wav	musica.wav
nave.gif	oceano.gif	photon.wav	test.gif
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Tutorial - Writing a Space Invaders game in Java

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