Tutorial - Writing a Space Invaders game in Java

Now that we have the tools to load and paint images, it's about time to see how we can make things happen in our Java game. Just as all Windows applications have a kernel called Message Loop, which is constantly receiving incoming events and responding to them, in a very similary way all games have a main loop which constantly performs the following:

In the first step is where all monster movements, player actions, level changes, etc... happen. Of course, this is not done in the same method, but it is a clearly different part from the next one, in which the player view of the world is redrawn.

Let's implement this system in our Java game. We'll call the method with the main loop game(). The method that will update the state of the world will be updateWorld() and finally we'll keep the paint() method for the moment as the method charged with redrawing the world.

In this step, "updating the world" will simply mean changing randomly the position of our monster. We'll need two variables called posX and posY that will store the current position, so that the updateWorld() can change them and the paint() can use them for painting the monster in the correct position


1     package version07;
2     /**
3      * Curso B?sico de desarrollo de Juegos en Java - Invaders
4      * 
5      * (c) 2004 Planetalia S.L. - Todos los derechos reservados. Prohibida su reproducci?n
6      * 
7      * http://www.planetalia.com
8      * 
9      */
10    
11    
12    import java.awt.Canvas;
13    import java.awt.Dimension;
14    import java.awt.Graphics;
15    import java.awt.event.WindowAdapter;
16    import java.awt.event.WindowEvent;
17    import java.awt.image.BufferedImage;
18    import java.net.URL;
19    import java.util.HashMap;
20    
21    import javax.imageio.ImageIO;
22    import javax.swing.JFrame;
23    import javax.swing.JPanel;
24    
25    public class Invaders extends Canvas {
26      public static final int WIDTH = 640;
27      public static final int HEIGHT = 480;
28      
29      public HashMap sprites;
30      public int posX,posY;
31      
32      public Invaders() {
33        sprites = new HashMap();
34        posX = WIDTH/2;
35        posY = HEIGHT/2;
36        
37        JFrame ventana = new JFrame("Invaders");
38        JPanel panel = (JPanel)ventana.getContentPane();
39        setBounds(0,0,WIDTH,HEIGHT);
40        panel.setPreferredSize(new Dimension(WIDTH,HEIGHT));
41        panel.setLayout(null);
42        panel.add(this);
43        ventana.setBounds(0,0,WIDTH,HEIGHT);
44        ventana.setVisible(true);
45        ventana.addWindowListener( new WindowAdapter() {
46          public void windowClosing(WindowEvent e) {
47            System.exit(0);
48          }
49        });
50        ventana.setResizable(false);
51      }
52      
53      public BufferedImage loadImage(String nombre) {
54        URL url=null;
55        try {
56          url = getClass().getClassLoader().getResource(nombre);
57          return ImageIO.read(url);
58        } catch (Exception e) {
59          System.out.println("No se pudo cargar la imagen " + nombre +" de "+url);
60          System.out.println("El error fue : "+e.getClass().getName()+" "+e.getMessage());
61          System.exit(0);
62          return null;
63        }
64      }
65      
66      public BufferedImage getSprite(String nombre) {
67        BufferedImage img = (BufferedImage)sprites.get(nombre);
68        if (img == null) {
69          img = loadImage("res/"+nombre);
70          sprites.put(nombre,img);
71        }
72        return img;
73      }
74      
75      
76      public void paint(Graphics g) {
77        g.drawImage(getSprite("bicho.gif"), posX, posY,this);
78      }
79      

80      public void updateWorld() {
81        posX = (int)(Math.random()*WIDTH);
82        posY = (int)(Math.random()*HEIGHT);
83      }

84      

85      public void game() {
86        while (isVisible()) {
87          updateWorld();
88          paint(getGraphics());
89        }
90      }

91      
92      public static void main(String[] args) {
93        Invaders inv = new Invaders();
94        inv.game();
95      }
96    }
97

We've also used this step to introduce a call to setResizable(false) in the constructor, to prevent the user from resizing our window.

If we run this program, we'll be surprised - our window will be almost instantly filled with a crowd of monsters:

Obviously we have a problem : since it's us who are calling the paint() method by hand, the window does not get erased prior to that, and the image of the monster in its old position remains drawn as the new image is shown.

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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
Thumbs.db

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Tutorial - Writing a Space Invaders game in Java

The Game Main Loop

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