bullet.java

一个j2me的游戏的实现.需要添加一个mathfp类来支持浮点数.

import net.jscience.math.kvm.MathFP;
import javax.microedition.lcdui.*;
import java.io.IOException;

/**
 * A Bullet Actor is a simple flying object fired from a Ship. It has a limited
 * lifetime of 3 seconds using code in the cycle method after which the object
 * is removed from the GameScreen.
 * @author Martin J. Wells
 */
public class Bullet extends Actor
{
   private static ImageSet bulletImageSet;  // the one and only imageset
   private Sprite bulletSprite;

   /**
    * Creates a new Bullet at the specified starting position and facing
    * direction. This is called by the Ship class when the player hits the fire
    * key.
    * @param worldArg The world this actor is in.
    * @param startX The starting x position.
    * @param startY The starting y position.
    * @param direction The starting direction.
    */
   public Bullet(World worldArg, int startX, int startY, int direction)
   {
      super(worldArg, startX, startY, direction, 16, MathFP.toFP("6.0"),
            MathFP.toFP("0.0"), MathFP.toFP("6.0"), MathFP.toFP("-1.0"), 0);

      if (bulletImageSet == null)
      {
         try
         {
            Image[] frames = ImageSet.extractFrames(Image.createImage(
                    "/general.png"), 0, 0, 4, 1, 3, 3);
            bulletImageSet = new ImageSet(1);
            bulletImageSet.addState(frames, 0);
         }
         catch (IOException ioe)
         {
            System.out.println("unable to load image");
         }
      }

      bulletSprite = new Sprite(bulletImageSet, 0, 0);
   }

   /**
    * The render method is basically the same as the Ship except that the bullet
    * graphic doesn抰 face a particular direction so you don抰 need to worry
    * about setting that.
    * @param graphics The graphics context upon which to draw the bullet.
    * @param offsetX The amount to offset the x drawing position by.
    * @param offsetY The amount to offset the y drawing position by.
    */
   public void render(Graphics graphics, int offsetX, int offsetY)
   {
      bulletSprite.draw(graphics, getX()-offsetX, getY()-offsetY);
   }

   /**
    * The cycle method calls the base class cycle (Actor) which handles the
    * movement, then we cycle the Sprite object so it will animate (progress to
    * the next frame) before checking the amount of time the object has been
    * around and destroy it by calling the GameScreen.removeActor method.
    * @param deltaMS The amount of time that has passed sine the last call to
    * cycle (in milliseconds).
    */
   public void cycle(long deltaMS)
   {
      super.cycle(deltaMS);
      bulletSprite.cycle(deltaMS);

      // terminate this bullet if it's older than 3 seconds
      if (bulletSprite.getTimeInCurrentState() > 3000)
         getWorld().removeActor(this);
   }

   /**
    * @return The height of the Bullet object (always 1 for this case).
    */
   public int getHeight() { return 1; }

   /**
    * @return The width of the Bullet object (always 1 for this case).
    */
   public int getWidth() { return 1; }
}