抛物线

 一。

 //Canvas类中的代码

 import java.io.IOException;
 import javax.microedition.lcdui.Graphics;
 import javax.microedition.lcdui.Image;
 import javax.microedition.lcdui.game.GameCanvas;
 import javax.microedition.lcdui.game.Sprite;
 import javax.microedition.lcdui.game.TiledLayer;

 public class MyCanvas extends GameCanvas implements Runnable
 {
  int x,y;         //小船的位置
  Sprite ship;     //小船精灵
  TiledLayer background;     //地图
  Image ship_IMG, sea_IMG, bullet_IMG;    //图片
   Bullets bullets;      //炮弹
  public MyCanvas()
  {
   super(true);
   //初始化船的位置
   x = 10;
   y = 60;
   try
   {
    ship_IMG = Image.createImage("/ship.png");
    sea_IMG = Image.createImage("/sea.png");
    bullet_IMG = Image.createImage("/bullet.png");
   }
   catch (IOException e)
   {
    e.printStackTrace();
   }
   //初始化炮弹
   bullets = new Bullets(bullet_IMG);
   //初始化小船
   ship = new Sprite(ship_IMG, 24, 30);
   ship.setPosition(x, y);
   //初始化地图
   background = new TiledLayer(20, 6, sea_IMG, 32, 32);
   background.createAnimatedTile(1);
   int[][] map = new int[][]
   {
     { 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4 },
     { -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1 },
     { -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1 },
     { -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1 },
     { -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1 },
     { 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6 }
   };
   //绘制地图
   for(int i = 0 ; i < map.length ; i ++){
    for(int j = 0 ; j < map[i].length ; j++){
     background.setCell(j, i, map[i][j]);
    }
   }
   Thread th = new Thread(this);
   th.start();
  }

  private int k = 0;
  public void run(){
   Graphics g = getGraphics();
   while(true){
    //显示出水的流动
    k++;
    if(k==10){
     if(background.getAnimatedTile(-1) == 1){
      background.setAnimatedTile(-1, 2);
     }
     else if(background.getAnimatedTile(-1) == 2){
      background.setAnimatedTile(-1, 1);
     }
     k = 0;
    }
    //主控制
    support(g);
    //绘制
    draw(g);
    try{
     Thread.sleep(35);
    }
    catch (InterruptedException e){
     e.printStackTrace();
    }
   }
  }

  public void support(Graphics g){
   //控制船的移动及发射
   int keyCode = getKeyStates();
   switch(keyCode){
      case UP_PRESSED:
       y = Math.max(0, y - 2);
       if(ship.getFrame() >= 2)
        ship.setFrame(0);
       else
        ship.nextFrame();
       break;
      case DOWN_PRESSED:
       y = Math.min(getHeight(), y + 2);
       if(ship.getFrame() <= 2 || ship.getFrame() >= 5)
        ship.setFrame(3);
       else
        ship.nextFrame();
       break;
      case LEFT_PRESSED:
       x = Math.max(0, x - 2);
       if(ship.getFrame() <= 5 || ship.getFrame() >= 8)
        ship.setFrame(6);
       else
        ship.nextFrame();
       break;
      case RIGHT_PRESSED:
       x = Math.min(getWidth(), x + 2);
       if(ship.getFrame() <= 8 || ship.getFrame() >= 11)
        ship.setFrame(9);
       else
        ship.nextFrame();
       break;
      case FIRE_PRESSED:
       if(!bullets.isfire){
        bullets.isfire = true;
        bullets.setPoint(x+13,y);
       }
       break;
   }
   ship.setPosition(this.x, this.y);
  }

  /**
   * 绘制
   * @param g
   */
  public void draw(Graphics g){
   g.setColor(0xffffff);
   g.fillRect(0,0,this.getWidth(),this.getHeight());
   g.setColor(0x000000);
   background.paint(g);
   bullets.drawBullets(g);
   ship.paint(g);
   flushGraphics();
  }
 }
 /**
  * 炮弹
  * <p>Title: Bullets</p>
  * <p>Description: </p>
  * <p>Copyright: Copyright (c) 2005</p>
  * <p>Date: 2006-3-2</p>
  * @author ZGY
  * @version 1.0
  */
 class Bullets extends Sprite{
  boolean isfire = false;
  //炮弹的发射点
  int bulletX, bulletY;
  //炮弹的射程
  int shotWidth = 50;
  //炮弹的射程高度
  int shotHeight = shotWidth/2-10;
  //炮弹的速度
  int shotSpeed = 3;
  //炮弹抛物线的系数
  double a,b,c;
  public Bullets(Image image){
   super(image, 15, 15);
  }

  /**
   * 初始化炮弹
   * @param x
   * @param y
   */
  public void setPoint(int x, int y){
   //初始化炮弹的发射点
   this.bulletX = x;
   this.bulletY = y;
   //根据炮弹的发射点、高度、射程计算出炮弹抛物线的三点
   int x1 = bulletX;
   int y1 = bulletY;
   int x2 = bulletX+shotWidth/2;
   int y2 = bulletY-shotHeight;
   int x3 = bulletX+shotWidth;
   int y3 = bulletY;
   //根据抛物线方程ax^2+bx+c=y，得方程组
   //ax1^2+bx1+c=y1
   //ax2^2+bx2+c=y2
   //ax3^2+bx3+c=y3
   //解方程组得抛物线的a,b,c
   b = ((y1-y3)*(x1*x1-x2*x2)-(y1-y2)*(x1*x1-x3*x3))/((x1-x3)*(x1*x1-x2*x2)-(x1-x2)*(x1*x1-x3*x3));
   a = ((y1-y2)-b*(x1-x2))/(x1*x1-x2*x2);
   c = y1-a*x1*x1-b*x1;
  }

  /**
   * 绘出炮弹
   * @param g
   */
  public void drawBullets(Graphics g){
   if(isfire) {
    int k = (int)(a*bulletX*bulletX+b*bulletX+c);
    setPosition(bulletX, k);
    paint(g);
    //炮弹的速度
    bulletX += shotSpeed;
    //炮弹将消失于小船的水平线
    if(k > bulletY){
     nextFrame();
     if(getFrame() == 2){
      isfire = false;
      setFrame(0);
     }
    }
   }
  }
 }

 二。更多学习资料

http://ajava.org/course/j2me/2475.html

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1.程序实现抛物线运动。

 x+=vx;

vy+=a;

y+=vy;

2.数学实现抛物线运动。

即根据发射点坐标与射程和射程高度。解方程。

缺点：必须支持小数。

3.物理。实现抛物线运动

发射的初始速度为V.与水平方向的夹角为@。

则水平方向的分速度：vx=v*cos@;

则竖直方向的分速度：vx=v*sin@;

水平方向运动：x+=vx;

竖直方向运动：v2=v1+g;

y+=(

v2^2-V1^2

)/(2*g);