java 跳动的小球

/* * To change this template, choose Tools | Templates * and open the template in the editor. */ package flickerball; import java.awt.*; import java.awt.image.BufferedImage; import javax.swing.*; /** * * @author zr */ public class FlickerBall extends JFrame { private int w; private int h; private int radius = 20; private float[] dx; private float[] dy; private int[] lastx, lasty; private int[] x, y; private static int numBalls = 20; final static float SPEED = 5.0f; final static double EPSLION = 1.0E-5; //add later Image offScreenImage = null; Graphics offG = null; public FlickerBall() { Toolkit tool = this.getToolkit(); Dimension d = tool.getScreenSize(); setSize(d); w = d.width; h = d.height; radius = Math.min(w, h) / 50; lastx = new int[numBalls]; lasty = new int[numBalls]; dx = new float[numBalls]; dy = new float[numBalls]; x = new int[numBalls]; y = new int[numBalls]; for (int i = 0; i < numBalls; i++) { lastx[i] = (int) (Math.random() * w); lasty[i] = (int) (Math.random() * h); dx[i] = (float) (2 * Math.random() - 1.0); dy[i] = (float) (2 * Math.random() - 1.0); double t = dx[i] * dx[i] + dy[i] * dy[i]; if (Math.abs(t) < EPSLION) { dx[i] = 1; dy[i] = 0; } else { t = Math.sqrt(t); dx[i] = (float) (t-0.1); dy[i] = (float) t; } } //offScreenImage = createImage(w, h); this code is wrong offScreenImage= new BufferedImage(w, h, BufferedImage.TYPE_INT_RGB); offG = offScreenImage.getGraphics(); } @Override public void paint(Graphics g) { offG.setColor(Color.black); offG.fillRect(0, 0, w, h); offG.setColor(Color.yellow); for (int i = 0; i < numBalls; i++) { offG.fillOval(lastx[i], lasty[i], radius, radius); } for (int i = 0; i < numBalls; i++) { x[i] = (int) (lastx[i] + SPEED * dx[i]); y[i] = (int) (lasty[i] + SPEED * dy[i]); if (x[i] < 0) { x[i] = 0; dx[i] = -dx[i]; } else { if (x[i] > w - radius) { x[i] = w - radius; dx[i] = -dx[i]; } } if (y[i] < 0) { y[i] = 0; dy[i] = -dy[i]; } else { if (y[i] > h - radius) { y[i] = h - radius; dy[i] = -dy[i]; } } lastx[i] = x[i]; lasty[i] = y[i]; } g.drawImage(offScreenImage, 0, 0, this); repaint(1000); } @Override public void update(Graphics g) { paint(g); } /** * @param args the command line arguments */ public static void main(String[] args) { FlickerBall frame = new FlickerBall(); frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); frame.setVisible(true); } }