graphapplet.java

这是《Java 2 简明教程(第2版)》一书配套的源代码。

import java.awt.*;
class GraphApplet extends Frame {
    GraphApplet() {
        super("Rectangle Example");
        add("Center", new ImageCanvas());
        setSize(200, 200);
        show();
    }

    public static void main(String[] args) {
        new GraphApplet();
    }
}

class ImageCanvas extends Canvas {
    int numCoins = 3;
    Rectangle[] coins = new Rectangle[numCoins];
    Rectangle[] slots = new Rectangle[numCoins];
    Color[] colors = {Color.red, Color.green, Color.blue};
    Image bbuf;
    Graphics bbufG;

    ImageCanvas() {
        for (int i=0; i<numCoins; i++) {
            coins[i] = new Rectangle(rand(50), rand(50), 50, 50);
            slots[i] = new Rectangle();
        }
    }

    int rand(int r) {
        return (int)Math.floor(Math.random()*r);
    }

    public void paint(Graphics g) {
        update(g);
    }

    public void update(Graphics g) {
        int w = getSize().width;
        int h = getSize().height;

        if (bbuf == null
            || w > bbuf.getWidth(null)
            || h > bbuf.getHeight(null)) {
            bbuf = createImage(w, h);
            bbufG = bbuf.getGraphics();
        }

        bbufG.setColor(Color.white);
        bbufG.fillRect(0, 0, getSize().width, getSize().height);

        // paint slots
        for (int i=0; i<numCoins; i++) {
            slots[i].setBounds(i*w/3, h-30, w/3, 30);
            slots[i].grow(-5, -2);
            bbufG.setColor(colors[i]);
            bbufG.fillRect(slots[i].x, slots[i].y, 
                slots[i].width, slots[i].height);
        }
            
        // paint coins
        for (int i=0; i<numCoins; i++) {
            if (!coins[i].isEmpty()) {
                if (dragging == i && coins[i].intersects(slots[i])) {
                    bbufG.setColor(Color.black);
                } else {
                    bbufG.setColor(colors[i]);
                }
                bbufG.fillOval(coins[i].x, coins[i].y, 
                    coins[i].width, coins[i].height);
            }
        }
        g.drawImage(bbuf, 0, 0, this);
    }

    int dragging = -1;
    Point offset;
    public synchronized boolean mouseDown(Event evt, int x, int y) {
for (int i=coins.length-1; i>=0; i--) {
    if (!coins[i].isEmpty() && coins[i].contains(x, y)) {
                dragging = i;
                offset = new Point(coins[i].x-x, coins[i].y-y);
                return true;
    }
}
        return false;
    }

    public synchronized boolean mouseDrag(Event evt, int x, int y) {
        if (dragging >= 0) {
            coins[dragging].setLocation(x, y);
            coins[dragging].translate(offset.x, offset.y);
            repaint();
            return true;
        }
        return false;
    }

    public synchronized boolean mouseUp(Event evt, int x, int y) {
        if (dragging >= 0) {
            if (coins[dragging].intersects(slots[dragging])) {
                coins[dragging].width = 0;    // make it empty
                dragging = -1;
            }
            repaint();
            return true;
        }
        return false;
    }
}