scroller.java

java applet 非常全的实例。比较利于初学者进行研究。

import java.awt.*;
import java.applet.*;

public class Scroller extends Applet {
   private Scrollbar vscroll;
   private Scrollbar hscroll;
   private MyCanvas canvas;
   private String name;
   private int imgW,imgH;

   public void init() {
      Image img;
      String param;
      MediaTracker imageTracker;
        
      imageTracker  = new MediaTracker(this);
        
      param = getParameter("image");
      if(param!=null)
         name = param;

      img = getImage(getCodeBase(), name);
      imageTracker.addImage(img,0);
        
      try{
         imageTracker.waitForID(0);
      }catch (InterruptedException e){}
      
      imgW = img.getWidth(this);
      imgH = img.getHeight(this);

      canvas = new MyCanvas(img,imgW,imgH);
      hscroll = new Scrollbar(Scrollbar.HORIZONTAL);
      vscroll = new Scrollbar(Scrollbar.VERTICAL);
      setLayout(new BorderLayout());
      add("Center", canvas);
      add("East", vscroll);
      add("South", hscroll);

      validate();
      R_hor();
      R_ver();
   }

   public boolean handleEvent(Event evt) {
      switch (evt.id) {
         case Event.SCROLL_LINE_UP:
         case Event.SCROLL_LINE_DOWN:
         case Event.SCROLL_PAGE_UP:
         case Event.SCROLL_PAGE_DOWN:
         case Event.SCROLL_ABSOLUTE:
         if (evt.target == vscroll) {
            canvas.dy = ((Integer)evt.arg).intValue();
            canvas.repaint();
         }
         if (evt.target == hscroll) {
            canvas.dx = ((Integer)evt.arg).intValue();
            canvas.repaint();
         }
      }
      return super.handleEvent(evt);
   }

   void R_hor() {
      int canvasW = canvas.size().width;
      if (canvasW <= 0) {
         return;
      }
      if ((canvas.dx + canvasW) > imgW) {
         int newtx = imgW - canvasW;
         if (newtx < 0) {
            newtx = 0;
         }
         canvas.dx = newtx;
      }
      hscroll.setValues(canvas.dx, (int)(canvasW * 0.9), 0, imgW- canvasW);
      hscroll.setPageIncrement((int)(canvasW * 0.9));
      return;
   }
  
   void R_ver() {
      int canvasH = canvas.size().height;
      if (canvasH <= 0) {
         return;
      }
      if ((canvas.dy + canvasH) > imgH) {
         int newty = imgH - canvasH;
         if (newty < 0) {
            newty = 0;
         }
         canvas.dy = newty;
      }
      vscroll.setValues(canvas.dy,(int)(canvasH * 0.9), 0,imgH - canvasH);
      vscroll.setPageIncrement((int)(canvasH * 0.9));
      return;
   }

   public void paint(Graphics g) {
      R_hor();
      R_ver();

      return;
    }
}

class MyCanvas extends Canvas {

   Image image,buffer;
   Graphics grap;
   int dx = 0;
   int dy = 0;
   int width;
   int height;

   MyCanvas(Image img,int w,int h){
      MediaTracker imageTracker;
      imageTracker = new MediaTracker(this);
      image = img;
      width = w;
      height = h;
      imageTracker.addImage(image,0);
      try{
         imageTracker.waitForID(0);
      }catch (InterruptedException e){}
   }
   public void update(Graphics g){
      paint(g);
   }

   public void paint(Graphics g) {
      buffer = createImage(image.getWidth(this),image.getHeight(this));
      grap = buffer.getGraphics();
      grap.translate(-dx, -dy);
      grap.drawImage(image,0,0,this);
      g.drawImage(buffer, 0, 0, getBackground(), this);
   }
}