pnviewer3.java

来自「java 完全探索的随书源码」· Java 代码 · 共 175 行

// PNViewer3.java

// Planetary Nebulae Viewer

import java.awt.*;
import java.awt.event.*;
import java.applet.Applet;

public class PNViewer3 extends Applet implements ItemListener
{
   private Choice c;

   private String [] imageFileNames =
   {
      "mycn18.jpg",
      "ngc2022.jpg",
      "ngc2867.jpg",
      "ngc3918.jpg",
      "ngc5307.jpg",
      "ngc6369.jpg",
      "ngc7009.jpg",
      "ngc7027.jpg"
   };

   private String [] imageTitleNames =
   {
      "Hourglass Nebula",
      "NGC 2022",
      "NGC 2867",
      "NGC 3918",
      "NGC 5307",
      "NGC 6369",
      "NGC 7009",
      "NGC 7027"
   };

   private Image [] im = new Image [imageFileNames.length];

   private Picture pic;

   public void init ()
   {
      // Create a MediaTracker object to track image loading.

      MediaTracker mt = new MediaTracker (this);

      for (int i = 0; i < im.length; i++)
      {
           // Get the next image.

           im [i] = getImage (getDocumentBase (), imageFileNames [i]);

           // Add image to MediaTracker object.

           mt.addImage (im [i], 0);
      }

      // Force all images to load.

      try
      {
           mt.waitForID (0);
      }
      catch (InterruptedException e) {}

      // Ensure that applet background has a lightgray appearance.

      setBackground (Color.lightGray);

      // By default, AWT applets use a flow layout manager.

      setLayout (new BorderLayout ());

      // Create picture component.

      pic = new Picture ();

      // Draw first nebula image.

      pic.draw (im [0]);

      // Add picture component to North region of applet container.
      // At this point, picture component will be visible on screen.

      add ("North", pic);

      // Create a Panel for holding Label and Choice components.

      Panel p = new Panel ();

      Label l = new Label ("Planetary Nebula:");

      // Ensure that Label background has a lightgray appearance.

      l.setBackground (Color.lightGray);

      // Add Label component to Panel.

      p.add (l);

      // Create a Choice component (a combobox) for holding textual
      // items.

      c = new Choice ();

      // When the user selects an item, an item state changed event
      // will be fired.  Register the current applet object to listen
      // for these events, so the image can be automatically updated.

      c.addItemListener (this);

      // Add nebulae names to Choice component.

      for (int i = 0; i < imageTitleNames.length; i++)
           c.add (imageTitleNames [i]);

      // Add Choice component to Panel.

      p.add (c);

      // Add Panel component to South region of applet container.
      // At this point, Label/Choice will be visible on screen.

      add ("South", p);
   }

   public void itemStateChanged (ItemEvent e)
   {
      // When item state changed event is fired, the code in the
      // itemStateChanged () method is called.  In turn, this code
      // will tell the picture component to draw the image that is
      // associated with the Choice component's current index.

      pic.draw (im [c.getSelectedIndex ()]);
   }
}

class Picture extends Canvas
{
   private Image image;     // The current image.

   private final Dimension size = new Dimension (420, 415);

   public void draw (Image image)
   {
      this.image = image;           // Save the current image.
      repaint ();                   // Draw the image.
   }

   public Dimension getPreferredSize ()
   {
      // When the layout manager calls picture's getPreferredSize ()
      // method, this method will return a Dimension object that
      // tells the layout manager how much room to reserve for the
      // picture component.  Because images range in size up to 400 x
      // 400 pixels, all picture components would request a size of
      // 420 x 415.

      return size;
   }

   public void paint (Graphics g)
   {
      // Determine the upper-left image corner (x, y) coordinates so
      // that the image will be centered.

      int x = (getSize ().width - image.getWidth (this)) / 2;
      int y = (getSize ().height - image.getHeight (this)) / 2;

      // Draw the image.

      g.drawImage (image, x, y, this);
   }
}