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import java.awt.BorderLayout;
import javax.swing.*;
import javax.swing.event.TreeSelectionListener;
import javax.swing.event.TreeSelectionEvent;
import javax.swing.tree.DefaultMutableTreeNode;
import java.util.Observable;
import java.util.Observer;

public final class LeftPanel extends JPanel
{// 把樹型選擇視圖布局在左邊
  private JTree tree;// 樹型選擇視圖
  private JScrollPane scroll;// 讓視圖可滾動
  private DefaultMutableTreeNode root, node1, node2;// 根節點及兩個葉子
  private Sensor sensor;// sensor是一個Observable，由于只能單根繼承，所以作為組合成員
  private String file;// 圖片文件名，與RightPanel通信的內容

  public LeftPanel(Observer observer)
  {
    file = "";
    sensor = new Sensor();
    sensor.addObserver(observer);// 向Observable注冊Observer
    root = new DefaultMutableTreeNode("Images");
    tree = new JTree(root);
    node1 = new DefaultMutableTreeNode("Rabbit");
    node2 = new DefaultMutableTreeNode("Devastator");
    root.add(node1);
    root.add(node2);
    tree.addTreeSelectionListener(new TreeSelectionListener()
    {// 樹節點選擇動作
      public void valueChanged(TreeSelectionEvent e)
      {
        Object obj = e.getPath().getLastPathComponent();
        if (obj instanceof DefaultMutableTreeNode)
        {
          DefaultMutableTreeNode node = (DefaultMutableTreeNode)obj;
          if (node == root)
            file = "";// 選擇根
          if (node == node1)
            file = "rabbit.jpg";// 選擇node1
          if (node == node2)
            file = "devastator.gif";// 選擇node2
          sensor.setData(file);// 改變Observable
          sensor.notifyObservers();// 通知observer，對象已改變
        }
      }
    });
    scroll = new JScrollPane(tree);
    add(scroll, BorderLayout.CENTER);
  }

  public Observable getSensor()
  {// 返回Observable對象，使Observer可以獲取
    return sensor;
  }
}

class Sensor extends Observable
{// 定義自己的Observable
  private Object data;

  public void setData(Object newData)
  {
    data = newData;
    setChanged();// 改變Observable
    System.out.println("Data changed!");
  }

  public Object getData()
  {
    return data;
  }
}

import java.awt.*;
import javax.swing.JPanel;
import java.util.Observer;
import java.util.Observable;

public class RightPanel extends JPanel implements Observer
{// 把圖片瀏覽視圖布局在右邊
  private Image image;

  public void update(Observable subject, Object obj)
  {// 定義接收到Observable變化后的響應動作
    String file = (String)((Sensor)subject).getData();
    if (!file.equals(""))
    {
      image = Toolkit.getDefaultToolkit().getImage(file);
      MediaTracker tracker = new MediaTracker(this);// 定義圖像跟蹤
      tracker.addImage(image, 0);
      try
      {
        tracker.waitForID(0);// 等待圖像的完全加載
      }
      catch (InterruptedException e)
      {
        e.printStackTrace();
      }
    }
    else
      image = null;
    repaint();// 重繪組件
  }

  public void paintComponent(Graphics g)
  {
    g.setColor(Color.LIGHT_GRAY);
    g.fillRect(0, 0, getWidth() - 1, getHeight() - 1);// 先將組件上的畫面清除
    if (image != null)
      g.drawImage(image, 0, 0, this);// 繪制新的圖像
  }
}

import java.awt.*;
import javax.swing.JFrame;

public class MainFrame extends JFrame
{// 演示窗口
  public static void main(String[] args)
  {
    MainFrame frame = new MainFrame();
    RightPanel right = new RightPanel();
    LeftPanel left = new LeftPanel(right);// 注冊Observer
    frame.getContentPane().add(left, BorderLayout.WEST);
    frame.getContentPane().add(right, BorderLayout.CENTER);
    frame.setTitle("Observer Test");
    frame.setSize(400, 300);
    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    frame.setVisible(true);
  }
}