Java WindowAdapter适配器类

  并不是所有的事件处理都像按钮点击那样简单。在正规的程序中，往往希望用户在确认没有丢失所做工作之后再关闭程序。当用户关闭框架时，可能希望弹出一个对话框来警告用户没有保存的工作有可能会丢失，只有在用户确认之后才退出程序。

  当程序用户试图关闭一个框架窗口时，JFrame对象就是WindowEvent的事件源。如果希望捕获这个事件，就必须有一个适合的监听器对象，并将它添加到框架的窗口监听器列表中。

  窗口监听器必须是实现WindowListener接口的类的一个对象。在WindowListener接口中包含7个方法。当发生窗口事件时，框架将调用这些方法响应7个不同的事件。从它们的名字就可以得知其作用，唯一的例外是在Windows下，通常将iconified称为minimized。

public interface WindowListener extends EventListener {
    /**
     * Invoked the first time a window is made visible.
     */
    public void windowOpened(WindowEvent e);

    /**
     * Invoked when the user attempts to close the window
     * from the window's system menu.
     */
    public void windowClosing(WindowEvent e);

    /**
     * Invoked when a window has been closed as the result
     * of calling dispose on the window.
     */
    public void windowClosed(WindowEvent e);

    /**
     * Invoked when a window is changed from a normal to a
     * minimized state. For many platforms, a minimized window 
     * is displayed as the icon specified in the window's 
     * iconImage property.
     * @see java.awt.Frame#setIconImage
     */
    public void windowIconified(WindowEvent e);

    /**
     * Invoked when a window is changed from a minimized
     * to a normal state.
     */
    public void windowDeiconified(WindowEvent e);

    /**
     * Invoked when the Window is set to be the active Window. Only a Frame or
     * a Dialog can be the active Window. The native windowing system may
     * denote the active Window or its children with special decorations, such
     * as a highlighted title bar. The active Window is always either the
     * focused Window, or the first Frame or Dialog that is an owner of the
     * focused Window.
     */
    public void windowActivated(WindowEvent e);

    /**
     * Invoked when a Window is no longer the active Window. Only a Frame or a
     * Dialog can be the active Window. The native windowing system may denote
     * the active Window or its children with special decorations, such as a
     * highlighted title bar. The active Window is always either the focused
     * Window, or the first Frame or Dialog that is an owner of the focused
     * Window.
     */
    public void windowDeactivated(WindowEvent e);
}

  在Java中，实现一个接口的任何类都必须实现其中的所有方法；在这里，意味着需要实现7个方法。然而这里如果我们只对名为windowClosing的方法感兴趣。

  当然，可以这样定义实现这个接口的类：在windowClosing方法中增加一个对System.out.println("TerminatorListener--windowClosing");的调用，其它6个方法不做任何事情：

 

class TerminatorListener implements WindowListener {

    public void windowClosing(WindowEvent e) {
        System.out.println("TerminatorListener--windowClosing");
    }

    @Override
    public void windowOpened(WindowEvent e) {
        // throw new UnsupportedOperationException("Not supported yet.");
    }

    @Override
    public void windowClosed(WindowEvent e) {
        //throw new UnsupportedOperationException("Not supported yet.");
    }

    @Override
    public void windowIconified(WindowEvent e) {
        //throw new UnsupportedOperationException("Not supported yet.");
    }

    @Override
    public void windowDeiconified(WindowEvent e) {
        //throw new UnsupportedOperationException("Not supported yet.");
    }

    @Override
    public void windowActivated(WindowEvent e) {
        //  throw new UnsupportedOperationException("Not supported yet.");
    }

    @Override
    public void windowDeactivated(WindowEvent e) {
        //  throw new UnsupportedOperationException("Not supported yet.");
    }
}

  书写6个没有任何操作的方法代码显然是一种乏味的工作。鉴于简化的目的，每个含有多个方法的AWT监听器接口都配有一个适配器（adapter）类，这个类实现了接口中的所有方法，但每个方法没有做任何事情。这意味着适配器类自动地满足了Java实现相关监听器接口的技术需求。可以通过扩展适配器类来指定对某些事件的响应动作，而不必实现接口中的每个方法（ActionListener这样的接口只有一个方法，因此没必要提供适配器类）。

  下面使用窗口适配器。首先定义一个WindowAdapter类的扩展类，其中包含继承的6个没有做任何事情的方法和一个覆盖的方法windowClosing：

class TerminatorAdapter extends WindowAdapter {

    public void windowClosing(WindowEvent e) {
        System.out.println("TerminatorAdapter---windowClosing");
    }
}

  现在，可以将一个Terminator对象注册为事件监听器：

WindowListener listenerAdapter = new TerminatorAdapter();
adapterFrame.addWindowListener(listenerAdapter);

  只要框架产生了窗口事件，就会通过调用7个方法之中的一个方法将事件传递给listener对象，其中6个方法没有做过任何事情；windowClosing方法调用System.exit(0)终止应用程序的执行。

  警告：如果在扩展适配器类时将方法名拼写错了，编译器不会捕获到这个错误。例如，如果在WindowAdapter类中定义一个windowIsClosing方法，就会得到一个包含8个方法的类，并且windowClosing方法没有做任何事情。

  创建一个扩展于WindowAdapter的监听器类是一个很好的改进，但是还可以继续改进。事实上，没有必要为listener对象命名。只需写成：

adapterFrame.addWindowListener(new WindowAdapter());

  不要就此止步！我们可以将监听器类定义为框架的匿名内部类。

adapterFrame.addWindowListener(new WindowAdapter() {

     public void windowClosing(WindowEvent e) {
         System.out.println("Terminator--windowClosing");
     }
});

  这段代码具有下列作用：

  定义了一个扩展于WindowAdapter类的无名类。

  将windowClosing方法添加到匿名类中。

  从WindowAdapter继承6个没有做任何事情的方法。

  创建这个类的一个对象，这个对象没有名字。

  将这个对象传递给addWindowListener方法。

完整示例代码如下：

import java.awt.EventQueue;
import javax.swing.JFrame;
import javax.swing.JPanel;
import java.awt.event.WindowListener;
import java.awt.event.WindowEvent;
import java.awt.event.WindowAdapter;

public class ListenerTest {

    public static void main(String[] args) {
        EventQueue.invokeLater(new Runnable() {

            public void run() {
                AdapterFrame adapterFrame = new AdapterFrame();
                WindowListener listenerListener = new TerminatorListener();
                adapterFrame.addWindowListener(listenerListener);
                
                WindowListener listenerAdapter = new TerminatorAdapter();
                 adapterFrame.addWindowListener(listenerAdapter);
                
                adapterFrame.addWindowListener(new WindowAdapter() {

                    public void windowClosing(WindowEvent e) {
                        System.out.println("Terminator--windowClosing");
                    }
                });

                adapterFrame.setVisible(true);
            }
        });
    }
}

class AdapterFrame extends JFrame {

    public AdapterFrame() {
        setTitle("AdapterTest");
        setSize(DEFAULT_WIDTH, DEFALUT_HEIGHT);

        adapterPanel = new JPanel();

        add(adapterPanel);
    }
    private JPanel adapterPanel;
    public static final int DEFAULT_WIDTH = 300;
    public static final int DEFALUT_HEIGHT = 200;
}

class TerminatorAdapter extends WindowAdapter {

    public void windowClosing(WindowEvent e) {
        System.out.println("TerminatorAdapter---windowClosing");
    }
}

class TerminatorListener implements WindowListener {

    public void windowClosing(WindowEvent e) {
        System.out.println("TerminatorListener--windowClosing");
    }

    @Override
    public void windowOpened(WindowEvent e) {
        // throw new UnsupportedOperationException("Not supported yet.");
    }

    @Override
    public void windowClosed(WindowEvent e) {
        //throw new UnsupportedOperationException("Not supported yet.");
    }

    @Override
    public void windowIconified(WindowEvent e) {
        //throw new UnsupportedOperationException("Not supported yet.");
    }

    @Override
    public void windowDeiconified(WindowEvent e) {
        //throw new UnsupportedOperationException("Not supported yet.");
    }

    @Override
    public void windowActivated(WindowEvent e) {
        //  throw new UnsupportedOperationException("Not supported yet.");
    }

    @Override
    public void windowDeactivated(WindowEvent e) {
        //  throw new UnsupportedOperationException("Not supported yet.");
    }
}

运行结果：

关闭AdapterTest窗口，显示结果：

TerminatorListener--windowClosing
TerminatorAdapter---windowClosing
Terminator--windowClosing