JAVA设计模式之适配器模式（adapter）

适配器模式使用案例：

• 数据库连接中的驱动
• java虚拟机
• zookeeper客户端
• redis客户端

误区：

• java中各种带Adapter后缀的类并不是使用适配器模式只是方便程序员编程使用。经典的应用场景如下：

package com.srr.dp.adapter;

import java.awt.*;;
import java.awt.event.*;

public class T extends Frame{

    public static void main(String[] args) {
        T t = new T();
        t.addKeyListener(new KeyAdapter(){
            public void keyPressed(KeyEvent e) {
            }
        });
        t.addWindowListener(new WindowAdapter() {
            @Override
            public void windowOpened(WindowEvent e) {
                super.windowOpened(e);
            }
        });
    }
}

其实我们可以直接使用KeyListener和WindowListener，代码如下：

package com.srr.dp.adapter;

import java.awt.*;;
import java.awt.event.*;

public class T2 extends Frame{

    public static void main(String[] args) {
        T2 t = new T2();
        t.addKeyListener(new KeyListener(){
            @Override
            public void keyTyped(KeyEvent e) {

            }
            public void keyPressed(KeyEvent e) {
            }
            @Override
            public void keyReleased(KeyEvent e) {

            }
        });

        t.addWindowListener(new WindowListener() {

            @Override
            public void windowOpened(WindowEvent e) {

            }

            @Override
            public void windowClosing(WindowEvent e) {

            }

            @Override
            public void windowClosed(WindowEvent e) {

            }

            @Override
            public void windowIconified(WindowEvent e) {

            }

            @Override
            public void windowDeiconified(WindowEvent e) {

            }

            @Override
            public void windowActivated(WindowEvent e) {

            }

            @Override
            public void windowDeactivated(WindowEvent e) {

            }
        });
    }
}

对比发现，使用KeyListener和WindowListener我们需要实现多个方法，想反使用KeyAdapter和WindowAdapter更方便只需实现较少的方法即可，查看源码发现并其实没有使用适配器模式而是

/*
 * Copyright (c) 1996, 2013, Oracle and/or its affiliates. All rights reserved.
 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 */

package java.awt.event;

/**
 * An abstract adapter class for receiving window events.
 * The methods in this class are empty. This class exists as
 * convenience for creating listener objects.
 * <P>
 * Extend this class to create a <code>WindowEvent</code> listener
 * and override the methods for the events of interest. (If you implement the
 * <code>WindowListener</code> interface, you have to define all of
 * the methods in it. This abstract class defines null methods for them
 * all, so you can only have to define methods for events you care about.)
 * <P>
 * Create a listener object using the extended class and then register it with
 * a Window using the window's <code>addWindowListener</code>
 * method. When the window's status changes by virtue of being opened,
 * closed, activated or deactivated, iconified or deiconified,
 * the relevant method in the listener
 * object is invoked, and the <code>WindowEvent</code> is passed to it.
 *
 * @see WindowEvent
 * @see WindowListener
 * @see <a href="https://docs.oracle.com/javase/tutorial/uiswing/events/windowlistener.html">Tutorial: Writing a Window Listener</a>
 *
 * @author Carl Quinn
 * @author Amy Fowler
 * @author David Mendenhall
 * @since 1.1
 */
public abstract class WindowAdapter
    implements WindowListener, WindowStateListener, WindowFocusListener
{
    /**
     * Invoked when a window has been opened.
     */
    public void windowOpened(WindowEvent e) {}

    /**
     * Invoked when a window is in the process of being closed.
     * The close operation can be overridden at this point.
     */
    public void windowClosing(WindowEvent e) {}

    /**
     * Invoked when a window has been closed.
     */
    public void windowClosed(WindowEvent e) {}

    /**
     * Invoked when a window is iconified.
     */
    public void windowIconified(WindowEvent e) {}

    /**
     * Invoked when a window is de-iconified.
     */
    public void windowDeiconified(WindowEvent e) {}

    /**
     * Invoked when a window is activated.
     */
    public void windowActivated(WindowEvent e) {}

    /**
     * Invoked when a window is de-activated.
     */
    public void windowDeactivated(WindowEvent e) {}

    /**
     * Invoked when a window state is changed.
     * @since 1.4
     */
    public void windowStateChanged(WindowEvent e) {}

    /**
     * Invoked when the Window is set to be the focused Window, which means
     * that the Window, or one of its subcomponents, will receive keyboard
     * events.
     *
     * @since 1.4
     */
    public void windowGainedFocus(WindowEvent e) {}

    /**
     * Invoked when the Window is no longer the focused Window, which means
     * that keyboard events will no longer be delivered to the Window or any of
     * its subcomponents.
     *
     * @since 1.4
     */
    public void windowLostFocus(WindowEvent e) {}
}

/*
 * Copyright (c) 1996, 2013, Oracle and/or its affiliates. All rights reserved.
 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 */

package java.awt.event;

/**
 * An abstract adapter class for receiving keyboard events.
 * The methods in this class are empty. This class exists as
 * convenience for creating listener objects.
 * <P>
 * Extend this class to create a <code>KeyEvent</code> listener
 * and override the methods for the events of interest. (If you implement the
 * <code>KeyListener</code> interface, you have to define all of
 * the methods in it. This abstract class defines null methods for them
 * all, so you can only have to define methods for events you care about.)
 * <P>
 * Create a listener object using the extended class and then register it with
 * a component using the component's <code>addKeyListener</code>
 * method. When a key is pressed, released, or typed,
 * the relevant method in the listener object is invoked,
 * and the <code>KeyEvent</code> is passed to it.
 *
 * @author Carl Quinn
 *
 * @see KeyEvent
 * @see KeyListener
 * @see <a href="https://docs.oracle.com/javase/tutorial/uiswing/events/keylistener.html">Tutorial: Writing a Key Listener</a>
 *
 * @since 1.1
 */
public abstract class KeyAdapter implements KeyListener {
    /**
     * Invoked when a key has been typed.
     * This event occurs when a key press is followed by a key release.
     */
    public void keyTyped(KeyEvent e) {}

    /**
     * Invoked when a key has been pressed.
     */
    public void keyPressed(KeyEvent e) {}

    /**
     * Invoked when a key has been released.
     */
    public void keyReleased(KeyEvent e) {}
}

KeyAdapter和WindowAdapter帮我们实现了KeyListener和WindowListener接口

接下来，java中经典的使用适配器模式的代码如下，其实我们经常用，请看下面代码：

package com.mashibing.dp.adapter;

import java.io.BufferedReader;
import java.io.FileInputStream;
import java.io.InputStreamReader;

/**
 * 适配器模式使用案例
 */
public class Main {
    public static void main(String[] args) throws Exception {
        FileInputStream fis = new FileInputStream("D:/test.text");
        /**
         *  InputStreamReader就扮演了适配器的角色，因为BufferedReader无法直接使用InputStream
         */
        InputStreamReader isr = new InputStreamReader(fis);
        BufferedReader br = new BufferedReader(isr);
        String line = br.readLine();
        while (line != null && !line.equals("")) {
            System.out.println(line);
        }
        br.close();
    }
}