A detailed introduction to the strategy pattern of Java design patterns

In 1984, I graduated from college with a degree in mechanical engineering and started my career as a software engineer. After self-learning C language, he engaged in the development of a 50,000-line graphical user interface (GUI) for Unix in 1985. The whole process was very relaxing and enjoyable.

At the end of 1985, my coding work was completed, and then I thought about starting other projects-or I thought I could start new projects. But soon I received a series of bug reports and requests for new additions, and I began to work hard to read these 50,000 lines of code to correct the errors. This job is very difficult.

The whole program is like a real house made of cards, which falls down almost every day. Even the smallest change would take me hours to restore stability to the program.

Maybe I happened to discover an important software engineering principle: be relaxed and happy during the development phase, and then find your next job after the project is deployed. However, my difficulties actually stem from my ignorance of the basic principles of object-oriented (OO) software development-Encapsulation. My program is a large collection of switch statements that call different functions in different situations - this leads to tight coupling of the code and makes it difficult for the entire software to adapt to changes.

In JavaDesign PatternsIn this article, I will discuss the strategy pattern, which may be the most basic design pattern. If I had known about the Strategy Pattern in 1984, a lot of this work could have been avoided.

Strategy Pattern

In the first chapter of GOF's design patterns book, the author discusses several OO design principles, which include the core of many design patterns. The Strategy Pattern embodies two principles - Encapsulating changes and Programming the interface rather than programming the implementation. The author of Design Pattern defines the strategy pattern as follows:

Define a family of algorithms, encapsulate each one, and make them interchangeable. [The] Strategy [pattern] lets the algorithm vary independently from clients that use it. (The strategy pattern defines a series of algorithms, encapsulates each algorithm, and makes them interchangeable. The strategy pattern allows the algorithm to vary independently from clients that use it.)

The Strategy pattern builds the entire software as a loosely coupled collection of interchangeable parts rather than a single tightly coupled system. Loosely coupled software is more scalable, easier to maintain, and more reusable.

To understand the strategy pattern, we first look at how Swing uses the strategy pattern to draw borders around components. Next, we discuss the benefits of using the Strategy Pattern in Swing, and finally explain how to implement the Strategy Pattern in your software.

Swing Border

Almost all Swing components can draw borders, including panels,

buttons, lists, etc. Swing also provides a variety of border types for components: bevel (beveled border), etched (embossed border), line (line border), titled (title border) and compound (compound border), etc. The border of the Swing component is drawn using the JComponent class, which is the base class of all Swing components and implements the common functions of all Swing components.

JComponent implements paintBorder(), which is used to draw the border around the component. If the creator of Swing uses a method similar to Example 1 to implement paintBorder():

// A hypothetical JComponent.paintBorder method
protected void paintBorder(Graphics g) {
   switch(getBorderType()) {
      case LINE_BORDER:   paintLineBorder(g);
                          break;
      case ETCHED_BORDER: paintEtchedBorder(g);
                          break;
      case TITLED_BORDER: paintTitledBorder(g);
                          break;
      ...
   }
}

Example 1 Wrong way to draw Swing borders

JComponent in Example 1 The .paintBorder() method hardcodes the drawing of the border in JComponent.

If you want to implement a new border type, you can imagine the result - you need to modify at least three places in the

JComponent class: First, add new fields related to the new border type The integer value. Second, add a case statement to the switch statement. Third, implement the paintXXXBorder() method, XXX represents the border type.

Obviously, extending the previous

paintBorder() is thankless. You will find that not only paintBorder() is difficult to extend new types, but the JComponent class is not the first place you modify it, it is part of the Swing toolkit, which means you will not have to Do not recompile classes and rebuild the entire toolkit. You must also require your users to use your own version of Swing instead of the standard version. This work will still need to be done after the next release of Swing. Also, because you added new border drawing functionality to the JComponent class, whether you like the current situation of every Swing component having access to that functionality or not - you can't restrict your new borders to specific components type.

It can be seen that if the

JComponent class uses the switch statement in example 1 to implement its function, the Swing component cannot be extended.

那么运用OO思想如何实现呢？使用策略模式解耦JComponent与边框绘制的代码，这样无需修改JComponent类就实现了边框绘制算法的多样性。使用策略模式封装变化，即绘制边框方法的变化，以及对接口编程而不是对实现编程，提供一个Border接口。接下来就看看JComponent如何使用策略模式绘制边框。示例2为JComponent.paintBorder()方法：

// The actual implementation of the JComponent.paintBorder() method
protected void paintBorder(Graphics g) {
   Border border = getBorder();
   if (border != null) {
      border.paintBorder(this, g, 0, 0, getWidth(), getHeight());
   }
}

示例2 绘制Swing边框的正确方式

前面的paintBorder()方法绘制了有边框物体的边框。在这种情况下，边框对象封装了边框绘制算法，而不是JComponent类。

注意JComponent把自身的引用传递给Border.paintBorder(),这样边框对象就可以从组件获取信息，这种方式通常称为委托。通过传递自身的引用，一个对象将功能委托给另一对象。

JComponent类引用了边框对象，作为JComponent.getBorder()方法的返回值，示例3为相关的setter方法。

...
private Border border;
...
public void setBorder(Border border) {
   Border oldBorder = this.border;
   this.border = border;
   firePropertyChange("border", oldBorder, border);
   if (border != oldBorder) {
      if (border == null || oldBorder == null || !(border.getBorderInsets(this).
                                    equals(oldBorder.getBorderInsets(this)))) {
         revalidate();
      }       
      repaint();
   }
}
...
public Border getBorder() {
   return border;
}

示例3 Swing组件边框的setter和getter方法

使用JComponent.setBorder()设置组件的边框时，JComponent类触发属性改变事件，如果新的边框与旧边框不同，组件重新绘制。getBorder()方法简单返回Border引用。

图1为边框和JComponent类之间关系的类图。

图1 Swing边框

JComponent类包含Border对象的私有引用。注意由于Border是接口不是类，Swing组件可以拥有任意类型的实现了Border接口的边框(这就是对接口编程而不是对实现编程的含义)。

我们已经知道了JComponent是如何通过策略模式实现边框绘制的，下面创建一种新边框类型来测试一下它的可扩展性。

创建新的边框类型

图2 新边框类型

图2显示了具有三个面板的Swing应用。每个面板设置自定义的边框，每个边框对应一个HandleBorder实例。绘图程序通常使用handleBorder对象来移动对象和改变对象大小。

示例4为HandleBorder类：

import java.awt.*;
import javax.swing.*;
import javax.swing.border.*;
public class HandleBorder extends AbstractBorder {
   protected Color lineColor;
   protected int thick;
   public HandleBorder() {
      this(Color.black, 6);
   }
   public HandleBorder(Color lineColor, int thick) {
      this.lineColor = lineColor;
      this.thick = thick;
   }
   public void paintBorder(Component component, 
                                  Graphics g, int x, int y, int w, int h) {
      Graphics copy = g.create();
      if(copy != null) {
         try {
            copy.translate(x,y);
            paintRectangle(component,copy,w,h);
            paintHandles(component,copy,w,h);
         }
         finally {
            copy.dispose();
         }
      }
   }
   public Insets getBorderInsets() {
      return new Insets(thick,thick,thick,thick);
   }
   protected void paintRectangle(Component c, Graphics g,
                           int w, int h) {
      g.setColor(lineColor);
      g.drawRect(thick/2,thick/2,w-thick-1,h-thick-1);
   }
   protected void paintHandles(Component c, Graphics g,
                           int w, int h) {
      g.setColor(lineColor);
      g.fillRect(0,0,thick,thick); // upper left
      g.fillRect(w-thick,0,thick,thick); // upper right
      g.fillRect(0,h-thick,thick,thick); // lower left
      g.fillRect(w-thick,h-thick,thick,thick); // lower right
      g.fillRect(w/2-thick/2,0,thick,thick); // mid top
      g.fillRect(0,h/2-thick/2,thick,thick); // mid left
      g.fillRect(w/2-thick/2,h-thick,thick,thick); // mid bottom
      g.fillRect(w-thick,h/2-thick/2,thick,thick); // mid right
   }   
}

示例4 HandleBorder类

HandleBorder类继承自javax.swing.border.AbstractBorder，覆盖paintBorder()和getBorderInsets()方法。尽管HandleBorder的实现不太重要，但是我们可以容易地创建新边框类型，因为Swing使用了策略模式绘制组件边框。

示例5为Swing应用。

import javax.swing.*;
import javax.swing.border.*;
import java.awt.*;
import java.awt.event.*;
public class Test extends JFrame {
   public static void main(String[] args) {
      JFrame frame = new Test();
      frame.setBounds(100, 100, 500, 200);
      frame.setDefaultCloseOperation(JFrame.DISPOSE_ON_CLOSE);
      frame.show();
   }
   public Test() {
      super("Creating a New Border Type");
      Container contentPane = getContentPane();
      JPanel[] panels = { new JPanel(), 
                     new JPanel(), new JPanel() };
      Border[] borders = { new HandleBorder(),
                     new HandleBorder(Color.red, 8),
                     new HandleBorder(Color.blue, 10) };
      contentPane.setLayout(
               new FlowLayout(FlowLayout.CENTER,20,20));
      for(int i=0; i < panels.length; ++i) {
         panels[i].setPreferredSize(new Dimension(100,100));
         panels[i].setBorder(borders[i]);
         contentPane.add(panels[i]);
      }
   }
}

示例5 使用handleBorder

前面的应用创建了三个面板(javax.swing.JPanel实例)和三个边框(HandleBorder实例)。注意通过调用JComponent.setBorder()可以为面板简单设置具体的边框。

回想一下示例2，当JComponent调用Border.paintBorder()时，组件引用传递给组件的边框——一种委托方式。正如我前面提到的，开发人员经常将策略模式与委托共同使用。该HandleBorder类未使用组件引用，但是其他边框会用到引用从组件获取信息。比如示例6为这种类型边框javax.swing.border.EtchedBorder的paintBorder()方法：

// The following listing is from
// javax.swing.border.EtchedBorder
public void paintBorder(Component component, Graphics g, int x, int y, 
                         int width, int height) {
   int w = width;
   int h = height;

   g.translate(x, y);

   g.setColor(etchType == LOWERED? getShadowColor(component) : getHighlightColor(component));
   g.drawRect(0, 0, w-2, h-2);

   g.setColor(etchType == LOWERED? getHighlightColor(component) : getShadowColor(component));
   g.drawLine(1, h-3, 1, 1);
   g.drawLine(1, 1, w-3, 1);

   g.drawLine(0, h-1, w-1, h-1);
   g.drawLine(w-1, h-1, w-1, 0);

   g.translate(-x, -y);
}

示例6 从组件获取信息的Swing边框

javax.swing.border.EtchedBorder.paintBorder()方法使用它的组件引用获取组件的阴影和高亮颜色信息。

实现策略模式

策略模式相对比较简单，在软件中容易实现：

1. 为你的策略对象定义Strategy接口

2. 编写ConcreteStrategy类实现Strategy接口

3. 在你的Context类中，保持对“`Strategy“对象的私有引用。

4. 在你的Context类中，实现Strategy对象的settter和getter方法。

Strategy接口定义了Strategy对象的行为；比如Swing边框的Strategy接口为javax.swing.Border接口。

具体的ConcreteStrategy类实现了Strategy接口；比如，Swing边框的LineBorder和EtchedBorder类为ConcreteStrategy类。Context类使用Strategy对象；比如JComponent类为Context对象。

你也可以检查一下你现有的类，看看它们是否是紧耦合的，这时可以考虑使用策略对象。通常情况下，这些包括switch语句的需要改进的地方与我在文章开头讨论的非常相似。

作业

一些Swing组件的渲染和编辑条件比其他的更加复杂。讨论如何在列表类(javax.swing.JList)使用策略模式渲染列表项。

上一次的作业

上一次的作业要求重新实现TableBubbleSortDecorator。在“装饰你的代码”一文首先讨论了JDK内建的对代理模式的支持。

简单来说，我创建了抽象类Decorator实现java.lang.reflect.InvocationHandler接口。Decorator类引用了装饰对象(或者说代理模式中的真实对象)。示例1H为Decorator类。

import java.lang.reflect.InvocationHandler;
public abstract class Decorator implements InvocationHandler {
   // The InvocationHandler interface defines one method:
   // invoke(Object proxy, Method method, Object[] args). That
   // method must be implemented by concrete (meaning not 
   // abstract) extensions of this class.
   private Object decorated;
   protected Decorator(Object decorated) {
      this.decorated = decorated;
   }
   protected synchronized Object getDecorated() {
      return decorated;
   }
   protected synchronized void setDecorated(Object decorated) {
      this.decorated = decorated;
   }
}

示例1H 抽象装饰器类

尽管Decorator类实现了InvocationHandler接口，但是它没有实现该接口的唯一方法invoke(Object proxy, Method method, Object[] methodArguments)。因为Decorator类是抽象的，Decorator的扩展是具体类的话必须实现invoke()方法。

Decorator类是所有装饰器的基类。示例2H为Decorator类的扩展，具体的表排序装饰器。注意TableSortDecorator没有实现invoke()方法，它是抽象的。

import javax.swing.table.TableModel;
import javax.swing.event.TableModelListener;
public abstract class TableSortDecorator 
                                 extends Decorator 
                                 implements TableModelListener {
   // Concrete extensions of this class must implement 
   // tableChanged from TableModelListener
   abstract public void sort(int column);
   public TableSortDecorator(TableModel realModel) {
      super(realModel);
   }
}

示例2H 修正的TableSortDecorator

现在可以使用JDK内建的对代理模式的支持实现TableBubbleSortDecorator：

import java.lang.reflect.Method;
import javax.swing.table.TableModel;
import javax.swing.event.TableModelEvent;
public class TableBubbleSortDecorator extends TableSortDecorator {
   private int indexes[];
   private static String GET_VALUE_AT = "getValueAt";
   private static String SET_VALUE_AT = "setValueAt";
   public TableBubbleSortDecorator(TableModel model) {
      super(model);
      allocate();
   }
   // tableChanged is defined in TableModelListener, which
   // is implemented by TableSortDecorator.
   public void tableChanged(TableModelEvent e) {
      allocate();   
   }
   // invoke() is defined by the java.lang.reflect.InvocationHandler
   // interface; that interface is implemented by the 
   // (abstract) Decorator class. Decorator is the superclass
   // of TableSortDecorator.
   public Object invoke(Object proxy, Method method, 
                                         Object[] args) {
      Object result = null;
      TableModel model = (TableModel)getDecorated();
      if(GET_VALUE_AT.equals(method.getName())) {
         Integer row = (Integer)args[0], 
                 col = (Integer)args[1];
         result = model.getValueAt(indexes[row.intValue()], 
                                           col.intValue());
      }
      else if(SET_VALUE_AT.equals(method.getName())) {
         Integer row = (Integer)args[1],
                 col = (Integer)args[2];
         model.setValueAt(args[0], indexes[row.intValue()],
                                   col.intValue());
      }
      else {
         try {
            result = method.invoke(model, args);
         }
         catch(Exception ex) {
            ex.printStackTrace(System.err);
         }
      }
      return result;
   }
   // The following methods perform the bubble sort ...
   public void sort(int column) {
      TableModel model = (TableModel)getDecorated();
      int rowCount = model.getRowCount();
      for(int i=0; i < rowCount; i++) {
         for(int j = i+1; j < rowCount; j++) {
            if(compare(indexes[i], indexes[j], column) < 0) {
               swap(i,j);
            }
         }
      }
   }
   private void swap(int i, int j) {
      int tmp = indexes[i];
      indexes[i] = indexes[j];
      indexes[j] = tmp;
   }
   private int compare(int i, int j, int column) {
      TableModel realModel = (TableModel)getDecorated();
      Object io = realModel.getValueAt(i,column);
      Object jo = realModel.getValueAt(j,column);
      int c = jo.toString().compareTo(io.toString());
      return (c < 0) ? -1 : ((c > 0) ? 1 : 0);
   }
   private void allocate() {
      indexes = new int[((TableModel)getDecorated()).
                          getRowCount()];
      for(int i=0; i < indexes.length; ++i) {
         indexes[i] = i;         
      }
   }
}

示例3H 修正的TableBubbleSortDecorator

使用JDK内建的对代理模式的支持和设计良好的基类，通过继承Decorator及实现invoke()方法很容易实现装饰器。

邮件

给我的一封邮件里这样写到：

根据我在树上选择的节点工具栏要显示特定的按钮。我创建了工具栏装饰器，它的构造函数参数为JToolBar工具栏。装饰器包含一个showButtonForNode()方法根据节点改变按钮。我调用在树的选择监听器的valueChanged()方法中调用showButtonForNode()方法。
这样使用装饰器模式正确吗？

很多设计模式可以达到功能扩展的目的；比如在Java设计模式中，你已经知道如何使用代理模式，装饰器模式和策略模式来扩展功能。由于他们都可以实现相同的目标(功能扩展)，在具体情况下使用哪个模式就很难判断。

装饰器模式的主要解决问题的点在于：在运行时结合多种行为；比如理解代理设计模式一文的“上一次得作业”部分，我展示了Swing表格排序和过滤相结合的方法。

TableSortDecorator sortDecorator = new TableBubbleSortDecorator(table.getModel());
TableFilterDecorator filterDecorator = new TableHighPriceFilter(sortDecorator);
table.setModel(filterDecorator);

前面的代码中，过滤装饰器装饰了排序装饰器，排序装饰器装饰了表格模型；结果表格模型可以排序和过滤数据。

对于邮件中的问题，使用工具栏按钮与其他行为组合不太合适，所以装饰器模式可能不合适。这种情况代理模式看来更好，在编译阶段而不是运行时就可以获取代理和真实对象的关系，从而扩展功能。

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