The Command Pattern in Java

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17 The Command Pattern
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The Chain of Responsibility pattern forwards requests along a chain of classes, but the Command pattern forwards a request only to a specific object It encloses the request for a specific action inside an object and gives it a known public interface It lets you give the client the ability to make requests without knowing anything about the actual action that will be performed and allows you to change that action without affecting the client program in any way
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Motivation
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When you build a Java user interface, you provide controls menu items, buttons, check boxes, and so on to allow the user to tell the program what to do When a user selects one of these controls, the program receives an ActionEvent, which it must trap by implementing the ActionListener interfaces' actionPerformed event Suppose that we build a very simple program that allows us to select the menu items File | Open and File | Exit and click on a button labeled Red that turns the background of the window red This program is shown in Figure 171 Figure 171 A simple program that receives actionPerformed events from the button and menu items
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The program consists of the File Menu object with the mnuOpen and mnuExit MenuItems added to it It also contains one button called btnRed Clicking any of these causes an ActionEvent, which generates a call to the actionPerformed method such as the following:
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public void actionPerformed(ActionEvent e) { Object obj = egetSource(); if (obj == mnuOpen) fileOpen(); //open file if (obj == mnuExit) exitClicked(); //exit from program if (obj == btnRed) redClicked(); //turn red }
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141 Team-Fly
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Here are the three private methods that we call from the actionPerformed method:
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private void exitClicked(){ Systemexit(0); } //-------------private void fileOpen() { FileDialog fDlg = new FileDialog(this, "Open a file", FileDialogLOAD); fDlgshow(); } //-------------private void redClicked() { psetBackground(Colorred); }
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Now, as long as there are only a few menu items and buttons, this approach works fine, but when there are dozens of menu items and several buttons, the actionPerformed code can get pretty unwieldy This also seems a little inelegant, since, when using an OO language such as Java, we want to avoid a long series of if statements to identify the selected object Instead, we want to find a way to have each object receive its commands directly
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Command Objects
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One way to ensure that every object receives its own commands directly is to use the Command pattern and create individual Command objects A Command object always has an Execute method that is called when an action occurs on that object Most simply, a Command object implements at least the following interface:
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public interface Command { public void Execute(); }
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We use this interface to reduce the actionPerformed method to the following:
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public void actionPerformed(ActionEvent e) { Command cmd = (Command)egetSource(); cmdExecute(); }
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Then we can provide the Execute method for each object that carries out the desired action, thus keeping the knowledge of what to do inside of the object where it belongs, instead of having another part of the program make these decisions One important purpose of the Command pattern is to keep the program and user interface objects completely separate from the actions that they initiate In other words, these program objects should be completely separate from each other and should not have to know how other objects work The user interface receives a command and tells a Command object to carry out whatever duties it has been instructed to do The GUI does not and should not need to know what tasks will be executed This decouples the UI class from the execution of specific commands, thereby making it possible to modify or completely change the action code without changing the classes that contain the user interface
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You can also use the Command object to tell the program to execute the command when the resources are available rather than immediately In such cases, you are queuing commands to be executed later Finally, you can use Command objects to remember operations so that you can support Undo requests
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