Chain of Responsibility Pattern in Java

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The Chain of Responsibility pattern allows a number of classes to attempt to handle a request, without any of them knowing about the capabilities of the other classes It provides a loose coupling between these classes; the only common link is the request that is passed between them The request is passed along until one of the classes can handle it One example of such a pattern is a Help system like the one shown in Figure 161 Here, every screen region of an application invites you to seek help, but in some window background areas, more generic help is the only suitable result Figure 161 A simple application where different screen areas provide different help messages
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When you select an area for help, that visual control forwards its ID or name to the chain Suppose that you selected the New button If the first module can handle the New button, it displays the help message; otherwise, it forwards the request to the next module Eventually, the message is forwarded to an All buttons class that can display a general message about how buttons work If no general button help is available, the message is forwarded to the general help module that tells you how the system works If that doesn't exist, the message is lost, and no information is displayed This is illustrated in Figure 162 Figure 162 A simple Chain of Responsibility pattern
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There are two significant points we can observe from this example:
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1 The chain is organized from most specific to most general 2 There is no guarantee that the request will produce a response in all cases We will see later that the Observer pattern defines how multiple classes can be notified of a change
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Applicability
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The Chain of Responsibility is a good example of a pattern that helps to keep separate the knowledge of what each object in a program can do That is, it reduces the coupling between objects so that they can act independently This also applies to the object that constitutes the main program and contains instances of the other objects You will find this pattern helpful when
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Several objects have similar methods that could be appropriate for the action that the program is requesting However, it is more appropriate for the objects to decide which one is to carry out the action than it is for you to build this decision into the calling code One of the objects might be most suitable, but you don't want to build in a series of if-else or switch statements to select a particular object There might be new objects that you want to add to the list of processing options while the program is executing Sometimes more than one object will have to act on a request, and you don't want to build knowledge of these interactions into the calling program
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Sample Code
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The help system described previously is a little involved for a first example So let's start with a simple visual command-interpreter program, shown in Figure 163, that illustrates how the chain works This program displays the results of typed commands While this first case is constrained to keep the example code tractable, we'll see that this Chain or Responsibility pattern is commonly used for parsers and even compilers Figure 163 A simple visual command-interpreter program that acts on one of four panels, depending on the command you type in
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In this example, the commands may be any of the following:
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Image filenames General filenames Color names Any other commands
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In the first three cases, we can display a concrete result of the request, and in the last case, we can display only the request text itself This system responds to commands as follows: 1 We type in "Mandrill," and see a display of the image Mandrilljpg 2 We type in "FileList," and that filename is highlighted in the center list box 3 We type in "blue," and that color is displayed in the lower center panel If we type in anything that is neither a filename nor a color, that text is displayed in the right-hand list box This sequence of steps is shown in Figure 164 Figure 164 The command chain for the program in Figure 163
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