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methods renamed as xView methods instead (eg, addView instead of addObserver) While it can be good to use the names described in a pattern, it will sometimes make more sense to rename them to fit the actual problem Finally, the ConcreteSubject and ConcreteObservers will be new classes derived from WmvcModel and WmvcView Figure 7-2 shows how everything fits together
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Figure 7-2 Wmvc implementation using Observer pattern Command Pattern in Wmvc One of the goals of the Wmvc Framework is to provide an easy to use interface to menu bar and tool bar commands Even though the details of implementing a menu item, a tool bar button, or other Java Swing components are different, Wmvc provides a uniform interface to each component This uniform interface is provided using the Command design pattern Figure 7-3 shows the structure of the
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Command pattern as given in Design Patterns
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Figure 7-3 Command Design Pattern Structure As we noted earlier, the Command pattern provides a way to encapsulate a command request as an object without the object that issues the command needing to know what the response will be In the case of Wmvc, it is the Swing library that issues a command after user input, and it is a Wmvc application that encapsulates the command response Within Wmvc, the class WmvcExecutor implements the Command class of the pattern Wmvc uses the name Executor rather than Command because it makes more sense within the framework Design patterns are outlines of solutions to problems that can be adapted and applied to specific problems In this case, we adapted the name
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Within the WmvcExecutor class is the execute() method which is invoked by the Swing system in response to user input This is the behavior encapsulated by the Command pattern Each command object (Swing component) invokes the execute() method, and is protected from knowing what happens then Other Patterns used in Wmvc and MovieCat The MovieEditor is implemented as a Singleton pattern The singleton pattern makes sure there will be only one MovieEditor object created We want only one editor active at a time The MovieModel class uses the Iterator pattern to provide access to all the movies This access is used by MovieListView to get the movies for the list The Iterator is also used within MovieModel itself to find the correct place to add a new movie object
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Design Patterns have become an important part of designing object-oriented software The primary reference is the Gang of Four Design Patterns book Patterns are defined using a description template There are twenty-three basic patterns arranged into three classification groups Wmvc and MovieCat use several patterns, including Observer, Command, Singleton, and Iterator
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Resources
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Java Design Patterns: A Tutorial, James W Cooper; AddisonWesley, 2000, ISBN 0-201-48539-7
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Pattern Hatching: Design Patterns Applied, John Vlissides, AddisonWesley, 1998, ISBN 0-201-43293-5 Patterns in Java, A Catalog of Reusable Design Patterns Illustrated with UML, Volume 1, Mark Grand, Wiley, 1998, ISBN 0471258393 Patterns Home Page - hillsidenet/patterns/patternshtml med for Flyheart
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8 Refactoring
Up to this point, we've focused on using object orientation to design and develop new programs Understanding objects, and using objectoriented techniques is one of the best ways to develop programs that are easy to understand, easy to write, and perhaps most importantly, easy to modify and maintain The fact is, most programming involves maintaining and modifying existing code If the existing code is not written in an object-oriented language, then using object orientation probably won't help much with changing that code If the existing code is written in an objectoriented language, then there is hope Unfortunately, as we've noted before, just because a program uses Java or C++ doesn't mean it uses OO techniques And even if the program started out with a decent OO design, if it has been maintained and modified over time, it is likely that it has lost some of its initial elegance So, what are you going to do One of the most recent object-oriented techniques to be formalized and developed into an essential programming tool is called Refactoring While programmers have always spent at least some time cleaning up their code, refactoring takes this a bit farther Refactoring is a specific, disciplined approach to improving the design of existing code With refactoring, the overall design and structure of an existing program is improved, while its observable functionality remains unchanged Once its design
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