What are Design Patterns in Java

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What are Design Patterns
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One big challenge in developing software is avoiding reinventing the wheel Using libraries and frameworks is one way to reuse useful tools for many software systems The tools provided by libraries and frameworks solve common, but low-level problems Design patterns deal with solving common design problems at a higher level They do not provide code that can be used directly for a specific problem, but rather a design solution to a software problem that has been found to come up repeatedly over many software projects A design pattern provides a description of a design solution that can be adapted and applied to a specific situation The Gang of Four (GoF) Design Patterns book describes twentythree patterns arranged into three groups The groups help classify how the patterns are used Creational patterns are used to help make a system independent of how the objects it uses are created Structural patterns are concerned with how classes and objects are organized and composed to build larger structures Behavioral patterns are used to deal with assignment of responsibilities to objects and communication between objects Using Design Patterns The complexity of different design patterns ranges from simple to hard For example, the Iterator design pattern is so simple and basic that it has become an essential feature of the standard Java object collections library Others, such as the Composite pattern, are somewhat more complex, and require more experience to recognize when they are useful Most of the patterns are not inherently difficult or complex In fact, good software designers have been using the solutions described by the patterns for years, but without formally recognizing that they really could be described as general design guidelines One of the
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goals of developing design patterns has been to pass on the experience and knowledge of pattern authors to other programmers to help them produce better designs Using design patterns takes some experience You must have at least a basic understanding of all the patterns available, and then be able to recognize when you have a design situation that can be helped by using a pattern Often, this will require nothing more than reading through the patterns to see if any fit the case at hand When a pattern fits, it tends to ring a little bell of recognition The longer you work with patterns, the easier they become to use For the rest of this chapter, we will cover how design patterns are described in the GoF book, go over each of the basic GoF patterns, and finally show how a few patterns have been used in the Wmvc framework and the MovieCat application Design Pattern Description Template The GoF developed a description template that is used in their book to define their patterns The GoF template has also been adopted for descriptions of patterns found in other sources The following table covers the sections included in the GoF pattern description template Each pattern in the GoF book is defined using this layout Section Description
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The name of the pattern, and its Pattern Name and Classification classification (Creational, Structural, or Behavioral) Intent A short statement about what the pattern does
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Alternate well known names for the pattern An illustrative design problem that shows how the pattern can solve the problem Situations where the pattern can be used A graphical (UML) representation showing the classes in the pattern The classes that participate in the pattern and their responsibilities How the participants collaborate Benefits and trade-offs of using the pattern Hints, pitfalls, and techniques that can be used to help implement the pattern Code illustrations of using the pattern Examples of the pattern used in real systems Other patterns closely related to the current one
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