Gang of four design patterns in java pdf free download

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Your email address will not be published. Pankaj I love Open Source technologies and writing about my experience about them is my passion. Follow Author. Comments chandra says:. December 2, at am. Laghmouchi says:. November 10, at am. Shubham Chaudhary says:. March 16, at pm. March 30, at am. March 2, at am. January 24, at pm. Quoc says:. June 15, at am. Dinesh says:. January 16, at am.

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April 15, at pm. April 6, at pm. Rohan says:. March 4, at pm. All actions the user can take with the document, ranging from entering text, changing formatting, quitting, saving, etc. Each menu item, rather than being instantiated with a list of parameters, is instead done with a Command object. Command is an abstract object that only has a single abstract Execute method.

Derivative objects extend the Execute method appropriately i. Execute would utilize the content's clipboard buffer. These objects can be used by widgets or buttons just as easily as they can be used by menu items.

To support undo and redo, Command is also given Unexecute and Reversible. In derivative classes, the former contains code that will undo that command, and the latter returns a boolean value that defines if the command is undoable.

Reversible allows some commands to be non-undoable, such as a Save command. All executed Commands are kept in a list with a method of keeping a 'present' marker directly after the most recently executed command.

A request to undo will call the Command. Unexecute directly before 'present', then move 'present' back one command. Conversely, a Redo request will call Command. Execute after 'present', and move 'present' forward one.

This Command approach is an implementation of the Command pattern. It encapsulates requests in objects, and uses a common interface to access those requests.

Thus, the client can handle different requests, and commands can be scattered throughout the application. This is the document editor's ability to textually analyze the contents of a document. Although there are many analyses that can be performed, spell check and hyphenation-formatting are the focus. Removing the integer-based index from the basic element allows for a different iteration interface to be implemented. This will require extra methods for traversal and object retrieval. These methods are put into an abstract Iterator interface.

Each element then implements a derivation of the Iterator , depending on how that element keeps its list ArrayIterator , LinkListIterator , etc. Functions for traversal and retrieval are put into the abstract Iterator interface. Future Iterators can be derived based on the type of list they will be iterating through, such as Arrays or Linked Lists.

Thus, no matter what type of indexing method any implementation of the element uses, it will have the appropriate Iterator. This is an implementation of the Iterator pattern. It allows the client to traverse through any object collection, without needing to access the contents of the collection directly, or be concerned about the type of list the collection's structure uses. Now that traversal has been handled, it is possible to analyze the elements of a structure. It is not feasible to build each type of analysis into the element structure themselves; every element would need to be coded, and much of the code would be the same for similar elements.

Instead, a generic CheckMe method is built into the element's abstract class. Each Iterator is given a reference to a specific algorithm such as spell check, grammar check, etc. When that Iterator iterates through its collection, it calls each element's CheckMe , passing the specified algorithm.

CheckMe then passes a reference to its element back to said algorithm for analysis. Thus, to perform a spell check, a front-to-end iterator would be given a reference to a SpellCheck object. The iterator would then access each element, executing its CheckMe method with the SpellCheck parameter. Each CheckMe would then call the SpellCheck , passing a reference to the appropriate element. In this manner, any algorithm can be used with any traversal method, without hard-code coupling one with the other.

For example, Find can be used as 'find next' or 'find previous', depending on if a 'forward' iterator was used, or a 'backwards' iterator. In addition, the algorithms themselves can be responsible for dealing with different elements. For example, a SpellCheck algorithm would ignore a Graphic element, rather than having to program every Graphic -derived element to not send themselves to a SpellCheck. Creational patterns are ones that create objects, rather than having to instantiate objects directly.

This gives the program more flexibility in deciding which objects need to be created for a given case. These concern class and object composition. They use inheritance to compose interfaces and define ways to compose objects to obtain new functionality. Most of these design patterns are specifically concerned with communication between objects.

Criticism has been directed at the concept of software design patterns generally, and at Design Patterns specifically. When I see patterns in my programs, I consider it a sign of trouble. The shape of a program should reflect only the problem it needs to solve. Any other regularity in the code is a sign, to me at least, that I'm using abstractions that aren't powerful enough-- often that I'm generating by hand the expansions of some macro that I need to write.

In an interview with InformIT in , Erich Gamma stated that the book authors had a discussion in on how they would have refactored the book and concluded that they would have recategorized some patterns, added a few additional ones and removed one of them Singleton altogether.

Introduction, Chapter 1 [ edit ] Chapter 1 is a discussion of object-oriented design techniques, based on the authors' experience, which they believe would lead to good object-oriented software design, including: 'Program to an 'interface', not an ' implementation'. Design Patterns Elements Of Reusable The authors refer to inheritance as white-box reuse , withwhite-box referring to visibility, because the internals of parent classes are often visible to subclasses.

The danger is stated as follows: 'Because inheritance exposes a subclass to details of its parent's implementation, it's often said that 'inheritance breaks encapsulation'. Gang of Four They warn that the implementation of a subclass can become so bound up with the implementation of its parent class that any change in the parent's implementation will force the subclass to change.

The authors admit that delegation and parameterization are very powerful but add a warning: 'Dynamic, highly parameterized software is harder to understand and build than more static software. The seven problems including their constraints and their solutions including the pattern s referenced , are as follows: Document Structure [ edit ] The document is 'an arrangement of basic graphical elements' such as characters, lines, other shapes, etc.

Problems and Constraints Text and graphics should be treated the same way that is, graphics aren't a derived instance of text, nor vice versa The implementation should treat complex and simple structures the same way. It should not have to know the difference between the two.

Specific derivatives of abstract elements should have specialized analytical elements. Solution and Pattern A recursive composition is a hierarchical structure of elements, that builds 'increasingly complex elements out of simpler ones' pp. Formatting [ edit ] Formatting differs from structure. Problems and Constraints Balance between formatting quality, speed and storage space Keep formatting independent uncoupled from the document structure. Solution and Pattern A Compositor class will encapsulate the algorithm used to format a composition.

Embellishing the User Interface [ edit ] The ability to change the graphical interface that the user uses to interact with the document.

Problems and Constraints Demarcate a page of text with a border around the editing area Scroll bars that let the user view different parts of the page User interface objects should not know about the embellishments Avoid an 'explosion of classes' that would be caused by subclassing for 'every possible combination of embellishments' and elements p. Problems and Constraints The editor must implement standards of multiple platforms so that it is portable Easily adapt to new and emergent standards Allow for run-time changing of look-and-feel i.

Iterator read tutorial Read Iterator Pattern Tutorial This pattern provides a way to access the elements of an aggregate object sequentially without exposing its underlying representation.

Mediator In this pattern communication between objects is encapsulated with a mediator object. Objects no longer communicate directly with each other, but instead communicate through the mediator.

Subject notifies the observers of any state changes. State read tutorial Click to Read State Design Pattern Tutorial State pattern allows an object to alter its behavior when its internal state changes. Strategy read tutorial Strategy Design Pattern Tutorial This pattern defines a family of algorithms, encapsulate each one, and make them interchangeable.

Template Method read tutorial Link to tutorial on Template Method Pattern Pattern defines steps of an algorithm as methods in an interface while allowing subclasses to override some steps in the overall algorithm.

Visitor read tutorial Link to Visitor Design Pattern Tutorial Pattern separates an algorithm from the object structure on which it operates, which provides the ability to add new operations to existing object structures without modifying those structures. Hi, I check your blog regularly. Yoour writinbg style is awesome, keep uup the goo work!

All original content on these pages is fingerprinted and certified by Digiprove. Sign-Up for JavaBrahmanWeekly! Get weekly summary of new articles in your inbox! Creational Design Patterns. Provides an interface for creating families of related or dependent objects without specifying their concrete classes.

Deals with the problem of creating related objects without specifying the exact class of object that will be created. This pattern ensures a class has only one instance and provides a global app-level point of access to it. Specify the kinds of objects to create using a prototypical instance, and create new objects by copying this prototype. Separates the construction of a complex object from its representation, thus enabling the same construction process to create various representations.

Structural Design Patterns.