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DESCRIPTION
design patterns,pptTRANSCRIPT
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*Design Patterns*How Design Pattern Solve Design ProblemsFinding Appropriate Objects:
Many objects in design come from the analysis model, but O-O designs often end up with additional higher-level classes that have no physical counterparts. Composite pattern can be used to support these high level abstraction that support flexible designs. Objects representing a process or algorithm dont occur in nature, yet are an important part of flexible designs. The Strategy pattern describes how to implement interchangeable families of algorithms. State pattern represents each state of an entity as an object. These objects are typically only discovered late in the course of making a design more flexible and reusable.
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*Design Patterns*
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*Design Patterns*
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*Design Patterns*
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*Design Patterns*
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*Design Patterns*
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*Design Patterns*
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*Design Patterns*
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*Design Patterns*
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*Design Patterns*
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*Design Patterns*
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*Design Patterns*
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*Design Patterns*
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*Design Patterns*
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*Design Patterns*
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*Design Patterns*
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*Design Patterns*
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*Design Patterns*
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*Design Patterns*
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*Design Patterns*
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*Design Patterns*
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*Design Patterns*How Design Pattern Solve Design ProblemsDetermining Object Granularity:
Objects can vary tremendously in size and number. They can represent a variety of things, from hardware to entire applications. How do we decide what should be an object? Design patterns can help to address this problem. Faade pattern describes how to represent complete subsystems as objects. Flyweight pattern describes how to support huge numbers of objects at the finest granularity. Abstract Factory and Builder yields objects whose only responsibilities are to create other objects. Visitor and Command patterns yield objects whose only responsibilities are to implement a request on another object or group of objects
Design Patterns
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*Design Patterns*How Design Pattern Solve Design ProblemsSpecifying Object Interfaces:
Operation (method) declaration for an object includes the operations name, objects it takes as parameters, the operations return value. Known as the operations signature. The set of all signatures for an object is called the interface to the object. An objects interface characterizes the complete set of requests that can be sent to the object. Any request that matches a signature in the objects interface may be sent to the object Design patterns help you define interfaces by identifying their key elements and the kinds of data that get sent across an interface. For example, both Decorator and Proxy patterns require the interfaces of Decorator and Proxy objects to be identical to the decorated and proxied objects. In a Visitor pattern, the Visitor interface must reflect all classes of objects that visitors can visit.
Design Patterns
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*Design Patterns*How Design Pattern Solve Design ProblemsSpecifying Object Implementations:
An objects implementation is defined by its class. An abstract class is one whose main purpose is to define a common interface for its subclasses. A abstract class is not instantiated. Class versus Interface Inheritance: Class inheritance defines an objects implementation in terms of another objects implementation. Its a mechanism for code and implementation sharing. In contrast, interface inheritance (or subtyping) describes when an object can be used in place of another. Many design patterns depend on this distinction. Chain of Responsibility must have a common type, but usually dont share a common implementation. In Composite pattern, Component object defines a common interface, but Composite object often defines a common implementation. Command, Observer, State and Strategy are often implemented with abstract classes that are pure interfaces.
Design Patterns
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*Design Patterns*How Design Pattern Solve Design ProblemsProgramming to an Interface, not an Implementation:
Two main benefits to manipulating objects solely in terms of the interface defined by abstract classes, as opposed to class inheritance: Clients remain unaware of the specific types of objects they use, as long as the objects adhere to the interface that clients expect. Clients remain unaware of the classes that implement these objects. Clients only know about the abstract class(es) defining the interface. This greatly reduces implementation dependencies between subsystems and leads to the following principle: Programming to an Interface, not an Implementation
Design Patterns
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*Design Patterns*How Design Pattern Solve Design ProblemsPutting Reuse Mechanisms to Work:
Inheritance versus Composition:
Reuse by subclassing is often referred to as white box reuse. Why?
Object composition is an alternative to class inheritance (subclassing).New functionality is obtained by composing objects. Called black boxreuse because no internal details of objects are visible.
Class Inheritance defined statically at compile time and is relatively straightforward to use. Inheritance exposes a subclass to details of itsparents implementation => inheritance breaks encapsulation.
Object composition defined dynamically at run-time through objectsacquiring references to other objects => respect for object interface.
Design Patterns
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*Design Patterns*How Design Pattern Solve Design ProblemsBecause (in composition) objects are accessed solely through theirinterfaces, encapsulation is broken. Any object can be replaced at run-time by another as long as it has the same type (interface). Favoring composition over inheritance helps keep each class encapsulatedand focused on one task. Class hierarchies will remain small andshallow. Favor object composition over class inheritance.
Delegation - a way of making composition as powerful as inheritance. In delegation, two objects are involved in handling a request: a receivingobject delegates its operations to its delegate.
Example: Consider an alternative to defining Window as a subclass of a Rectangle.
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*Design Patterns*How Design Pattern Solve Design ProblemsDisadvantage of delegation approach, highly parameterized software isharder to understand than more static software.
State, Strategy, and Visitor patterns depend on delegation.
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*Design Patterns*Designing for Change through PatternsThe key to maximizing reuse lies in anticipating new requirements and changes to existing requirements, and in designing your systemsso that they can evolve accordingly.Some common causes of redesign and patterns to address them:
1 Creating an object by specifying a class explicitly. Specifying a class name when you create an object commits you to a particularimplementation instead of a particular interface. This commitmentcan complicate future changes. To avoid it, create objects indirectly. Design patterns: Abstract Factory, Factory Method, Prototype.
2 Dependence on specific operations. By avoiding hard-coded requests, you make it easier to change the way a request gets satisfied both at compile-time and at run-time. Design patterns: Chain of Responsibility, Command.
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*Design Patterns*Designing for Change through Patterns3 Dependence on hardware and software platform. In general,it's important to design your system to limit its platform dependencies.
Design patterns: Abstract Factory, Bridge.
4 Dependence on object representations or implementations. Clients that know how an object is represented, stored, located, or implemented might need to be changed when the object changes.Hiding this information from clients keeps changes from cascading.
Design patterns: Abstract Factory, Bridge, Memento, Proxy.
5 Algorithmic dependencies. Objects that depend on an algorithm will have to change when the algorithm changes. Therefore algorithms that are likely to change should be isolated.
Design patterns: Builder, Iterator, Strategy, Template, Method, Visitor.
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*Design Patterns*Designing for Change through Patterns6 Tight coupling. Classes that are tightly coupled are hard toreuse in isolation, since they depend on each other.
Design patterns: Abstract Factory, Bridge, Chain of Responsibility,Command, Facade, Mediator, Observer.
7 Extending functionality by subclassing.
Design patterns: Bridge, Chain of Responsibility, Composite, Decorator, Observer, Strategy.
8 Inability to alter classes conveniently.
Design patterns: Adapter, Decorato, Visitor.
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*Design Patterns*How to Select a Design Patterns1. Consider how design patterns solve design problems
2. Scan Intent sections.
3. Study how patterns interrelate.
4. Study patterns of like purpose.
5. Examine a cause of redesign.
6. Consider what should be variable in your design.
Design Patterns
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*Design Patterns*Designing for Change through PatternsPurposeDesign Pattern Aspect(s) That Can Vary
CreationalAbstract Factory families of product objectsBuilder how a composite object gets createdFactory Method subclass of object that is instantiatedPrototype class of object that is instantiatedSingleton the sole instance of a classStructuralAdapter interface to an objectBridge implementation of an objectComposite structure and composition of an objectDecorator responsibilities of an object without subclassingFacade interface to a subsystemFlyweight storage costs of objectsProxy how an object is accessed; its location
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*Design Patterns*Designing for Change through Patterns BehavioralChain of Resp. object that can fulfill a requestCommand when and how a request is fulfilled Interpreter grammar and interpret. of a languageIterator how an aggregate's elements are accessed, traversedMediator how and which objects interact with each otherMemento what private information is stored outside an object, and whenObserver number of objects that depend on another object State states of an objectStrategy an algorithmTemplate Method steps of an algorithmVisitor operations that can be applied to object(s) without changing their class(es)
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*Design Patterns*How to Use a Design Patterns1. Read the pattern once through for an overview
2. Go back and study the Structure, Participants, and Collaborations sections.
3. Look at the Sample Code section to see a concrete example of the pattern in code
4. Choose names for pattern participants that are meaningful in the application context
5. Define the classes.
6. Define application-specific names for operations in the pattern.
7. Implement the operations to carry out the responsibilities and collaborations in the pattern
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*Design Patterns*How Not to Use a Design PatternsDesign patterns should not be applied indiscriminately.Often they achieve flexibility and variability by introducingadditional levels of interaction, that can complicate adesign and/or cost you some performance. A design pattern should only be applied when the flexibility itaffords is actually needed. The Consequences sectionsare most helpful when evaluating a patterns benefitsand liabilities.
Design Patterns