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Overview[ edit ] The Factory Method  design pattern is one of the twenty-three well-known GoF design patterns that describe how to solve recurring design problems to design flexible and reusable object-oriented software, that is, objects that are easier to implement, change, test, and reuse. The Factory Method design pattern solves problems like: How can an object be created so that subclasses can redefine which class to instantiate? How can a class defer instantiation to subclasses?
Creating an object directly within the class that requires uses the object is inflexible because it commits the class to a particular object and makes it impossible to change the instantiation independently from without having to change the class. The Factory Method design pattern describes how to solve such problems: Define a separate operation factory method for creating an object.
Create an object by calling a factory method. This enables writing of subclasses to change the way an object is created to redefine which class to instantiate. See also the UML class diagram below. Definition[ edit ] "Define an interface for creating an object, but let subclasses decide which class to instantiate. The Factory method lets a class defer instantiation it uses to subclasses. The factory method design pattern handles these problems by defining a separate method for creating the objects, which subclasses can then override to specify the derived type of product that will be created.
The factory method pattern relies on inheritance, as object creation is delegated to subclasses that implement the factory method to create objects. Instead, the Creator refers to a separate factoryMethod to create a product object, which makes the Creator independent of which concrete class is instantiated.
Subclasses of Creator can redefine which class to instantiate. In this example, the Creator1 subclass implements the abstract factoryMethod by instantiating the Product1 class.
MazeGame declares the abstract factory method to produce such a base product. MagicRoom or OrdinaryRoom are subclasses of the base product implementing the final product. Thus factory methods decouple callers MazeGame from the implementation of the concrete classes.
Java[ edit ] A maze game may be played in two modes, one with regular rooms that are only connected with adjacent rooms, and one with magic rooms that allow players to be transported at random this Java example is similar to one in the book Design Patterns.
The MazeGame uses Rooms but it puts the responsibility of creating Rooms to its subclasses which create the concrete classes. The regular game mode could use this template method: It refers to the makeRoom factory method that encapsulates the creation of rooms such that other rooms can be used in a subclass. To implement the other game mode that has magic rooms, it suffices to override the makeRoom method: It is important to note that the factory method can also be defined as public and called directly by the client code in contrast with the Java example above.
NET[ edit ] Factory pattern deals with the instantiation of objects without exposing the instantiation logic. In other words, a Factory is actually a creator of objects which have a common interface.
Based on the type passed into the Factory object, we are returning the original concrete object as the interface IPerson. A factory method is just an addition to Factory class. It creates the object of the class through interfaces but on the other hand, it also lets the subclass decide which class is instantiated. As a result, you can easily call DoSomething from it to get the IProduct.
You might also write your custom logic after getting the object in the concrete Factory Method. The GetObject is made abstract in the Factory interface.
CreateParameter is an example of the use of factory method to connect parallel class hierarchies. In Qt , QMainWindow:: In Java , several factories are used in the javax.
Design Patterns , the highly influential book Design pattern , overview of design patterns in general Abstract factory pattern , a pattern often implemented using factory methods.