Graphical code analysis of factory pattern in Java

This article mainly introduces the factory mode_related information compiled by the Power Node Java Academy. Friends in need can refer to it

Simple factory mode

　

When an addition class is needed, call CreateOperate() of the factory class and specify the Product to be manufactured

For example, the operation factory example given in Dahua Design Pattern P11:

public static Operation createOperate(string operate)
{
Operation oper = null;
12
switch (operate)
{
case "+":
{
oper = new OperationAdd();
break;
}
case "-":
{
oper = new OperationSub();
break;
}
case "*":
{
oper = new OperationMul();
break;
}
case "/":
{
oper = new Operationp();
break;
}
}
return oper;
}
}

To call the factory, you need createOperator("/") to return the division operator.

Advantages: The client does not need to modify the code.

Disadvantages: When a new operation class needs to be added, not only the addition operation class is required, but also the factory class is modified, which violates the opening and closing principle.

　 Factory method pattern

The UML class diagram is as follows:

　

This is different from a simple factory, which is a simple factory The pattern has only one factory, and the factory method pattern has a corresponding factory for each product

Benefits: To add an operation class (such as the Nth power class), you only need to add the operation class and the corresponding factory, two class, there is no need to modify the factory class.

Disadvantages: Adding operation classes will modify the client code. The factory method only moves the internal logical judgment of a simple factory to the client.

Abstract Factory Pattern:

The UML class diagram is as follows:

It can be seen from the diagram that this and The factory method pattern is very similar, but there are several differences:

Abstract factory pattern, a specific factory can manufacture several products, for example, Microsoft factory (equivalent to SqlserverFactory) can manufacture Microsoft mouse (belongs to the mouse class, Under the mouse category, there are HP mouse, Microsoft mouse, etc., which are equivalent to the SqlserverDepartment and AccessDepartment under IDepartment in the above picture). You can also make Microsoft keyboards (belonging to the keyboard category. Under the keyboard category, there are HP keyboards, Microsoft keyboards, etc., which are equivalent to the above picture. SqlserverUser and AccessUser below IUser in .

工厂方法模式：一个抽象产品类，可以派生出多个具体产品类。 
　　一个抽象工厂类，可以派生出多个具体工厂类。 
　　每个具体工厂类只能创建一个具体产品类的实例。 
　　抽象工厂模式：多个抽象产品类，每个抽象产品类可以派生出多个具体产品类。 
　　一个抽象工厂类，可以派生出多个具体工厂类。 
　　每个具体工厂类可以创建多个具体产品类的实例。 

　　区别：工厂方法模式只有一个抽象产品类，而抽象工厂模式有多个。
　　工厂方法模式的具体工厂类只能创建一个具体产品类的实例，而抽象工厂模式可以创建多个。

Advantages: Easy to exchange product families, such as switching between Access and SQLServer databases.

Disadvantages: For example, to add a chassis product, you not only need to add three categories "chassis class, Microsoft chassis, HP chassis", but also modify the HP factory. The Microsoft factory supports manufacturing chassis. When adding a Lenovo factory, you only need to add three classes. To use Lenovo factory, you still need to modify the client code.

In "Dahua Design Patterns", a method of using the simple factory pattern to improve the abstract factory pattern is proposed.

　

　The code of DataAccess is as follows: When changing the database (or brand), modify the code in DataAccess.

Of course, you can use reflection to modify the database. In this way, you only need to modify the configuration file. This perfectly improves the shortcomings of the abstract factory, and there is no need to modify the code on the client side.

Reflection implementation:

To modify the db attributes:

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