GoLang设计模式12|GoLang设计模式12 - 空对象模式

空对象设计模式是一种行为型设计模式,主要用于应对空对象的检查。使用这种设计模式可以避免对空对象进行检查。也就是说,在这种模式下,使用空对象不会造成异常。
空对象模式的组件包括:

  • Entity:接口,定义了子struct需要实现的方法
  • ConcreteEntity:实现了Entity 的具体struct
  • NullEntity:这个就表示了空对象,虽然也实现了Entity接口,但它的值都是空的
  • Client:这个类会获取Entity接口实现类的实例并使用它。这里并不关注实现类是ConcreteEntity 还是 NullEntity,对二者会进行相同的处理。
用个例子来说一下:假设有一所大学,大学有多个系,每个系都有一定数量的教授。
系(department)可以用一个接口来表示:
type department interface { getNumberOfProfessors() int getName() string }

大学(college)也是一个接口:
type college struct { departments []department }

现在假设有一个机构想统计下大学每个系的教授数量。
在这个例子里,假设大学里没有某个系,我们就会用到空对象模式。这里定义了一个nullDepartment来表示不存在的系。
nullDepartment.go:
type nullDepartment struct { numberOfProfessors int }func (c *nullDepartment) getNumberOfProfessors() int { return 0 }func (c *nullDepartment) getName() string { return "nullDepartment" }

统计的代码在agency.go里:
func main() { college1 := createCollege1() college2 := createCollege2() totalProfessors := 0 departmentArray := []string{"computerScience", "mechanical", "civil", "electronics"} for _, departmentName := range departmentArray { d := college1.getDepartment(departmentName) totalProfessors += d.getNumberOfProfessors() } fmt.Printf("Total number of professors in college1 is %d\n", totalProfessors) //Reset the professor count totalProfessors = 0 for _, departmentName := range departmentArray { d := college2.getDepartment(departmentName) totalProfessors += d.getNumberOfProfessors() } fmt.Printf("Total number of professors in college2 is %d\n", totalProfessors) }func createCollege1() *college { college := &college{} college.addDepartment("computerScience", 4) college.addDepartment("mechanical", 5) return college }func createCollege2() *college { college := &college{} college.addDepartment("computerScience", 2) return college }

【GoLang设计模式12|GoLang设计模式12 - 空对象模式】注意这段代码:
  • agency.go 并不关心某个系在大学里是否存在。当这个系不存在时,大学只需要返回一个nullDepartment对象即可
  • agency.go 对nullDepartment对象和其他department实现类的对象做了相同处理,这之中不需要对空值进行检查,直接调用getNumberOfProfessors()就可以了
以上就是使用空对象模式的好处了。
下面是其他的代码。
college.go:
type college struct { departments []department }func (c *college) addDepartment(departmentName string, numOfProfessors int) { if departmentName == "computerScience" { computerScienceDepartment := &computerScience{numberOfProfessors: numOfProfessors} c.departments = append(c.departments, computerScienceDepartment) } if departmentName == "mechanical" { mechanicalDepartment := &mechanical{numberOfProfessors: numOfProfessors} c.departments = append(c.departments, mechanicalDepartment) } return }func (c *college) getDepartment(departmentName string) department { for _, department := range c.departments { if department.getName() == departmentName { return department } } //Return a null department if the department doesn't exits return &nullDepartment{} }

计算机系,computerscience.go:
type computerScience struct { numberOfProfessors int }func (c *computerScience) getNumberOfProfessors() int { return c.numberOfProfessors }func (c *computerScience) getName() string { return "computerScience" }

数学系,mechanical.go:
type mechanical struct { numberOfProfessors int }func (c *mechanical) getNumberOfProfessors() int { return c.numberOfProfessors }func (c *mechanical) getName() string { return "mechanical" }

执行agency.go,输出内容如下:
Total number of professors in college1 is 9 Total number of professors in college2 is 2

End!!

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