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。
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
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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