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
}
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
代码已上传至GitHub： zhyea / go-patterns / null-object-pattern
End！！