go - 如何调用实现接口(interface)的结构的特定方法

Question

我有以下接口(interface)和一些实现它的结构:

package main

import "fmt"

type vehicle interface {
    vehicleType() string
    numberOfWheels() int
    EngineType() string
}

// -------------------------------------------

type truck struct {
    loadCapacity int
}

func (t truck) vehicleType() string {
    return "Truck"
}

func (t truck) numberOfWheels() int {
    return 6
}

func (t truck) EngineType() string {
    return "Gasoline"
}

// -------------------------------------------
type ev struct {
    capacityInKWh int
}

func (e ev) vehicleType() string {
    return "Electric Vehicle"
}

func (e ev) numberOfWheels() int {
    return 4
}

func (e ev) EngineType() string {
    return "Electric"
}

func (e ev) Capacity() int {
    return e.capacityInKWh
}

// -------------------------------------------

type dealer struct{}

func (d dealer) sell(automobile vehicle) {
    fmt.Println("Selling a vehicle with the following properties")
    fmt.Printf("Vehicle Type: %s \n", automobile.vehicleType())
    fmt.Printf("Vehicle Number of wheels: %d \n", automobile.numberOfWheels())
    fmt.Printf("Vehicle Engine Type: %s \n", automobile.EngineType())

    if automobile.EngineType() == "Electric" {
        fmt.Printf("The battery capacity of the vehicle is %d KWh", automobile.Capacity())
        //fmt.Printf("Here")
    }
}

func main() {

    volvoTruck := truck{
        loadCapacity: 10,
    }

    tesla := ev{
        capacityInKWh: 100,
    }

    myDealer := dealer{}
    myDealer.sell(volvoTruck)
    fmt.Println("---------------------------")
    myDealer.sell(tesla)

}

Sell我的 dealer{} 中的方法struct 接收一个接口(interface)。在这个方法中，我想调用一个只存在于实现接口(interface)的结构之一上而不存在于其他结构上的方法:

if automobile.EngineType() == "Electric" {
            fmt.Printf("The battery capacity of the vehicle is %d KWh", automobile.Capacity())
        }

请注意 Capacity()仅存在于 ev{}但不在 truck{} 中.有没有办法做到这一点，而不必将此方法添加到强制所有实现使用它的接口(interface)？

Answer 1

您可以使用 type assertion 检查方法是否存在.检查该值(或更具体地说，它的类型)是否具有您要查找的方法，如果有，您可以调用它。
可以通过检查值是否使用该单一方法实现接口(interface)来实现检查方法:

if hc, ok := automobile.(interface {
    Capacity() int
}); ok {
    fmt.Printf("The battery capacity of the vehicle is %d KWh", hc.Capacity())
}

然后输出将是(在 Go Playground 上尝试):

Selling a vehicle with the following properties
Vehicle Type: Truck 
Vehicle Number of wheels: 6 
Vehicle Engine Type: Gasoline 
---------------------------
Selling a vehicle with the following properties
Vehicle Type: Electric Vehicle 
Vehicle Number of wheels: 4 
Vehicle Engine Type: Electric 
The battery capacity of the vehicle is 100 KWh

如果为它创建一个命名的接口(interface)类型会更好:

type HasCapacity interface {
    Capacity() int
}

进而:

if hc, ok := automobile.(HasCapacity); ok {
    fmt.Printf("The battery capacity of the vehicle is %d KWh", hc.Capacity())
}

输出将是相同的，在 Go Playground 上试试这个.