Golang中有哪些方法可以实现多态？

type Animal interface {
    Speak() string
}

type Dog struct{}

func (d Dog) Speak() string {
    return "汪汪汪"
}

type Cat struct{}

func (c Cat) Speak() string {
    return "喵喵喵"
}

func main() {
    animals := []Animal{Dog{}, Cat{}}
    for _, animal := range animals {
        fmt.Println(animal.Speak())
    }
}

type Shape interface {
    Area() float64
}

type Circle struct {
    radius float64
}

func (c Circle) Area() float64 {
    return math.Pi * c.radius * c.radius
}

type Rectangle struct {
    length float64
    width  float64
}

func (r Rectangle) Area() float64 {
    return r.length * r.width
}

func main() {
    shapes := []Shape{Circle{radius: 2}, Rectangle{length: 3, width: 4}}
    for _, shape := range shapes {
        fmt.Println(shape.Area())
    }
}