Interfaces in Go

Interfaces are collections of method signatures. A type "implements" an interface if it has all of the methods of the given interface defined on it.

In the following example, a "shape" must be able to return its area and perimeter. Both rect and circle fulfill the interface.

type shape interface {
  area() float64
  perimeter() float64
}

type rect struct {
    width, height float64
}
func (r rect) area() float64 {
    return r.width * r.height
}
func (r rect) perimeter() float64 {
    return 2*r.width + 2*r.height
}

type circle struct {
    radius float64
}
func (c circle) area() float64 {
    return math.Pi * c.radius * c.radius
}
func (c circle) perimeter() float64 {
    return 2 * math.Pi * c.radius
}

When a type implements an interface, it can then be used as the interface type.

Assignment

The birthdayMessage and sendingReport structs have already implemented the getMessage methods. The getMessage method simply returns a string, and any type that implements the method can be considered a message.

First, add the getMessage() method as a requirement on the method interface.

Second, complete the sendMessage function. It should print a message's message, which it obtains through the interface method. Notice that your code doesn't need to worry at all about whether a specific message is a birthdayMessage or a sendingReport!