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.

Interface Implementation

Interfaces are implemented implicitly. A type never declares that it implements a given interface. Unlike in many other languages, there is no explicit declaration of intent, there is no "implements" keyword. If an interface exists and a type has the proper methods defined, then the type automatically fulfills that interface. Implicit interfaces decouple the definition of an interface from its implementation. You may add methods to a type and in the process be unknowingly implementing various interfaces, and that's okay.

Note

Remember, 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.

type shape interface {
  area() float64
}

If a type in your code implements an area method, with the same signature (e.g. accepts nothing and returns a float64), then that object is said to implement the shape interface.

type circle struct{
	radius int
}

func (c *circle) area() float64 {
  return 3.14 * c.radius * c.radius
}

This is different from most other languages, where you have to explicitly assign an interface type to an object, like with Java:

class Circle implements Shape