Interfaces

Interfaces are named collections of method signatures.

package main

import (
    "fmt"
    "math"
)

// Here’s a basic interface for geometric shapes.
type geometry interface {
    area() float64
    perim() float64
}

// For our example we’ll implement this interface on rect and circle types.
type rect struct {
    width, height float64
}
type circle struct {
    radius float64
}

// To implement an interface in Go, we just need to implement
// all the methods in the interface. Here we implement geometry on rects.
func (r rect) area() float64 {
    return r.width * r.height
}
func (r rect) perim() float64 {
    return 2*r.width + 2*r.height
}

// The implementation for circles.
func (c circle) area() float64 {
    return math.Pi * c.radius * c.radius
}
func (c circle) perim() float64 {
    return 2 * math.Pi * c.radius
}

// If a variable has an interface type, then we can call methods
// that are in the named interface.
// Here’s a generic measure function taking advantage of this to work on any geometry.
func measure(g geometry) {
    fmt.Println(g)
    fmt.Println(g.area())
    fmt.Println(g.perim())
}

func main() {
    r := rect{width: 3, height: 4}
    c := circle{radius: 5}

    // The circle and rect struct types both implement
    // the geometry interface so we can use instances of these structs as arguments to measure.
    measure(r)
    measure(c)
}

$ go run interfaces.go
{3 4}
12
14
{5}
78.53981633974483
31.41592653589793

To learn more about Go’s interfaces, check out this great blog post.