Go by Example: Interfaces

Interfaces are named collections of method signatures.

 
package main

import "fmt"
import "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 square and circle types.

 
type square 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 squares.

 
func (s square) area() float64 {
    return s.width * s.height
}
func (s square) perim() float64 {
    return 2*s.width + 2*s.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() {
    s := square{width: 3, height: 4}
    c := circle{radius: 5}

The circle and square struct types both implement the geometry interface so we can use instances of these structs as arguments to measure.

 
    measure(s)
    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.

Next example: Errors.

@gobyexample | feedback | source | license