Go Example

package main

import (
    "fmt"
)

type areaCalculator struct {
    area int
}

func (a *areaCalculator) visitForSquare(s *square) {
    // Calculate area for square.
    // Then assign in to the area instance variable.
    fmt.Println("Calculating area for square")
}

func (a *areaCalculator) visitForCircle(s *circle) {
    fmt.Println("Calculating area for circle")
}
func (a *areaCalculator) visitForrectangle(s *rectangle) {
    fmt.Println("Calculating area for rectangle")
}

package main

type circle struct {
    radius int
}

func (c *circle) accept(v visitor) {
    v.visitForCircle(c)
}

func (c *circle) getType() string {
    return "Circle"
}

package main

import "fmt"

type middleCoordinates struct {
    x int
    y int
}

func (a *middleCoordinates) visitForSquare(s *square) {
    // Calculate middle point coordinates for square.
    // Then assign in to the x and y instance variable.
    fmt.Println("Calculating middle point coordinates for square")
}

func (a *middleCoordinates) visitForCircle(c *circle) {
    fmt.Println("Calculating middle point coordinates for circle")
}
func (a *middleCoordinates) visitForrectangle(t *rectangle) {
    fmt.Println("Calculating middle point coordinates for rectangle")
}

package main

type rectangle struct {
    l int
    b int
}

func (t *rectangle) accept(v visitor) {
    v.visitForrectangle(t)
}

func (t *rectangle) getType() string {
    return "rectangle"
}

package main

type shape interface {
    getType() string
    accept(visitor)
}

package main

type square struct {
    side int
}

func (s *square) accept(v visitor) {
    v.visitForSquare(s)
}

func (s *square) getType() string {
    return "Square"
}

package main

type visitor interface {
    visitForSquare(*square)
    visitForCircle(*circle)
    visitForrectangle(*rectangle)
}

package main

import "fmt"

func main() {
    square := &square{side: 2}
    circle := &circle{radius: 3}
    rectangle := &rectangle{l: 2, b: 3}

    areaCalculator := &areaCalculator{}

    square.accept(areaCalculator)
    circle.accept(areaCalculator)
    rectangle.accept(areaCalculator)

    fmt.Println()
    middleCoordinates := &middleCoordinates{}
    square.accept(middleCoordinates)
    circle.accept(middleCoordinates)
    rectangle.accept(middleCoordinates)
}

Calculating area for square
Calculating area for circle
Calculating area for rectangle

Calculating middle point coordinates for square
Calculating middle point coordinates for circle
Calculating middle point coordinates for rectangle