Adding example for interfaces in GO.

2022/01/golang_demo3/Program.go

@@ -0,0 +1,57 @@
+package main
+
+// interfaces example
+
+import (
+	"fmt"
+	"math"
+)
+
+type Shape interface {
+	area() float64
+	perim() float64
+}
+
+type Circle struct {
+	r float64
+}
+
+func (c Circle) area() float64 {
+	return c.r * c.r * math.Pi
+}
+
+func (c Circle) perim() float64 {
+	return 2 * math.Pi * c.r
+}
+
+type Rect struct {
+	a float64
+	b float64
+}
+
+func (r Rect) area() float64 {
+	return r.a * r.b
+}
+
+func (r Rect) perim() float64 {
+	return (r.a + r.b) * 2.0
+}
+
+func main() {
+	c := Circle{
+		r: 5.0,
+	}
+
+	r := Rect{
+		a: 2.0,
+		b: 3.0,
+	}
+
+	showShape("Circle", c)
+	showShape("Rect", r)
+}
+
+func showShape(name string, s Shape) {
+	fmt.Printf("%v area %.2f\n", name, s.area())
+	fmt.Printf("%v area %.2f\n\n", name, s.perim())
+}