update book code

eBook/exercises/chapter_11/interface_poly3.go

@@ -0,0 +1,68 @@
+// interface_poly3.go
+package main
+
+import (
+	"fmt"
+	"math"
+)
+
+type Shaper interface {
+	Area() float32
+}
+
+type Shape struct {}
+func (sh Shape) Area() float32 {
+	return -1 // the shape is indetermined, so we return something impossible
+}
+
+type Square struct {
+	side float32
+	Shape
+}
+
+func (sq *Square) Area() float32 {
+	return sq.side * sq.side
+}
+
+type Rectangle struct {
+   length, width float32
+   Shape
+}
+
+func (r *Rectangle) Area() float32 {
+   return r.length * r.width
+}
+
+type Circle struct {
+	radius float32
+	Shape
+}
+
+func (c *Circle) Area() float32 {
+   return math.Pi * c.radius * c.radius 
+}
+
+func main() {
+   s := Shape{}
+   r := &Rectangle{5, 3, s} // Area() of Rectangle needs a value
+   q := &Square{5, s}      // Area() of Square needs a pointer
+   c := &Circle{2.5, s}
+   shapes := []Shaper{r, q, c, s}
+   fmt.Println("Looping through shapes for area ...")
+   for n, _ := range shapes {
+       fmt.Println("Shape details: ", shapes[n])
+       fmt.Println("Area of this shape is: ", shapes[n].Area())
+   }
+}
+/* Output:
+Looping through shapes for area ...
+Shape details:  {5 3}
+Area of this shape is:  15
+Shape details:  &{5}
+Area of this shape is:  25
+Shape details:  &{2.5}
+Area of this shape is:  19.634954
+Shape details:  {}
+Area of this shape is:  -1
+*/
+