go by example part 2

5-go-by-example/20-interfaces.go

@@ -0,0 +1,55 @@
+// Interfaces are named collections of method signatures
+
+package main
+
+import (
+	"fmt"
+	"math"
+)
+
+// basic interface for geometric shapes
+type geometry interface {
+	area() float64
+	perim() float64
+}
+
+// implement interface on rect and circle types
+type rect struct {
+	width, height float64
+}
+type circle struct {
+	radius float64
+}
+
+// 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
+}
+
+// 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
+}
+
+// call methods that are in the named interface
+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}
+
+	// circle and rect struct types both implement the geometry interface
+	// use instances of these structs as arguments to measure
+	measure(r)
+	measure(c)
+}