update book code

eBook/examples/chapter_11/interfaces.go

@@ -0,0 +1,30 @@
+package main
+
+import "fmt"
+
+type Shaper interface {
+	Area() float32
+	// Perimeter() float32
+}
+
+type Square struct {
+	side float32
+}
+
+func (sq *Square) Area() float32 {
+	return sq.side * sq.side
+}
+
+func main() {
+	sq1 := new(Square)
+	sq1.side = 5
+
+	// var areaIntf Shaper
+	// areaIntf = sq1
+	// shorter, without separate declaration:
+	// areaIntf := Shaper(sq1) 
+	// or even:
+	areaIntf := sq1
+	fmt.Printf("The square has area: %f\n", areaIntf.Area())
+}
+// The square has area: 25.000000