update book code

eBook/examples/chapter_11/duck_dance.go

@@ -0,0 +1,32 @@
+package main
+
+import "fmt"
+
+type IDuck interface {
+	Quack()
+	Walk()
+}
+
+func DuckDance(duck IDuck) {
+	for i := 1; i <= 3; i++ {
+		duck.Quack()
+		duck.Walk()
+	}
+}
+
+type Bird struct {
+	// ...
+}
+
+func (b *Bird) Quack() {
+	fmt.Println("I am quacking!")
+}
+
+func (b *Bird) Walk()  {
+	fmt.Println("I am walking!")
+}
+
+func main() {
+	b := new(Bird)
+	DuckDance(b)
+}