update book code

eBook/examples/chapter_16/pointer_interface.go

@@ -0,0 +1,31 @@
+// nexter.go
+package main
+
+import (
+	"fmt"
+)
+
+type nexter interface {
+    next() byte
+}
+
+
+func nextFew1(n nexter, num int) []byte {
+	var b []byte
+	for i:=0; i < num; i++ {
+		b[i] = n.next()
+	}
+	return b
+}
+
+func nextFew2(n *nexter, num int) []byte {
+	var b []byte
+	for i:=0; i < num; i++ {
+		b[i] = n.next() // compile error:  n.next undefined (type *nexter has no field or method next)
+	}
+	return b
+}
+
+func main() {
+	fmt.Println("Hello World!")
+}