add: string interface

interfaces/09-stringer/book.go

@@ -0,0 +1,50 @@
+// For more tutorials: https://blog.learngoprogramming.com
+//
+// Copyright © 2018 Inanc Gumus
+// Learn Go Programming Course
+// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
+//
+
+package main
+
+import (
+	"fmt"
+	"strconv"
+	"time"
+)
+
+type book struct {
+	product
+	published interface{}
+}
+
+// book satisfies the fmt.Stringer
+func (b *book) String() string {
+	p := format(b.published)
+
+	// product.String has a pointer receiver.
+	// that's why we need to take its address here.
+	//
+	// when you put a value in an interface, the interface
+	// cannot run that value's type's value-receiver methods.
+	return fmt.Sprintf("%s - (%v)", &b.product, p)
+}
+
+func format(v interface{}) string {
+	var t int
+
+	switch v := v.(type) {
+	case int:
+		t = v
+	case string:
+		t, _ = strconv.Atoi(v)
+	default:
+		return "unknown"
+	}
+
+	// Mon Jan 2 15:04:05 -0700 MST 2006
+	const layout = "2006/01"
+
+	u := time.Unix(int64(t), 0)
+	return u.Format(layout)
+}