Stringer interface(Golang)

package main

import (
	"fmt"
)

// Animal has a Name and an Age to represent an animal.
type Animal struct {
	Name string
	Age  uint
}

// String makes Animal satisfy the Stringer interface.
func (a Animal) String() string {
	return fmt.Sprintf("%v (%d)", a.Name, a.Age)
}

func main() {
	a := Animal{
		Name: "Gopher",
		Age:  2,
	}
	fmt.Println(a)
}

<String() string実装>
Gopher (2)

<デフォルト>
{Gopher 2}