Can someone explain this interface example in Go? - json

Question

From http://jordanorelli.com/post/32665860244/how-to-use-interfaces-in-go there an example illustrating a possible use of interfaces in Go. Code as below:
package main
import (
"encoding/json"
"fmt"
"reflect"
"time"
)
// start with a string representation of our JSON data
var input = `
{
"created_at": "Thu May 31 00:00:01 +0000 2012"
}
`
type Timestamp time.Time
func (t *Timestamp) UnmarshalJSON(b []byte) error {
v, err := time.Parse(time.RubyDate, string(b[1:len(b)-1]))
if err != nil {
return err
}
*t = Timestamp(v)
return nil
}
func main() {
// our target will be of type map[string]interface{}, which is a pretty generic type
// that will give us a hashtable whose keys are strings, and whose values are of
// type interface{}
var val map[string]Timestamp
if err := json.Unmarshal([]byte(input), &val); err != nil {
panic(err)
}
fmt.Println(val)
for k, v := range val {
fmt.Println(k, reflect.TypeOf(v))
}
fmt.Println(time.Time(val["created_at"]))
}
with a result like this:
map[created_at:{63474019201 0 0x59f680}]
created_at main.Timestamp
2012-05-31 00:00:01 +0000 UTC
I am struggling to understand how the function call
json.Unmarshal([]byte(input), &val){...}
relates to the method defined earlier
func (t *Timestamp) UnmarshalJSON(b []byte) error{...}
Reading the doc at http://golang.org/pkg/encoding/json/#Unmarshal is confusing me even more.
I am obviously missing something here, but I can't figure it out.

Answer 1

In Go an interface is implemented just by implementing its methods. It is so much different from the most other popular languages (Java, C#, C++) in which the class interfaces should be explicitly mentioned in the class declaration.
The detailed explanation of this concept you can find in the Go documentation: https://golang.org/doc/effective_go.html#interfaces
So the func (t *Timestamp) UnmarshalJSON(...) defines a method and in a same time implements the interface. The json.Unmarshal then type asserts the elements of val to the Unmarshaler interface (http://golang.org/pkg/encoding/json/#Unmarshaler) and call the UnmarshalJSON method to construct them from the byte slice.