json.go

//
// Go by Example: JSON - gobyexample.com
// json.go
//
package main

import (
	"encoding/json"
	"fmt"
	"os"
)

// We’ll use these two structs to demonstrate encoding
// and decoding of custom types below.
type response1 struct {
	Page   int
	Fruits []string
}

// Only exported fields will be encoded/decoded in JSON.
// Fields must start with capital letters to be exported.
type response2 struct {
	Page   int      `json:"page"`
	Fruits []string `json:"fruits"`
}

//  main
func main() {

	// First we’ll look at encoding basic data types to JSON strings.
	// Here are some examples for atomic values.
	bolB, _ := json.Marshal(true)
	fmt.Println(string(bolB))

	intB, _ := json.Marshal(1)
	fmt.Println(string(intB))

	fltB, _ := json.Marshal(2.34)
	fmt.Println(string(fltB))

	strB, _ := json.Marshal("gopher")
	fmt.Println(string(strB))

	// And here are some for slices and maps,
	// which encode to JSON arrays and objects as you’d expect.
	slcD := []string{"apple", "peach", "pear"}
	slcB, _ := json.Marshal(slcD)
	fmt.Println(string(slcB))

	mapD := map[string]int{"apple": 5, "lettuce": 7}
	mapB, _ := json.Marshal(mapD)
	fmt.Println(string(mapB))

	// The JSON package can automatically encode your custom data types.
	// It will only include exported fields in the encoded output
	// and will by default use those names as the JSON keys.
	res1D := &response1{
		Page:   1,
		Fruits: []string{"apple", "peach", "pear"}}
	res1B, _ := json.Marshal(res1D)
	fmt.Println(string(res1B))

	// You can use tags on struct field declarations to customize
	// the encoded JSON key names.
	// Check the definition of response2 above to see an example of
	// such tags.
	res2D := &response2{
		Page:   1,
		Fruits: []string{"apple", "peach", "pear"}}
	res2B, _ := json.Marshal(res2D)
	fmt.Println(string(res2B))

	// Now let’s look at decoding JSON data into Go values.
	// Here’s an example for a generic data structure.
	byt := []byte(`{"num":6.13,"strs":["a","b"]}`)

	// We need to provide a variable where the JSON package
	// can put the decoded data.
	// This map[string]interface{} will hold a map of strings
	// to arbitrary data types.
	var dat map[string]interface{}

	// Here’s the actual decoding, and a check for associated errors.
	if err := json.Unmarshal(byt, &dat); err != nil {
		panic(err)
	}
	fmt.Println(dat)

	// In order to use the values in the decoded map,
	// we’ll need to convert them to their appropriate type.
	// For example here we convert the value in num to the expected float64 type.
	num := dat["num"].(float64)
	fmt.Println(num)

	// Accessing nested data requires a series of conversions
	strs := dat["strs"].([]interface{})
	str1 := strs[0].(string)
	fmt.Println(str1)

	// We can also decode JSON into custom data types.
	// This has the advantages of adding additional type-safety
	// to our programs and eliminating the need for type assertions
	// when accessing the decoded data.
	str := `{"page": 1, "fruits": ["apple", "peach"]}`
	res := response2{}
	json.Unmarshal([]byte(str), &res)
	fmt.Println(res)
	fmt.Println(res.Fruits[0])

	// In the examples above we always used bytes and strings
	// as intermediates between the data and JSON representation
	// on standard out. We can also stream JSON encodings directly
	// to os.Writers like os.Stdout or even HTTP response bodies.
	enc := json.NewEncoder(os.Stdout)
	d := map[string]int{"apple": 5, "lettuce": 7}
	enc.Encode(d)

}