47_json.go

package gobyexample

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

// Define structs to demonstrate encoding and decoding
// of custom types

type response1 struct {
	Page   int
	Fruits []string
}

type response2 struct {
	Page   int      `json:"page"`
	Fruits []string `json:"fruits"`
}

// JSONDemo - demonstrates JSON encoding and decoding in Go
func JSONDemo() {
	// encoding basic types
	boolB, _ := json.Marshal(true)
	fmt.Println(string(boolB))

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

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

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

	// slices and maps encode to JSON arrays and objects as expected
	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 our custom data types.
	// It will only include exported fields in the encoded output and
	// will by default use those fields' names as the JSON keys.
	res1D := &response1{
		Page:   1,
		Fruits: []string{"apple", "peach", "pear"}}
	res1B, _ := json.Marshal(res1D)
	fmt.Println(string(res1B))

	// We 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 we 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 with a check for 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 cast them to their
	// appropriate type.
	num := dat["num"].(float64)
	fmt.Println(num)

	// Accessing nested data requires a series of casts.
	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 above examples we always used bytes and strings as intermediates between the
	// data and JSON representation on stdout. 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)
}