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Summary - encoding/json

Combine Marshal, Unmarshal and tags to write a record.

encoding/json summary

In memory: json.Marshal / json.Unmarshal(&v). In streams: json.NewEncoder(w).Encode / json.NewDecoder(r).Decode. Tags: json:"ad", omitempty, -. Data of unknown shape: map[string]any (numbers are float64) or json.RawMessage. Pointer fields to tell missing from zero.

Custom MarshalJSON

Keeping a money amount in cents but showing decimal text in JSON:

type Para int64  // cents

func (p Para) MarshalJSON() ([]byte, error) {
    return []byte(fmt.Sprintf(`"%d.%02d"`, p/100, p%100)), nil
}

json.Marshal(map[string]Para{"tutar": 1250})  // {"tutar":"12.50"}

A MarshalJSON written with a value receiver works for both values and pointers.

Practice Money type

Write the Para type and its MarshalJSON method above. Encode map[string]Para{"tutar": 1250} and print it.

Starter code

main.go
package main

import (
    "encoding/json"
    "fmt"
)

func main() {
    // buraya yaz
}

Sample solution

main.go
package main

import (
    "encoding/json"
    "fmt"
)

type Para int64

func (p Para) MarshalJSON() ([]byte, error) {
    return []byte(fmt.Sprintf(`"%d.%02d"`, p/100, p%100)), nil
}

func main() {
    b, err := json.Marshal(map[string]Para{"tutar": 1250})
    if err != nil {
        fmt.Println(err)
        return
    }
    fmt.Println(string(b))
}

Expected output

{"tutar":"12.50"}

Questions and answers

Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.

1 How does a type define its own JSON format?

  • Only with tags
  • It isn't possible
  • By implementing MarshalJSON / UnmarshalJSON methods Correct answer
  • By writing String()

Explanation: Types implementing the json.Marshaler and json.Unmarshaler interfaces control encoding entirely themselves.

2 Readable output

Question 
b, _ := json.____(v, "", "  ")
Answer 
b, _ := json.MarshalIndent(v, "", "  ")

Explanation: MarshalIndent writes JSON on multiple lines with the given indentation.

3 Group summary

Term Meaning
Marshal Go → JSON
Unmarshal JSON → Go
omitempty Skip zero
MarshalJSON Custom format

All lessons in Intermediate

1 Goroutines

  1. What is a goroutine?
  2. The go keyword
  3. Goroutine and function
  4. The main goroutine
  5. Number of goroutines
  6. Summary - Goroutines

2 Channels

  1. What is a channel?
  2. Channel with make
  3. Sending data
  4. Receiving data
  5. Buffered channel
  6. Summary - Channels

3 Select

  1. What is select?
  2. Multiple channels
  3. default and non-blocking
  4. Select with timeout
  5. Closing a channel
  6. Summary - Select

4 sync package

  1. sync.Mutex
  2. sync.RWMutex
  3. sync.WaitGroup
  4. sync.Once
  5. sync.Pool
  6. Summary - sync package

5 Project: Concurrency

  1. Worker pool skeleton
  2. Job queue
  3. Collecting results
  4. Error handling

6 Context

  1. context.Background
  2. WithCancel
  3. WithTimeout
  4. WithValue
  5. WithDeadline
  6. Summary - Context

7 io package

  1. io.Reader
  2. io.Writer
  3. os.Stdin and os.Stdout
  4. bufio.Scanner
  5. io.Copy
  6. Summary - io package

8 Testing

  1. Test function
  2. t.Error and t.Fatal
  3. Table-driven tests
  4. Test helpers
  5. Example tests
  6. Summary - Testing

9 Benchmark

  1. Benchmark function
  2. b.ResetTimer
  3. Memory benchmark
  4. Sub-benchmark
  5. Comparing benchmarks
  6. Summary - Benchmark

10 fmt package

  1. fmt.Printf
  2. Format verbs
  3. fmt.Sprintf
  4. fmt.Errorf
  5. fmt.Fprintln
  6. Summary - fmt package

11 encoding/json

  1. The encoding/json package
  2. Struct tags
  3. Marshal and Unmarshal
  4. Encoder and Decoder
  5. JSON and map
  6. Summary - encoding/json

12 net/http

  1. Client with net/http
  2. http.Get
  3. http.Handler
  4. http.HandleFunc
  5. JSON API
  6. Summary - net/http

13 flag package

  1. The flag package
  2. flag.String and flag.Int
  3. flag.Parse
  4. Subcommand
  5. os.Args
  6. Summary - flag package

14 Intermediate final

  1. Challenge 1 - Concurrency
  2. Challenge 2 - HTTP service
  3. Challenge 3 - Test and benchmark
  4. Challenge 4 - CLI tool