Convert List Object to JSON java

In this Dart/Flutter JSON encode tutorial, I will show you ways to convert Object, Nested Object, List, List of JSON objects into JSON string. Finally, you can convert a complex Nested Object (that contains some fields and a List of objects field) into JSON string.

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Overview

dart:convert library provides a built-in top-level function called jsonEncode that can convert many types of Object to JSON string.

We have 3 steps to convert an Object/List to JSON string:

• create the class
• create toJson() method which returns a JSON object that has key/value pairs corresponding to all fields of the class
• get JSON string from JSON object/List using jsonEncode() function

For every type of Object, Nested Object, simple List, List of Objects, or Nested Object containing List, we will implement toJson() method with some little difference.

Dart/Flutter convert simple Object to JSON string

Assume that we have a User class with fields: name & age.

class User { String name; int age; User(this.name, this.age); }

If you call jsonEncode() function without creating toJson() method. You will get an error:

Unhandled exception: Converting object to an encodable object failed: Instance of 'User' ...

Lets create toJson() method like the following code:

class User { String name; int age; User(this.name, this.age); Map toJson() => { 'name': name, 'age': age, }; }

Now we can use dart:convert librarys built-in jsonEncode() function to convert the User object to JSON string:

import 'dart:convert'; main() { User user = User('bezkoder', 21); String jsonUser = jsonEncode(user); print(jsonUser); }

The result will show a string like this.

{"name":"bezkoder","age":21}

Dart/Flutter convert Nested Object to JSON string

If we have a Tutorial class and author is a field that has User class type.
Were gonna create toJson() method with the author is the result when calling Users toJson() method.

class User { String name; int age; ... } class Tutorial { String title; String description; User author; Tutorial(this.title, this.description, [this.author]); Map toJson() { Map author = this.author != null ? this.author.toJson() : null; return {'title': title, 'description': description, 'author': author}; } }

Remember to check if author property is null or not.
Now we can easily call jsonEncode() like this.

import 'dart:convert'; main() { User user = User('bezkoder', 21); Tutorial tutorial = Tutorial('Dart Tut#1', 'Tut#1 Description', user); String jsonTutorial = jsonEncode(tutorial); print(jsonTutorial); }

Check the result, you can see our JSON string:

{"title":"Dart Tut#1","description":"Tut#1 Description","author":{"name":"bezkoder","age":21}}

Dart/Flutter convert List to JSON string

We can easily do JSON encode a List without the need of creating any class.

import 'dart:convert'; main() { List tags = ['tagA', 'tagB', 'tagC']; String jsonTags = jsonEncode(tags); print(jsonTags); }

The result shows a JSON string array:

["tagA","tagB","tagC"]

Dart/Flutter convert List of objects to JSON string

How about more complicated List? For example, every item in the List is a Tag object.
The Tag class has 2 fields: name & quantity. So we also need to create toJson() method that return a Map.

class Tag { String name; int quantity; Tag(this.name, this.quantity); Map toJson() => { 'name': name, 'quantity': quantity, }; }

Now everything becomes simple with jsonEncode() function.

import 'dart:convert'; main() { List tags = [Tag('tagA', 3), Tag('tagB', 6), Tag('tagC', 8)]; String jsonTags = jsonEncode(tags); print(jsonTags); }

JSON string looks like an array:

[{"name":"tagA","quantity":3},{"name":"tagB","quantity":6},{"name":"tagC","quantity":8}]

Dart/Flutter convert complex Nested Object to JSON string

Yeah! Youre in the last section of this tutorial.
We will convert a complex Nested Object that contains some fields and a List of objects field into JSON string.

For example, Tutorial class has title, description, author (User class), tags (List):

class Tutorial { String title; String description; User author; List tags; Tutorial(this.title, this.description, [this.author, this.tags]); }

Were gonna define the toJson() method like this:

class Tutorial { ... Map toJson() { Map author = this.author != null ? this.author.toJson() : null; List