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Learn how to use the JSONObject class to encode key/value pairs into JSON strings.

Revised: Thu Jun 02 19:24:40 CDT 2016

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Table of contents

Preface

This is a page from the book titled The json-simple Java Library . The book explains how to use the json-simple Java library to generate, transform, and query JSON text. This page explains how to use the JSONObject class to encode key/value pairs into JSON strings.

Viewing tip

I recommend that you open another copy of this module in a separate browser window and use the following links to easily find and view the Figuresand Listings while you are reading about them.

Figures

Listings

  • Listing 1 . Create ArrayList containers.
  • Listing 2 . Create and populate a JSONObject object.
  • Listing 3 . Create and populate two more JSONObject objects.
  • Listing 4 . Encode the data into JSON strings.
  • Listing 5 . Display the JSON strings.
  • Listing 6 . The program named Code.java.

General background information

As you learned in the earlier page titled Json0210: Structure of the json-simple Java Library , the class named JSONObject extends the standard Java class named HashMap . Therefore, once you have an object of the class named JSONObject , you can call any of the methods defined in or inherited into the JSONObject class on that object.

In the sample program that follows, we will use the put method that is inherited from the HashMap class to populate the object with key/value pairs. We will use the toJSONString method that is defined in the JSONObject class to transform the populated object into a JSON string.

Discussion and sample code

The program named Code (see Listing 6 ) constructs three JSONObject objects, populates the objects with key/value pairs, and saves the JSONObject objects in an ArrayList object. (Actually it saves references to the JSONObject objects in the ArrayList object.)

Then it transforms each JSONObject object into a JSON string. At this point, the JSON strings could be written to an outputstream and transferred to a different programming environment. However, to keep the program simple, the program simply saves the JSON strings in a second ArrayList object for later display.

Then the program displays the JSON strings for comparison with the code that populated the JSONObject objects in the first place.

Create ArrayList containers

A complete listing of the program named Code is provided in Listing 6 near the end of the page. I will discuss and explain the code in fragments.The first fragment is shown in Listing 1 .

Listing 1 . Create ArrayList containers. import org.json.simple.JSONObject; import java.util.ArrayList;import java.util.Iterator; class Code {public static void main(String[] args){//Create a container for several JSON objects. ArrayList<JSONObject>listA = new ArrayList<JSONObject>(); //Create a container for several JSON stringsArrayList<String>listB = new ArrayList<String>();

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