You are missing json-simple-1.1.1.jar from your classpath.

if you are using Maven, add below into your pom.xml.

<dependency>
    <groupId>com.googlecode.json-simple</groupId>
    <artifactId>json-simple</artifactId>
    <version>1.1.1</version>
</dependency>

Or you can download it from here.

Probably your simple json.jar file isn't in your classpath.

If you are using LibGdx, it does not come with a Gradle setup, making multi-project builds streamlined.

You need to add implementation 'org.json:json:<version>' to the dependencies block of your top-level build.gradle.

To find the latest version, see http://mvnrepository.com/artifact/org.json/json/

For example, using the latest version at the time of writing:

dependencies {
   implementation 'org.json:json:20200518'
}

Download the ZIP file from this URL and extract it to get the Jar. Add the Jar to your build path. To check the available classes in this Jar use this URL.

To Add this Jar to your build path Right click the Project > Build Path > Configure build path> Select Libraries tab > Click Add External Libraries > Select the Jar file Download

I hope this will solve your problem

Bit late, but I wanted to share my opinion on this.

I faced this problem recently when I found a Java project with both libraries and they were used at the same time.

I think that org.json is easier to read and to use, for 2 main reasons (for my needs):

1. JsonObject is immutable. You can't add new key/value pairs to an already existing JsonObject (reference here: javax.json: Add new JsonNumber to existing JsonObject)

2. It takes a few lines to pretty print a JsonObject or JsonArray, while it only takes 1 line to do it with JSONObject or JSONArray. Example:

   CopyStringWriter sw = new StringWriter();
   Map<String, Object> properties = new HashMap<>();
   properties.put(JsonGenerator.PRETTY_PRINTING, true);
   
   JsonWriterFactory writerFactory = Json.createWriterFactory(properties);
   JsonWriter jsonWriter = writerFactory.createWriter(sw);
   
   jsonWriter.writeObject(jsonObject); //JsonObject created before
   jsonWriter.close();
   String prettyPrintedJSON = sw.toString();
   

That is the code I use to get an indented JSON to write to a file. And with org.json I only need jsonObject.toString(4).

Another difference is the constructors. You will need a JsonObjectBuilder to create a JSON with javax.json. One step more that can be avoided.

I'm sure there are more differences (not sure if it's possible to create a JsonObject from a String) but these are my thoughts.

In my case i was using maven build tool and got this error so had to add below dependency from here like below and error resolved.

<dependency>
    <groupId>org.json</groupId>
    <artifactId>json</artifactId>
    <version>20180130</version>
</dependency>