You can use:

JSONObject jsonObject = new JSONObject();
jsonObject.toString();

And if you want to get a specific value, you can use:

jsonObject.getString("msg");

or Integer value

jsonObject.getInt("codeNum");

For long Integers

jsonObject.getLong("longNum");

Check out Google's Gson: http://code.google.com/p/google-gson/

From their website:

Gson gson = new Gson(); // Or use new GsonBuilder().create();
MyType target2 = gson.fromJson(json, MyType.class); // deserializes json into target2

You would just need to make a MyType class (renamed, of course) with all the fields in the json string. It might get a little more complicated when you're doing the arrays, if you prefer to do all of the parsing manually (also pretty easy) check out http://www.json.org/ and download the Java source for the Json parser objects.

I know this is old (and I am new to java), but I ran into the same problem. And the answers were not as clear to me as a newbie... so I thought I would add what I learned.

I used a third-party library to aid in the endeavor: org.codehaus.jackson All of the downloads for this can be found here.

For base JSON functionality, you need to add the following jars to your project's libraries: jackson-mapper-asl and jackson-core-asl

Choose the version your project needs. (Typically you can go with the latest stable build).

Once they are imported in to your project's libraries, add the following import lines to your code:

 import org.codehaus.jackson.JsonGenerationException;
 import org.codehaus.jackson.map.JsonMappingException;
 import com.fasterxml.jackson.databind.ObjectMapper;

With the java object defined and assigned values that you wish to convert to JSON and return as part of a RESTful web service

User u = new User();
u.firstName = "Sample";
u.lastName = "User";
u.email = "sampleU@example.com";

ObjectMapper mapper = new ObjectMapper();
    
try {
    // convert user object to json string and return it 
    return mapper.writeValueAsString(u);
}
catch (JsonGenerationException | JsonMappingException  e) {
    // catch various errors
    e.printStackTrace();
}

The result should looks like this: {"firstName":"Sample","lastName":"User","email":"sampleU@example.com"}

To anyone still looking for an answer:

JSONParser parser = new JSONParser();
JSONObject json = (JSONObject) parser.parse(stringToParse);