Use a library called org.json, it is honestly the best java json library.
for example:
import org.json.JSONObject;
private static void createJSON(boolean prettyPrint) {    
   JSONObject tomJsonObj = new JSONObject();
   tomJsonObj.put("name", "Tom");
   tomJsonObj.put("birthday", "1940-02-10");
   tomJsonObj.put("age", 76);
   tomJsonObj.put("married", false);
// Cannot set null directly
tomJsonObj.put("car", JSONObject.NULL);
tomJsonObj.put("favorite_foods", new String[] { "cookie", "fish", "chips" });
// {"id": 100001, "nationality", "American"}
JSONObject passportJsonObj = new JSONObject();
passportJsonObj.put("id", 100001);
passportJsonObj.put("nationality", "American");
// Value of a key is a JSONObject
tomJsonObj.put("passport", passportJsonObj);
if (prettyPrint) {
    // With four indent spaces
    System.out.println(tomJsonObj.toString(4));
} else {
    System.out.println(tomJsonObj.toString());
}
}