Create the [JSONObject](<https://developer.android.com/reference/org/json/JSONObject.html>) using the empty constructor and add fields using the [put()](<https://developer.android.com/reference/org/json/JSONObject.html#put(java.lang.String,%20double)>) method, which is overloaded so that it can be used with different types:

try {
    // Create a new instance of a JSONObject
    final JSONObject object = new JSONObject();
    
    // With put you can add a name/value pair to the JSONObject
    object.put("name", "test");
    object.put("content", "Hello World!!!1");
    object.put("year", 2016);
    object.put("value", 3.23);
    object.put("member", true);
    object.put("null_value", JSONObject.NULL);

    // Calling toString() on the JSONObject returns the JSON in string format.
    final String json = object.toString();
    
} catch (JSONException e) {
    Log.e(TAG, "Failed to create JSONObject", e);
}

The resulting JSON string looks like this:

{  
   "name":"test",
   "content":"Hello World!!!1",
   "year":2016,
   "value":3.23,
   "member":true,
   "null_value":null
}