Empty Mysql field (NULL) ends up being a string “null” in Java

Question

I have some data extracted from a MySQL database where some fields are NULL . Not accidentally as a string but properly stored as NULL . When I send these null-data JSON-encoded to my android app, they end up being a string "null" of length 4. So I rebuilt this problem condensed to the essential code:

PHP:

$string = null;
echo $array[0]['alt_names'] = $string;
echo json_encode($array);

Java: (My PHP class returns a string, in this case jsonResult )

Log.i("Tag", "result = " + jsonResult);          // result = [{"alt_names":null}]

JSONArray jsonArray = new JSONArray(jsonResult);
Log.i("Tag", "jsonArray = " + jsonArray);        // jsonArray = [{"alt_names":null}]

JSONObject jsonObject = new JSONObject(jsonArray.getJSONObject(0).toString());
Log.i("Tag", "jsonobject = " + jsonObject);      // jsonobject = {"alt_names":null}

String test = jsonObject.get("alt_names").toString();
Log.i("Tag", "test: " + test);                   // test: null
Log.i("Tag", "test.length(): " + test.length()); // test.length(): 4

The missing quotation marks (not) enclosing null in the Log-output show me, that this is not a string "null" butt actually null . Nevertheless the string's length is 4 and this is true:

if (test.equals("null")) {Log.i("Tag", "true");} // true

What do I not understand? Thanks in advance!

Answer 1

Don't do a toString() on the object return by jsonObject.get("alt_names") . It is actually the static instance of JSONObject.NULL

Object test = jsonObject.get("alt_names");
System.out.println(test == JSONObject.NULL); // true
System.out.println(test.equals(null)); // true
System.out.println(jsonObject.isNull("alt_names")); // true

From javadoc :

It is sometimes more convenient and less ambiguous to have a NULL object than to use Java's null value. JSONObject.NULL.equals(null) returns true. JSONObject.NULL.toString() returns "null".