json 空值不输出是什么原因

FlightSegment //类

//字段
private String dep;

private String arr;

private String date;

private String clock;

//有set和get方法

//Test

FlightSegment f = new FlightSegment();
f.setArr("CAN");
f.setClock("0000-123456");
f.setDate("2012-10-10");

//其中有dep我不赋值,我想调用JSONObject.fromObject(f)不格式化，应该则么样
JSONObject json = JSONObject.fromObject(f);
System.out.println(json); //结果{"arr":"CAN","clock":"0000-","date":"2012-10-10","dep":""}

//我希望上面的结果是

{"arr":"CAN","clock":"0000-","date":"2012-10-10"}