HashMapに関する小さなバグ

Map, Object> map = new HashMap, Object>();

Map, List> param = new HashMap, List>();

param.put("class", Arrays.asList(1,2,3,4));
param.put("student", Arrays.asList(1, 2, 3, 4));
map.put("lineChart", param);
System.out.println(JSONObject.fromObject(map).toString());     

{"lineChart":{"student":[1,2,3,4]}}

Map, Object> map = new HashMap, Object>();

Map, List> param = new HashMap, List>();

param.put("grade", Arrays.asList(1,2,3,4));
param.put("student", Arrays.asList(1, 2, 3, 4));
map.put("lineChart", param);

System.out.println(JSONObject.fromObject(map).toString());
印刷結果:
{"lineChart":{"student":[1,2,3,4],"grade":[1,2,3,4]}}
つまり、classキーのmapを再び1つのmapに入れ、そのキー値ペアが消える