Append data in Json file

Append data in Json file

I have this type of JSON:
{
"stampi":
[
{
"nome": "Ovale Piccolo 18.2x13.5cm",
"lunghezza": 18.2,
"larghezza": 13.5,
"altezza": 4,
"volume": 786.83
},
{
"nome": "Ovale Grande 22.5x17.4cm",
"lunghezza": 22.5,
"larghezza": 17.4,
"altezza": 4,
"volume": 1246.54
}
]
}
and normally I read with this code:
StringBuffer sb = new StringBuffer();
BufferedReader br = null;
try {
br = new BufferedReader(new
InputStreamReader(getAssets().open("stampi.json")));
String temp;
while ((temp = br.readLine()) != null)
sb.append(temp);
} catch (IOException e) {
e.printStackTrace();
} finally {
try {
br.close(); // stop reading
} catch (IOException e) {
e.printStackTrace();
}
}
myjson_stampi = sb.toString();
and after use the array inside the program. I have create a menu that add
new value inside the JSON file but i have a problem ...this is the code:
StringBuffer sb = new StringBuffer();
BufferedReader br = null;
try {
br = new BufferedReader(new
InputStreamReader(getAssets().open("stampi.json")));
String temp;
while ((temp = br.readLine()) != null)
sb.append(temp);
} catch (IOException e) {
e.printStackTrace();
} finally {
try {
br.close(); // stop reading
} catch (IOException e) {
e.printStackTrace();
}
}
myjson_stampi = sb.toString();
try {
// Creating JSONObject from String
JSONObject jsonObjMain = new JSONObject(myjson_stampi);
// Creating JSONArray from JSONObject
JSONArray objNames = jsonObjMain.names();
System.out.println(objNames.toString());
jsonArray_stampi = jsonObjMain.getJSONArray("stampi");
int num_elem = jsonArray_stampi.length();
jsonObjMain.put( "nome","prova");
jsonObjMain.put( "lunghezza",22);
jsonObjMain.put( "larghezza", 10);
jsonObjMain.put( "altezza", 4);
jsonObjMain.put( "volume", 10.5);
jsonArray_stampi.put( jsonObjMain );
try {
FileWriter file = new FileWriter("c:\\test.json");
//file.write(jsonArray_stampi.);
file.write( JSON.stringify(jsonArray_stampi) );
file.flush();
file.close();
} catch (IOException e) {
e.printStackTrace();
}
} catch (JSONException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
} });
why can't work correctly? the num_elem variable are 2 always..help me!
thx Andrea