Use a JSON library to parse the string and retrieve the value.
The following very basic example uses the built-in JSON parser from Android.
String jsonString = "{ \"name\" : \"John\", \"age\" : \"20\", \"address\" : \"some address\" }";
JSONObject jsonObject = new JSONObject(jsonString);
int age = jsonObject.getInt("age");
More advanced JSON libraries, such as jackson, google-gson, json-io or genson, allow you to convert JSON objects to Java objects directly.