在给定的for和while循环中使用哪种数据结构代替Arraylist？

Question

JSONObject jObject = new JSONObject(jsonString);
        Iterator<String> keys = jObject.keys();
        String key;
        JSONArray jsonArray;
        String value;
        while (keys.hasNext()) {
            key = keys.next();
            countriesList.add(key);
            jsonArray = jObject.getJSONArray(key);

            for (int i = 0; i < jsonArray.length(); i++) {
                value = jsonArray.getString(i);
                citiesList.add(value);
            }
            countryCities.put(key, citiesList);
            citiesList.clear();

Assume the following JSON file:

    {
  "China": [
    "Guangzhou",
    "Fuzhou",
    "Beijing"
  ],
  "Nepal": [
    "Pokhara",
    "Kathmandu",
    "Hetauda"
  ],
  "India": [
    "Delhi",
    "Mumbai",
    "Chennai"
  ]
}

There is a test method :

    private void CountriesListIterator(){

        ArrayList<String> country= countryCities.get("China");
        Log.d("Country size", String.valueOf(country.size()));


}

The test method when called from onCreate() returns : 06-18 09:05:37.245 17603-17603/com.example.user.statusok D/Country size: 0

The test method logs 0 because I have cleared the citiesList after I have put the citiesList in a HashMap. If I remove the citiesList.clear() line, the test method logs 9. Is there a way in which if I run the test method it returns 3 instead of 0 or 9 ? What should I use instead of ArrayList to achieve the desired result?

Answer 1

The easiest solution is to create a new ArrayList , that way it will be a different reference; and the behavior will be as your probably expected. Change

citiesList.clear();

to

citiesList = new ArrayList<>();

Answer 2

You should not clear. You're getting zero because you are.

When you don't, you're getting 9 because there's only ever one list you add to.

Both are a symptom of holding onto the same object reference.

You need to make a new ArrayList for every country

while (keys.hasNext()) {
    List<String> cities = new ArrayList<>();

Answer 3

When you do countryCities.put(key, citiesList); , you add a reference to the list to your map. Since you reuse the same list, each reference points to the exact same list object. Instead, you need to create a new list of cities to add to the map. If you have many cities such that memory usage becomes a concern, you will also need to implement some kind of caching mechanism so that only a subset of your cities are loaded into memory at any given moment.