ArrayList.clear（）正在更新List <>的先前值

Question

I am getting JSONArray from server. The JSON I am getting has the following format.

[ {
   "area_name":"abc",
   "coordinates":[
     {
     "lat":1.123,
     "lng":2.123,
     },
     {
     "lat":1.123,
     "lng":2.123,
     },
     .
     .
     ]
},
{
.
.
.
}
] 

In order to plot the areas on map I am adding areas to areaList and their corresponding coordinate in coordinatesList

ArrayList<HashMap<String, LatLng>> coordinatesList; // initialised in onCreate
List<ArrayList<HashMap<String, LatLng>>> areaList; // initialised in onCreate

for (int i = 0; i < areaArray.size(); i++) {
     coordinatesList.clear();
     //coordinatesList = new ArrayList<>();
     //  parsing of json and other non-related stuff

      for (int j =0; j < pointsArray.size(); j++) {
          // Getting data from json and saving to hashmap and adding to coordinatesList
          coordinatesList.add(pointsHashMap);
     }

     areaList.add(coordinatesList);
}

Now if I try to get values from this areaList at a certain function. I get the same values for all position.

log.d(TAG, "area " + areaList.get(i).get(0).get("lat_lng)); // its in a loop

for any value of i I get the last added value by areaList.add(coordinatesList); . All the previously added values in the areaList are replaced.

But when I comment coordinatesList.clear(); and uncomment coordinatesList = new ArrayList<>(); everything works as expected.

Can someone please elaborate this, why is it happening like this?

Answer 1

因为每次您在循环中使数组列表满足需要时，都尝试在onCreate（）中使数组满足需要。

coordinatesList = new ArrayList<>();

Answer 2

As this is initialised in our onCreate method, which will be called once the activity is pushed to backstack. ArrayList<HashMap<String, LatLng>> coordinatesList;

Now the instance of your coordinatesList is valid throughout your Class / Activity .

So if you want to get new results all the time while invoking your function for coordinatesList implementation, either you need to reinitialize the ArrayList object or clear it to get new results poplated.

Answer 3

You definitely need to add a new ArrayList to areaList each iteration through the loop. Right now you're adding the same instance of coordinatesList to areaList each time through the loop. That same instance is getting cleared at the top of the list.

Adding the same coordinatesList to the areaList doesn't copy the elements, it merely adds the reference to the existing list.

Answer 4

The following should work

public void onCreate(Bundle b){
    super(b);

    parseResponse();
}

private final void parseResponse(){

    areaList = new ArrayList<ArrayList<HashMap<String, LatLng>>>>(jsonArray.length());
    for(int i=0;i<areaList.size();i++){

        List<HashMap<String,LatLng>> coordinatesList = new ArrayList<>();
        //  parsing of json and other non-related stuff

        for (int j =0; j < pointsArray.size(); j++) {
            // Getting data from json and saving to hashmap and adding to coordinatesList
            coordinatesList.add(pointsHashMap);
        }

     areaList.set(i,coordinatesList);
}

Note: The length of the JSON Array is the same as the length of areaList so we can initialize areaList to the same length when parsing. This can be more efficient then initializing a default-sized ArrayList

Code Review:

This snippet of code is rather unreadable:

ArrayList<HashMap<String, LatLng>> coordinatesList; 
List<ArrayList<HashMap<String, LatLng>>> areaList; 

I think it would be a lot cleaner if you declared an Area class that encapsulated the pertinent details. For example:

public class Area{
   private String name;
   private List<LatLng> coordinates; 

   public Area(String name, List<LatLng> coordinates){//...}
}

This would then reduce the code in your Activity to:

List<Area> areaList; 

private final void parseResponse(){

    areaList = new ArrayList<Area>(jsonArray.length());
    for(int i=0;i<areaList.size();i++){

        List<LatLng> coordinatesList = new ArrayList<>();
        //  parsing of json and other non-related stuff
        // Getting area_name and coordinates from json

         areaList.add(new Area(areaName,coordinatesList));
    }

}