Question

I am working on an android project and I am facing a problem and the problem is:

Arraylist empty when I return it.

Here is my java code:

 ArrayList<ArrayList<Object>> container = new ArrayList<ArrayList<Object>>();
            ArrayList<Object> itemRow = new ArrayList<Object>();
            JSONObject jsonObj =  new JSONObject(result);
            JSONArray allElements = jsonObj.getJSONArray("Table");
            Log.i("allElements", "" + allElements);
            for (int i = 0; i < allElements.length(); i++) {
                itemRow.add(allElements.getJSONObject(i).getString("ParentName").toString());
                itemRow.add(allElements.getJSONObject(i).getString("ParentEmailID").toString());
                itemRow.add(allElements.getJSONObject(i).getString("ParentContact").toString());
                itemRow.add(allElements.getJSONObject(i).getString("ParentAddress").toString());
                itemRow.add(allElements.getJSONObject(i).getString("ParentProfilePictureName").toString());
                itemRow.add(allElements.getJSONObject(i).getString("StudentName").toString());
                Log.i("itemRow", "itemRow at index: " + i + ", " + itemRow);
                container.add(((i*2)/2), itemRow);
                itemRow.clear();
            }

            return container;

In this code I have two Arraylist one for contain all the elements and another one for storing single row of elements. These Arraylist are loaded from JSONArray, all is working fine and I can print data from item row (Arraylist which take single row) and store into main Arraylist (container).

But when I return this Arraylist (container) and print in logcat it shows empty Arraylist like

[[], [], [], [], []].

I cannot understand why this happen please help me to solve this issue.

Thanks.

Answer 1

Stop clearing the list, and it won’t be empty anymore:

itemRow.clear();

You should create a new list at each iteration. Put the following line of code inside the for loop:

ArrayList<Object> itemRow = new ArrayList<Object>();

Remember that Java passes references to objects. So the container list holds a reference to the list you add to it. It doesn’t make a copy of the list. So your current code adds several references to the same list object to the container list, and you clear the list each time you add it. It thus contains N references to the same empty list at the end of the loop.