How to initialize inner ArrayList of a two-dimensional ArrayList

Question

The below ArrayList is a two-dimensional ArrayList of size parts. I'm divding the storeIds into parts of ArrayList and add them to the inner ArrayList of the 2D ArrayList.

ArrayList<ArrayList<String>> partStoreIds = new ArrayList<ArrayList<String>>(parts);    
for(int i = 0; i < parts; i++)
    {
        System.out.println("Executing part: " + i);
        int maxIndex = Math.min(storeIds.size(), querySize*(i+1));
        //The below line is throwing an exception
        partStoreIds.addAll(storeIds.subList(querySize*i, maxIndex));           

    }

Please post the full stack trace. Also, which type does storeIds have? — Turing85
– Turing85, Commented Sep 24, 2016 at 11:14
Sorry about that @Turing85. storeIds is of type List<String> — Vineeth Chitteti
– Vineeth Chitteti, Commented Sep 26, 2016 at 8:33

Nicolas Filotto · Accepted Answer · 2016-09-24 11:23:25Z

2

What you try to achieve can be done as next:

partStoreIds.add(new ArrayList<>(storeIds.subList(querySize*i, maxIndex)));

Indeed, as partStoreIds is an ArrayList of ArrayList only ArrayList instances can be added and since storeIds.subList(querySize*i, maxIndex) returns a List, you need to convert it first as an ArrayList using the constructor new ArrayList(Collection).

But a much simpler approach would be to declare your partStoreIds as a List of List, then you can add your subList directly as next:

List<List<String>> partStoreIds = new ArrayList<>(parts);
...
    partStoreIds.add(storeIds.subList(querySize*i, maxIndex));

edited Sep 24, 2016 at 11:23

answered Sep 24, 2016 at 11:17

Nicolas Filotto

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Pavneet_Singh · Accepted Answer · 2016-09-24 11:14:11Z

2

You need to create a new ArraraList and then add items to it

ArrayList<String> temp=new ArrayList<String>();
temp.addAll(storeIds.subList(querySize*i, maxIndex));
partStoreIds.add(temp);

edited Sep 24, 2016 at 11:14

answered Sep 24, 2016 at 11:11

Pavneet_Singh

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1 Comment

Turing85 Over a year ago

Some explanation would be beneficial.

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