Convert ArrayList<ArrayList<Integer>> to List<List<Integer>>

Question

I have my Integer data in Array Lists of Arrays Lists and I want to convert that data to Lists of List format. How can I do it?

public List<List<Integer>> subsetsWithDup(int[] nums) {

    ArrayList<ArrayList<Integer>> ansList = new ArrayList<ArrayList<Integer>>();

    String arrStr = nums.toString();
    ArrayList<Integer> tempList = null;

    for(int i = 0 ; i < arrStr.length()-1 ; i++){
        for(int j = i+1 ; j<arrStr.length() ; j++){

            tempList = new ArrayList<Integer>();

            tempList.add(Integer.parseInt(arrStr.substring(i,j)));

        }

        if(!ansList.contains(tempList)){
            ansList.add(tempList);
        }

    }
    return ansList;
}

List is the base class of ArrayList,so you can either write List<ArrayList<Integer>> cc = new ArrayList<ArrayList<Integer>>() where your generic type ArrayList<Integer> has to be same. — Kavita Patil
– Kavita Patil, Commented Mar 18, 2019 at 5:44
How about this List<List<Integer>> listList = new ArrayList<>(arrayLists); — soorapadman
– soorapadman, Commented Mar 18, 2019 at 5:55
@Thilo an ArrayList<ArrayList<Integer>> is not a List<List<Integer>> — Ferrybig
– Ferrybig, Commented Mar 18, 2019 at 6:19

KevinO · Accepted Answer · 2019-03-18 06:15:56Z

It would be best to share the code where you have this issue. However, you should declare the variables as List<List<Integer>>, and then instantiate using an ArrayList.

For example:

public static void main (String[] args) throws java.lang.Exception
{

   List<List<Integer>> myValues = getValues();
   System.out.println(myValues);

}

public static List<List<Integer>> getValues() {
   List<List<Integer>> lst = new ArrayList<>();
   List<Integer> vals = new ArrayList<>();

   vals.add(1);
   vals.add(2);
   lst.add(vals);

   vals = new ArrayList<>();
   vals.add(5);
   vals.add(6);
   lst.add(vals);       

   return lst;
}

In general, program to an interface (such as List).

Based upon the edit to the OP's question, one can see that, as originally suggested, one can change:

ArrayList<ArrayList<Integer>> ansList = new ArrayList<ArrayList<Integer>>();

to

List<List<Integer>> ansList = new ArrayList<>();

And

 ArrayList<Integer> tempList = null;

to

List<Integer> tempList = null;

And the code will then conform to the method's return signature of List<List<Integer>>.

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