How to pass 2D arraylist to a function accepting 2D List

Question

After trying for 2 Days, finally asking on stackoverflow: How to pass a 2D Arraylist to 2 D List

Below is the code:

public class Main {

    static void fn1(List<List<Integer>> x){
        System.out.println(x);
    }
    public static void main(String[] args) {

        // Here aList is an ArrayList of ArrayLists
        ArrayList<ArrayList<Integer>> aList =
                new ArrayList<ArrayList<Integer>>();

        // Create n lists one by one and append to the
        // master list (ArrayList of ArrayList)
        ArrayList<Integer> a1 = new ArrayList<Integer>();
        a1.add(1);
        a1.add(2);
        aList.add(a1);

        ArrayList<Integer> a2 = new ArrayList<Integer>();
        a2.add(5);
        aList.add(a2);

        ArrayList<Integer> a3 = new ArrayList<Integer>();
        a3.add(10);
        a3.add(20);
        a3.add(30);
        aList.add(a3);

        for (int i = 0; i < aList.size(); i++) {
            for (int j = 0; j < aList.get(i).size(); j++) {
                System.out.print(aList.get(i).get(j) + " ");
            }
            System.out.println();
        }

        fn1(aList);
    }
}

I am receiving the error:

Error:(41, 13) java: incompatible types: java.util.ArrayList<java.util.ArrayList<java.lang.Integer>> cannot be converted to java.util.List<java.util.List<java.lang.Integer>>

I have a constrain that I can't change the signature of fn1.

I don't think its duplicate, that qn is pretty generic and complex — Devender Goyal, Aug 19 '19 at 14:53
Is casting allowed? If so, you can simply call your method as follows: `fn1((List)aList);` — Eritrean, Aug 19 '19 at 14:53
@Eritrean: Thanks this works with minimal change :), I didn't thought of casting — Devender Goyal, Aug 19 '19 at 14:55

score 4 · Accepted Answer · edited Aug 19 '19 at 14:43

4

Initialise the Object using the List interface instead of the concrete class.

public static void main(String[] args) {
        // Here aList is an ArrayList of ArrayLists
        List<List<Integer>> aList =
            new ArrayList<List<Integer>>();

edited Aug 19 '19 at 14:43

Lomtrur

1,703
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answered Aug 19 '19 at 14:35

Pete

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ArrayList implements List. List is an interface so when you ask how to pass from one to the other, it is a tad confusing. Please add additional info if my answer is not helpful. – Pete Aug 19 '19 at 14:42
Thanks pete, If outside ArrayList gets converted to List then why can't the inside ArrayList gets converted to List ? – Devender Goyal Aug 19 '19 at 14:49
I know am being a little dumb here, but am unable to wrap my head around it – Devender Goyal Aug 19 '19 at 14:49
Could you elaborate why was I getting errror and why this solution is correct :) – Devender Goyal Aug 19 '19 at 14:50

How to pass 2D arraylist to a function accepting 2D List

1 Answers1