Creating a Matrix with ArrayLists

I want to create an ArrayList-matrix with n-rows and m-columns, for example

   1 2 3 4 5

   1 2 3 4 5 

   1 2 3 4 5

   1 2 3 4 5

   1 2 3 4 5

I’ve already written a code for creating such a matrix but my code doesn’t display the values when I clear the list containing the column-data. Here is my

package testproject;

import java.util.ArrayList;

public class TestProject {

    public static void main(String[] args) {
        ArrayList<Integer> intList = new ArrayList<>();
        ArrayList<ArrayList<Integer>> mainList = new ArrayList<>();
        for (int i = 0; i < 10; i++) {
            for (int k = 0; k < 5; k++) {
                intList.add(k);
            }
            mainList.add(intList);
            intList.clear(); //this line is probably the problem
        }
        for (int row = 0; row < mainList.size(); row++) {
            for (int col = 0; col < mainList.get(0).size(); col++) {
                System.out.print(mainList.get(row).get(col) + ",");
            }
            System.out.println("");
        }
    }
}

Is there any possibility to clear the contents of intList without clearing the contents of mainList??

Answer

When calling mainList.add(intList); you are adding a reference pointing to the intList object into your mainList rather than copying the values. You will have to create one instance of “intList” per row, and not clear anything.