Jagged Arrays in Java: Converting char[][] and ArrayList<ArrayList<char>>

Question

How can I convert the following so that it is essentially a jagged nested ArrayList of ArrayLists, with each row/col a list of chars?

   //Following excerpt from https://www.geeksforgeeks.org/jagged-array-in-java/
   int r = 5;
        //Need this to have capacity to hold list of chars (eg, each row/col index can be empty, //have 1 char, or multiple chars
        char toFill = new Variable(" ");

        // Declaring 2-D array with 5 rows... need it to be ArrayList<ArrayList<char>>
        char matrix[][] = new Variable[r][];


        // Creating a 2D array such that first row
        // has 1 element, second row has two
        // elements and so on.
        for (int i=0; i<matrix.length; i++) {
            matrix[i] = new char[i + 1];
        }

        // Initializing array
        int count = 0;
        for (int i=0; i<matrix.length; i++) {
            for (int j = 0; j < matrix[i].length; j++) {
                matrix[i][j] = toFill;
            }
        }

Does this answer your question? Transfer a Two-Dimensional array to Two-Dimensional ArrayList? — kasptom
– kasptom, Commented Feb 23, 2020 at 23:44
@kasptom That's helpful but I still have the problem of each index (row/col) only holding one value or character. I want each row/col index to be a list itself. For each row I want to be able to add to each col index any amount of characters when looping through a string. Currently I can't because each row/col allows for only one item, not multiple. Example of what I would want my first row to look like: [[S,R], [S,R,E], [], [E], [R,E,M]]. — Zuckerbrenner
– Zuckerbrenner, Commented Feb 24, 2020 at 0:28

kriegaex · Accepted Answer · 2020-02-25 00:41:51Z

Actually lists are dynamic structures, you don't need to pre-define their sizes as with static structures like arrays. He is a sample program directly translating the jagged array into a jagged list of lists. I tried to keep as much of the structure as possible so as to make it easier for you to understand. The code still looks kind of "array-ish" (which I actually don't like because it is somewhat unnatural), but I hope you get the idea.

package de.scrum_master.stackoverflow.q60367936;

import java.util.ArrayList;
import java.util.List;

/**
 * Demonstrate 2-D jagged array/list such that first row has 1 element,
 * second row has two elements and so on.
 */
class Main {
  private static void jaggedArray() {
    int r = 5;

    // Declaring 2-D array with 5 rows
    int arr[][] = new int[r][];

    // Creating a 2D array such that first row has 1 element, second row has two elements and so on.
    for (int i = 0; i < arr.length; i++)
      arr[i] = new int[i + 1];

    // Initializing array
    int count = 0;
    for (int i = 0; i < arr.length; i++)
      for (int j = 0; j < arr[i].length; j++)
        arr[i][j] = count++;

    // Displaying the values of 2D Jagged array
    System.out.println("Contents of 2D Jagged Array");
    for (int i = 0; i < arr.length; i++) {
      for (int j = 0; j < arr[i].length; j++)
        System.out.print(arr[i][j] + " ");
      System.out.println();
    }
  }

  private static void jaggedArrayList() {
    int r = 5;

    // Declaring 2-D list of lists
    List<List<Integer>> arr = new ArrayList<>();

    // Adding empty sub list to main list
    for (int i = 0; i < r; i++)
      arr.add(new ArrayList<Integer>());

    // Initializing 2-D list
    int count = 0;
    for (int i = 0; i < r; i++)
      for (int j = 0; j <= i; j++)
        arr.get(i).add(count++);

    // Displaying the values of 2D Jagged list
    System.out.println("Contents of 2D Jagged ArrayList");
    for (List<Integer> list : arr) {
      for (Integer i : list)
        System.out.print(i + " ");
      System.out.println();
    }
  }

  public static void main(String[] args) {
    jaggedArray();
    System.out.println("\n------------------------------\n");
    jaggedArrayList();
  }
}

Console log:

Contents of 2D Jagged Array
0 
1 2 
3 4 5 
6 7 8 9 
10 11 12 13 14 

------------------------------

Contents of 2D Jagged ArrayList
0 
1 2 
3 4 5 
6 7 8 9 
10 11 12 13 14

As you can see, both the array and the list variant yield identical results. Whether you use a List<List<Integer>> or a List<List<Char>> does not matter for the algorithm.

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