Getting the array length of a 2D array in Java

Question

Consider

public static void main(String[] args) {

    int[][] foo = new int[][] {
        new int[] { 1, 2, 3 },
        new int[] { 1, 2, 3, 4},
    };

    System.out.println(foo.length); //2
    System.out.println(foo[0].length); //3
    System.out.println(foo[1].length); //4
}

Column lengths differ per row. If you're backing some data by a fixed size 2D array, then provide getters to the fixed values in a wrapper class.

A 2D array is not a rectangular grid. Or maybe better, there is no such thing as a 2D array in Java.

import java.util.Arrays;

public class Main {
  public static void main(String args[]) {

    int[][] test; 
    test = new int[5][];//'2D array'
    for (int i=0;i<test.length;i++)
      test[i] = new int[i];

    System.out.println(Arrays.deepToString(test));

    Object[] test2; 
    test2 = new Object[5];//array of objects
    for (int i=0;i<test2.length;i++)
      test2[i] = new int[i];//array is a object too

    System.out.println(Arrays.deepToString(test2));
  }
}

Outputs

[[], [0], [0, 0], [0, 0, 0], [0, 0, 0, 0]]
[[], [0], [0, 0], [0, 0, 0], [0, 0, 0, 0]]

The arrays test and test2 are (more or less) the same.

It was really hard to remember that

    int numberOfColumns = arr.length;
    int numberOfRows = arr[0].length;

Let's understand why this is so and how we can figure this out when we're given an array problem. From the below code we can see that rows = 4 and columns = 3:

    int[][] arr = { {1, 1, 1, 1}, 

                    {2, 2, 2, 2}, 

                    {3, 3, 3, 3} };

arr has multiple arrays in it, and these arrays can be arranged in a vertical manner to get the number of columns. To get the number of rows, we need to access the first array and consider its length. In this case, we access [1, 1, 1, 1] and thus, the number of rows = 4. When you're given a problem where you can't see the array, you can visualize the array as a rectangle with n X m dimensions and conclude that we can get the number of rows by accessing the first array then its length. The other one (arr.length) is for the columns.

Java allows you to create "ragged arrays" where each "row" has different lengths. If you know you have a square array, you can use your code modified to protect against an empty array like this:

if (row > 0) col = test[0].length;