4.11 - 2D Array Creation and Access

A 2D array is basically an array of arrays. Its size is locked in at creation, just like a regular array, and it can hold either primitives or object references.

On the AP exam, 2D arrays are always rectangular: every row has the same number of columns. 2D arrays whose rows have different lengths are outside the scope of the course.


int[][] grid = new int[3][4]; // 3 rows, 4 columns

In new int[3][4], the first number is how many rows there are and the second is how many columns. Every row is its own 1D array of length 4.

Just like a 1D array, every element starts at the default value for its type when you create it with new.


double[][] prices = new double[2][2];  // every element is 0.0
boolean[][] taken = new boolean[3][5]; // every element is false
String[][] seats = new String[2][3];   // every element is null

You can also use a nested initializer list:


int[][] arr2D = { {1, 2, 3}, {4, 5, 6} };

Each inner list is one row, so arr2D has 2 rows and 3 columns:

	col 0	col 1	col 2
row 0	1	2	3
row 1	4	5	6

Square brackets with [row][col] let you access or modify a specific element. The first index is the row, the second is the column.


System.out.println(arr2D[1][2]); // 6, second row, third column
arr2D[0][0] = 100; // changes the top-left element

You can grab an entire row (as its own 1D array) using just one set of brackets:


int[] firstRow = arr2D[0]; // {1, 2, 3}

firstRow refers to the actual row inside arr2D, not a copy. So firstRow[0] = 50; would change arr2D[0][0] to 50 as well.

To find dimensions:


int numRows = arr2D.length;       // number of rows
int numCols = arr2D[0].length;    // number of columns (based on row 0)

Same rule as before, going out of range throws an ArrayIndexOutOfBoundsException.

Valid row indices run from 0 to arr2D.length - 1, and valid column indices run from 0 to arr2D[0].length - 1. For arr2D above, that’s rows 0 to 1 and columns 0 to 2:


System.out.println(arr2D[1][2]); // 6, the bottom-right element
System.out.println(arr2D[2][0]); // ArrayIndexOutOfBoundsException, there's no row 2
System.out.println(arr2D[0][3]); // ArrayIndexOutOfBoundsException, there's no column 3

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