A grid of values

An array keeps its values in a line, and one index says where a value is. Some data goes in two directions. A cinema's seats are in rows, and each row has seats from left to right. A spreadsheet has rows and columns, and so does a picture made of pixels. For data like that, Java has two-dimensional arrays, where each value has two indexes: its row and its column.

Here the values are the brightness of the pixels of a tiny picture, from 0 for black to 9 for white:

int[][] image = new int[3][4];
image[1][2] = 9;
image[0][3] = 4;
System.out.println(image[1][2]);
System.out.println(image[0][3]);
System.out.println(image[2][0]);

9
4
0

• int[][] is the type "two-dimensional array of ints". Two pairs of brackets, two indexes.
• new int[3][4] creates a grid of 3 rows with 4 columns each: 12 ints, all 0 to start with.
• image[1][2] = 9; stores 9 in row 1, column 2. The row comes first, then the column.
• image[1][2] reads the value back. Both indexes start from 0, as always.

This is the grid after the two assignments:

The order of the indexes matters: image[2][1] is row 2, column 1, a different cell from image[1][2]. Here it still holds 0.

A cell works like a variable

Each cell behaves like a variable of the element type, just as the elements of an array do. You can use its value in a calculation and change it with =, +=, ++ or --, and the indexes can be any whole-number expressions:

int[][] image = new int[3][4];
image[1][2] = 9;
image[1][2]--;
int row = 2;
image[row][row + 1] = image[1][2] / 2;
System.out.println(image[1][2] + " " + image[2][3]);

8 4

Default values

A new grid starts with the same default values as a new array: 0 for int, 0.0 for double, false for boolean and null for String. That suits a cinema's seat map. In a boolean[][] where true means booked, every seat starts free:

boolean[][] booked = new boolean[5][8];
booked[2][3] = true;
booked[2][4] = true;
System.out.println(booked[2][3]);
System.out.println(booked[2][5]);
System.out.println(booked[0][0]);

true
false
false

People count rows and seats from 1, but the indexes start from 0. The ticket for row 3, seat 4 is booked[2][3], so a program that reads seats the way people write them subtracts one from each number: booked[row - 1][seat - 1].

The short way

When you know the values as you write the program, list them row by row: braces around each row, commas between the rows, and braces around them all.

int[][] image = {
    {0, 3, 7, 3},
    {3, 8, 9, 8},
    {0, 3, 7, 3}
};
System.out.println(image[1][1]);
System.out.println(image[2][2]);

8
7

The first row in braces is row 0, the next one row 1, and so on. Java counts the rows, and the values in each, to make a grid of the right size. As with a one-dimensional array, the braces alone work only in the line that declares the variable.