Which comes first in a 2D array, rows or columns?

Java specifies arrays similar to that of a "row major" configuration, meaning that it indexes rows first. This is because a 2D array is an "array of arrays".

For example:

int[ ][ ] a = new int[2][4];  // Two rows and four columns.a[0][0] a[0][1] a[0][2] a[0][3]a[1][0] a[1][1] a[1][2] a[1][3]

It can also be visualized more like this:

a[0] ->  [0] [1] [2] [3]a[1] ->  [0] [1] [2] [3]

The second illustration shows the "array of arrays" aspect. The first array contains {a[0] and a[1]}, and each of those is an array containing four elements, {[0][1][2][3]}.

TL;DR summary:

Array[number of arrays][how many elements in each of those arrays]

For more explanations, see also Arrays - 2-dimensional.

While Matt B's may be true in one sense, it may help to think of Java multidimensional array without thinking about geometeric matrices at all. Java multi-dim arrays are simply arrays of arrays, and each element of the first-"dimension" can be of different size from the other elements, or in fact can actually store a null "sub"-array. See comments under this question

Instinctively one thinks geometrically: horizontal (X) axis and then vertical (Y) axis. This is not, however, the case with a 2D array, rows come first and then columns.

Consider the following analogy: in geometry one walks to the ladder (X axis) and climbs it (Y axis). Conversely, in Java one descends the ladder (rows) and walks away (columns).