Java: A two dimensional array is stored in column-major or row-major order?

Question

In Java, is a multidimensional array stored in column-major or row-major order?

Answer 1

Neither. What we may sometimes think of as two-dimensional array in Java is actually an array of references to arrays. It's not stored linearly in memory.

The Java Language specification notes this in the introduction:

The language supports arrays of arrays, rather than multidimensional arrays.

This has several implications.

• Arrays of arrays can be jagged -- member arrays can have different lengths.
• The members of an outer array are references, and can be null.
• Cloning an outer array is shallow -- the member arrays are shared between the original and the clone.

From the JLS, section 10.2, "Array Variables":

A single variable of array type may contain references to arrays of different lengths, because an array's length is not part of its type.

From the JLS, section 10.7, "Array Members":

A clone of a multidimensional array is shallow, which is to say that it creates only a single new array. Subarrays are shared.