They're not the same thing. A flat array is fixed in dimension (modulo sum each size = total size). An array of arrays can have its sub-arrays replaced.

They're not the same thing. A flat array is fixed in dimension (modulo sum each size = total size). An array of arrays can have its sub-arrays replaced. long[][] foo = new long[2][]; foo[0] = new long[5]; foo[1] = new long[3]; // ... foo[0] = new long[7]; Although I think as a developer you probably almost always want to index a single flat flat array if the dimensions are fixed. This is much faster.