how calculate java array memory usage

you'll probably get the best approximation from this: http://java.sun.com/javase/6/docs/api/java/lang/instrument/Instrumentation.html#getObjectSize(java.lang.Object)

This article, and this comprehensive one demonstrates/details this approach

I know I'm kinda late to the party, but it's really not extremely hard to compute the memory footprint.

Lets take your first example: int c[] = new int[N];

According to the 64-bit memory model, an int is 4 bytes, so all the elements will be 4*N bytes in size. In addition to that, Java has a 24 bytes array overhead and there's also 8 bytes for the actual array object. So that's a total of 32 + 4 * N bytes.

For a 2 dimensional array: int a[][] = new int[N][M];

It's basically the same just that each element in the first array is another array of size M, so instead of 4 we have 32 + 4 * M, so the total size is 32 + (32 + 4 * M) * N.

It's true that a generalization for D dimensions is pretty complicated, but you get the idea.

The int[] or int[][] is not a primitive data type. It is an Object in Java. And with an Object, the size cannot be calculated straight away.

If you want an accurate answer, you can't. At least not in any easy way. This thread explains more.

The trouble with Bragaadeesh's and Bakkal's answers are that they ignore overhead. Each array also stores things like the number of dimensions it has, how long it is and some stuff the garbage collector uses.

For a simple estimate, you should be fine by using the calculations from the other answers and adding 100-200 bytes.