零长度多维数组

Question

I know that similar questions have been asked several times and they were nicely answered but... they were about zero length array of 1 dimension like:

int[] array = new int[0];

Seems that there is a purpose for such arrays in case when null should not / cannot be used. But why Java allows to create things like that:

int[][][] multiDims = new int[5][0][9];

Of course like in simple 1D case we get nothing from such array if we try to iterate or something and I am asking only because it looks extremely nasty for me. :-) How much memory is allocated for such a pointless creature?

Answer 1

As for why Java allows this - from the point of view of the language (not the concepts you're trying to express with it), why should it specifically disallow this? If you allow a zero-length array of any type, why specifically disallow a zero-length array of int[9] ? Disallowing it would necessitate more compiler checks to enforce a rule that's basically useless, because even without the rule the behavior is well defined.

Bottom line, compiler checks are not here to ensure your program makes sense. They're here only to check it's unambiguous.

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Edited to add:

As pointed out in a comments, such check is not even really possible, since array length is not part of the type information and can be given at run time. So, apart of a "special case" when there's int[0] directly in the source code, compiler doesn't even have any means to know whether it is a zero-length array.

Answer 2

This will create 6 objects - 5 empty arrays and an array containing these arrays.

Why is it allowed? For the same reason as in case of 1-dimensional arrays. If you create an array like this:

int[][][] multiDims = new int[p][q][r];

where each p , q and r can be sometimes 0 , handling these special cases would be very difficult. Instead you get a legitimate object which can be used in a loop (a very short loop - a one that ends immediately, but without errors).

Answer 3

The [5] actually does something useful: The outermost array will have five element arrays with length 0.

But you're right about the [9] . It doesn't do anything: Since all of the five intermediate arrays will be empty no array of length 9 will be created. Any integer could be put there to exactly the same effect.

The language could have been designed so that if there was one level of nested array with dimension set to literal 0 , then all subsequent levels would have to be set to literal 0 as well. But that would create yet another a special case in the specification and implementation and the benefit would be very small.