Why doesn't Java have true multidimensional arrays?

Question

The TL;DR version, for those who don\'t want the background, is the following specific question:

Question

Why doesn\'t Java have an implemen

Answer 1

but it seems that this is really not what one might have expected.

Why?

Consider that the form T[] means "array of type T", then just as we would expect int[] to mean "array of type int", we would expect int[][] to mean "array of type array of type int", because there's no less reason for having int[] as the T than int.

As such, considering that one can have arrays of any type, it follows just from the way [ and ] are used in declaring and initialising arrays (and for that matter, {, } and ,), that without some sort of special rule banning arrays of arrays, we get this sort of use "for free".

Now consider also that there are things we can do with jagged arrays that we can't do otherwise:

1. We can have "jagged" arrays where different inner arrays are of different sizes.
2. We can have null arrays within the outer array where appropriate mapping of the data, or perhaps to allow lazy building.
3. We can deliberately alias within the array so e.g. lookup[1] is the same array as lookup[5]. (This can allow for massive savings with some data-sets, e.g. many Unicode properties can be mapped for the full set of 1,112,064 code points in a small amount of memory because leaf arrays of properties can be repeated for ranges with matching patterns).
4. Some heap implementations can handle the many smaller objects better than one large object in memory.

There are certainly cases where these sort of multi-dimensional arrays are useful.

Now, the default state of any feature is unspecified and unimplemented. Someone needs to decide to specify and implement a feature, or else it wouldn't exist.

Since, as shown above, the array-of-array sort of multidimensional array will exist unless someone decided to introduce a special banning array-of-array feature. Since arrays of arrays are useful for the reasons above, that would be a strange decision to make.

Conversely, the sort of multidimensional array where an array has a defined rank that can be greater than 1 and so be used with a set of indices rather than a single index, does not follow naturally from what is already defined. Someone would need to:

1. Decide on the specification for the declaration, initialisation and use would work.
2. Document it.
3. Write the actual code to do this.
4. Test the code to do this.
5. Handle the bugs, edge-cases, reports of bugs that aren't actually bugs, backward-compatibility issues caused by fixing the bugs.

Also users would have to learn this new feature.

So, it has to be worth it. Some things that would make it worth it would be:

1. If there was no way of doing the same thing.
2. If the way of doing the same thing was strange or not well-known.
3. People would expect it from similar contexts.
4. Users can't provide similar functionality themselves.

In this case though:

1. But there is.
2. Using strides within arrays was already known to C and C++ programmers and Java built on its syntax so that the same techniques are directly applicable
3. Java's syntax was based on C++, and C++ similarly only has direct support for multidimensional arrays as arrays-of-arrays. (Except when statically allocated, but that's not something that would have an analogy in Java where arrays are objects).
4. One can easily write a class that wraps an array and details of stride-sizes and allows access via a set of indices.

Really, the question is not "why doesn't Java have true multidimensional arrays"? But "Why should it?"

Of course, the points you made in favour of multidimensional arrays are valid, and some languages do have them for that reason, but the burden is nonetheless to argue a feature in, not argue it out.

(I hear a rumour that C# does something like this, although I also hear another rumour that the CLR implementation is so bad that it's not worth having... perhaps they're just rumours...)

Like many rumours, there's an element of truth here, but it is not the full truth.

.NET arrays can indeed have multiple ranks. This is not the only way in which it is more flexible than Java. Each rank can also have a lower-bound other than zero. As such, you could for example have an array that goes from -3 to 42 or a two dimensional array where one rank goes from -2 to 5 and another from 57 to 100, or whatever.

C# does not give complete access to all of this from its built-in syntax (you need to call Array.CreateInstance() for lower bounds other than zero), but it does for allow you to use the syntax int[,] for a two-dimensional array of int, int[,,] for a three-dimensional array, and so on.

Now, the extra work involved in dealing with lower bounds other than zero adds a performance burden, and yet these cases are relatively uncommon. For that reason single-rank arrays with a lower-bound of 0 are treated as a special case with a more performant implementation. Indeed, they are internally a different sort of structure.

In .NET multi-dimensional arrays with lower bounds of zero are treated as multi-dimensional arrays whose lower bounds just happen to be zero (that is, as an example of the slower case) rather than the faster case being able to handle ranks greater than 1.

Of course, .NET could have had a fast-path case for zero-based multi-dimensional arrays, but then all the reasons for Java not having them apply and the fact that there's already one special case, and special cases suck, and then there would be two special cases and they would suck more. (As it is, one can have some issues with trying to assign a value of one type to a variable of the other type).

Not a single thing above shows clearly that Java couldn't possibly have had the sort of multi-dimensional array you talk of; it would have been a sensible enough decision, but so also the decision that was made was also sensible.