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The java.lang.OutOfMemoryError: Requested array size exceeds VM limit can appear as a result of either of the following situations:

  • Your arrays grow too big and end up having a size between the platform limit and the Integer.MAX_INT
  • You deliberately try to allocate arrays larger than 2^31-1 elements to experiment with the limits.

In the first case, check your code base to see whether you really need arrays that large. Maybe you could reduce the size of the arrays and be done with it. Or divide the array into smaller bulks and load the data you need to work with in batches fitting into your platform limit.

In the second case – remember that Java arrays are indexed by int. So you cannot go beyond 2^31-1 elements in your arrays when using the standard data structures within the platform. In fact, in this case you are already blocked by the compiler announcing “error: integer number too large” during compilation.

But if you really work with truly large data sets, you need to rethink your options. You can load the data you need to work with in smaller batches and still use standard Java tools, or you might go beyond the standard utilities. One way to achieve this is to look into the sun.misc.Unsafe class. This allows you to allocate memory directly like you would in C.