What is causing it?

The error is thrown by the native code within the JVM. It happens before allocating memory for an array when the JVM performs a platform-specific check: whether the allocated data structure is addressable in this platform. This error is less common than you might initially think.

The reason you only seldom face this error is that Java arrays are indexed by int. The maximum positive int in Java is 2^31 – 1 = 2,147,483,647. And the platform-specific limits can be really close to this number – for example on my 64bit MB Pro on Java 1.7 I can happily initialize arrays with up to 2,147,483,645 or Integer.MAX_VALUE-2 elements.

Increasing the length of the array by one to Integer.MAX_VALUE-1 results in the familiar OutOfMemoryError:

Exception in thread "main" java.lang.OutOfMemoryError: Requested array size exceeds VM limit

But the limit might not be that high – on 32-bit Linux with OpenJDK 6, you will hit the “java.lang.OutOfMemoryError: Requested array size exceeds VM limit” already when allocating an array with ~1.1 billion elements. To understand the limits of your specific environments run the small test program described in the next chapter.