Sunday, May 2, 2010

java array memory allocation

See if anyone can shed some light. Not an interview question. Just my own curiosity.

Q: When i declare a Serializable[2] array, how does JVM decide how many contiguous bytes to reserve for 2 array elements of type Serializable? The actual object can have any number of fields in it, so the object can occupy any amount of memory.

Now the background. In C, i remember array element addresses are always uniform and contiguous, so we can use pointer arithmetic for random access. Someone mentioned java array is similar to C array. For example, int[2] would need exactly 32 bytes * 2 = 64 bytes, probably same in java vs C.

There's more circumstantial evidence that java arrays occupy contiguous memory -- when inserting an element in the middle, java need to shift or copy data in memory, that's why linked list is known to be more efficient for this operation.

It's possible to measure runtime memory usage before and after declaring a Serializable[10000] but I'd like to understand how system decides how much memory to allocate.

Thanks.

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