Question

In Java, what is faster and less in memory: int[n] or boolean[n] or maybe Bitset(n) ?

The question is applicable for arrays of small (n is up to 1000), medium (n is between 1000 and 100000) and huge (n is greater than 100000) sizes. Thank you.

I want to achieve flags (1/0) storage.

Answer 1

On most JVMs; an array or object has a 12-16 byte overhead. An int use 4 bytes and a boolean uses a byte (it doesn’t have to but it does with OpenJDK/HotSpot) BitSet uses two objects and more memory for small sets, but only one bit per have. So for small collections an int[] can be smaller than a BitSet but as the size grows, BitSet will be the smallest.

If the data structure is smaller than your cache the fastest is int[] then boolean[] then BitSet This is because there is non-trival overhead in breaking int into byte or a bit.

However once your cache size becomes important, it can be that the overhead of BitSet fades compared to the overhead of using a slower cache or main memory.

In short: if in doubt use BitSet as this is clearer as to your intent and its likely to be faster.