Question

I am writing a program which accepts 400 numbers of type long and will modify some of them depending on conditions at runtime and I want to know whether to use ArrayList<Long> or long[].

Which will be faster to use? I am thinking of using long[] because size is fixed.

Answer 1

When the size is fixed, long[] is faster, but it allows a less maintainable API, because it does not implement the List interface.

Note a long[] is faster for 2 reasons:

1. Uses primitive longs and not box object Longs (also enables better cache performace, since the longs are allocated contigously and the Longs aren’t guaranteed to)
2. An array is much simpler and more efficient DS.

Nevertheless, for simpler maintainability – I would have used a List<Long>, unless performace is very critical at this part of the program.

If you use this collection very often in a tight loop – and your profiler says it is indeed a bottleneck – I would then switch to a more efficient long[].