Using a long as ArrayList index in java

Question

I am writing this java program to find all the prime numbers up to num using the Sieve of Eratosthenes, but when I try to compile, it says I can\'t use a long var as an arra

Answer 1

The Java specification limits arrays to at most Integer.MAX_VALUE elements. While a List may contain more elements (this is true for Collections in general), you can only add/get/remove/set them using an int index.

Assuming you have the memory for that many elements (very unlikely I think), you could write your own data structure consisting of "concatenated" arrays. The get() and set() methods would take a long index and figure out the corresponding array and int index within that array.

Also, I would suggest using booleans to represent the state of each number, instead of storing/removing each number explicitly. This would be better because (1) booleans take less space than longs, and (2) shifting elements (as done in ArrayList) during element removal can be expensive.