Set<Integer> set = ...;

int[] arr = new int[set.size()];

int index = 0;

for( Integer i : set ) {
  arr[index++] = i; //note the autounboxing here
}

Integer.intValue();

Set<Integer> myIntegers; // your set
int[] ints = new int[myInts.size()];
int index = 0;
for(Integer i : myIntegers){
    ints[index++] = i;
}

int[] array = set.stream().mapToInt(i->i).toArray();