Using Stream:

// int[] nums = {1,2,3,4,5}
Set<Integer> set = Arrays.stream(nums).boxed().collect(Collectors.toSet())

string[] doesn't exist, I guess you mean String[].

For converting a Set<Integer> to int[] you'd have to iterate over the set manually.

Like this:

Set<Integer> set = ...;

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

int index = 0;

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

Note that sets don't have any particular order, if the order is important, you'd need to use a SortedSet.

You can create an int[] from any Collection<Integer> (including a HashSet<Integer>) using Java 8 streams:

int[] array = coll.stream().mapToInt(Number::intValue).toArray();

The library is still iterating over the collection (or other stream source) on your behalf, of course.

In addition to being concise and having no external library dependencies, streams also let you go parallel if you have a really big collection to copy.