code: public int[] toArray() {
  		int[] a = new int[this.collection.size()];
  		int i = 0;
  		for (int col : this.collection) {
  			a[i++] = col;
  		}
  		return a;
  	}
docs: Transforms the field set into an array of field IDs. Whether the IDs are ordered
  or unordered depends on the specific subclass of the field set.
  
  @return An array of all contained field IDs.