Error message

Hi, I am having problems when I call getUnion.
The error message are "getUnion(IntergerSet) in IntergerSet cannot be
applied to ()" and "incompatible types"
where the arrows are.
Here is my code:

public void inputSet ()
    {
        Scanner input = new Scanner (System.in);

     int value;
     boolean arraySet[] = new boolean[100];
     do
     {
     System.out.println("\nPlease enter a value or -1 to finish: ");
     value = input.nextInt();
     if(validEntry(value))
             arraySet[value] = true;
            else
             System.out.printf("\nYou enter %d to finish entering values",
value, "\n");
     }while(value != -1);

     System.out.println("\n\nIf you want to find the union of two sets
press 'u': ");
        String unionSets = input.next();
        if("u".equalsIgnoreCase(unionSets))
---> System.out.printf("\nUNION", getUnion());

    }

    //Finding the union of two sets
    public String getUnion(IntergerSet other)
    {
     IntergerSet res = new IntergerSet();

     for(int i=0; i<SIZE; i++)
     {
     res.arraySet[i] = this.arraySet[i]||other.arraySet[i];
     }
---> return res;
    }