Question

private ArrayList<HashSet<Integer>> sets = new ArrayList<HashSet<Integer>>();

            Iterator i = this.sets.iterator();
            while (i.hasNext()){

                if(i.next().containsAll(union)){
                    return true;
                }
            }

I get the following exception as it asssumes .next() gets an object of type Object

Exception in thread "main" java.lang.Error: Unresolved compilation problems: 
    Type mismatch: cannot convert from Object to HashSet<Integer>
    The method containsAll(HashSet<Integer>) is undefined for the type Object

While I get the object Hash set

        while (i.hasNext()){
            System.out.println(i.next().getClass().getName());
        }

prints

java.util.HashSet
java.util.HashSet
java.util.HashSet

What’s going on ?

Answer 1

Make sure your Iterator also as type of HashSet

Iterator<HashSet<Integer>> i = this.sets.iterator();