Understanding Java Iterator

Question

If I run the following code, it will print out 3 times duplicate, but when I remove the if statement inside the while loop (just to see how many times it will iterate) it starts an infinite loop.

How does actually this hasNext() method working? I thought that will iterate only 5 times as I have 5 items in the list.

    public class ExerciseOne {
    public static void main(String []args){
        String []colors = {"MAGENTA","RED","WHITE","BLUE","CYAN"};
        List<String> list = new ArrayList<String>();
        for(String color : colors)
            list.add(color);
        String[] removeColors = {"RED","WHITE","BLUE"};
        List<String> removeList = new ArrayList<String>();
        for(String color : removeColors)
            removeList.add(color);

        removeColors(list,removeList);
        System.out.printf("%n%nArrayList after calling removeColors:%n");
        for(String color : list)
        {
            System.out.printf("%s ",color);
        }
    }

    private static void removeColors(Collection<String> collection1, Collection<String> collection2)
    {
        Iterator<String> iterator = collection1.iterator();

            while(iterator.hasNext()){
                if(collection2.contains(iterator.next()))
                    System.out.println("duplicate");
            }
    }

}

Answer 1

It is pretty simple, actually

while(iterator.hasNext()){
    if(collection2.contains(iterator.next()))
       System.out.println("duplicate");
}

Imagine that the iterator is a pointer to an element of your list.

When you call next(), you're moving this pointer one step ahead.

If you don't move the pointer, hasNext() will always be true because you're still in the beginning of the list.

So you have to call the iterator's next() until there isn't any remaining element in the list.