Java Iterator – infinite loop



What am I doing wrong? Why does this iterator keep running?

default List<T> greedyAlgorithm() {
    List<T> lst = new ArrayList<>();
    T element = selection().next();
        
    while(selection().hasNext()) {
        if(feasibility(lst ,element)) {
            assign(lst, element);
        } else {
            element = selection().next();
        }

        if(solution(lst)){
            return lst;
        }
    }   

    return null;
}

The feasibility function checks if the element is viable, if yes assign the element to the list. Then the solution checks if this is the ‘algo’ solution and returns the list, else check the next element in the list.

Answer

next should always be executed inside the loop.

If the feasibility condition isn’t false or solution true, then it’ll keep running without advancing.

I suggest rearranging the loop so you unconditionally call next.

There may also be a problem if selection returns a new Iterator every call.

Perhaps something like this:

default List<T> greedyAlgorithm() {
    List<T> lst = new ArrayList<>();
    
    for (
        Iterator<T> selection = selection();
        selection.hasNext();
    ) {
        T element = selection.next();
        if (feasibility(lst ,element)) {
            assign(lst, element);
            if (solution(lst)) {
                return lst;
            }
        }
    }   
    return null;
}

If selection() returned an Iterable you could use a posh for loop. Generally returning a type such as Iterable that (kind of) doesn’t change internal state is better than a type that does, such as Iterator (although there is remove so Iterable.iterator isn’t entirely innocent).



Source: stackoverflow