Java Iterator Loop once, start in middle

user_2139340 0
Viewed 1858

I have an Iterator - placed somewhere in the middle of my collection.

I want to loop over the collection from there on, returning a special element if I find it.

If I reach !hasNext() then I want to start at the begining, but I want to stop if I reach the place where I started first.

The best Idea how to solve that seems currently to save the "first" (wherever the iterator pointed when I started) element and then stop if I find that one again.

And that works as long as items can occure only once in my collection (i.e. in a HasSet).

Any better ideas how to do such thing? Iterator doesn't seem to supply a pointer/number back to me that I could compare.

iterator collections loops java
4 Answers

With Guava Iterators (Java 7):

Iterator<YourType> iterator = Iterators.concat(yourIteratorPlaceSomewhereInMiddle, Iterators.limit(yourCollection.iterator(), positionOfSomewhereInMiddle));

With Streams (Java 8):

Optional<YourType> theAnswer = IntStream.iterate(positionOfSomewhereInMiddle, pos -> ++pos)
                                        .map(pos -> pos % sizeOfCollection)
                                        .mapToObj(yourCollection::get)
                                        .limit(sizeOfCollection)
                                        .filter(element -> element is 42)
                                        .findFirst();
user_20896750

If the collection is a list, then it can be done with below code

private void Object traverseList(List namesList, int startIndex ) {
    int index = 0;
    Iterator iterator = namesList.listIterator(startIndex);
    while (iterator.hasNext() && index < namesList.size()){
        Object element = iterator.next();
        /*if(element.isSpecialElement()){
         return element;
         }
        */
        index++;
        System.out.println(element);
        if(!iterator.hasNext()){
            iterator = namesList.iterator();
        }
    }
}

Since you want to transverse all the elements in the list, I am making use of list size. And during iteration, if the special element is found, it can be returned.

user_22127680

To start to iterate a Set from a definedValue I do :

    public void iterate(Set<Sample> set, Sample definedValue){
        Iterator<Sample> iterator = set.iterator();
        while(iterator.hasNext()){
            Sample currentSample = iterator.next();
            if(currentSample.equals(definedValue)){
                while(iterator.hasNext()){
                Sample sampleToConsider = iterator.next();
                //Then do anything
                }
            }
        }
    }
user_165994180

The solution becomes more simple if you:

  • Don't use Iterator, it makes it more complicated

  • Use a List instead of Set as Set is ill-suited for this use case 

    public static void main(String[] args) {
    Set<String> yourStartingCollection = new HashSet<>();
    List<String> fooList = new ArrayList<>(yourStartingCollection);

    Optional<String> specialElement = findSpecialElementStartingFromMiddle(fooList);
    if (specialElement.isPresent()) {
        // found it!
    }
}

private static Optional<String> findSpecialElementStartingFromMiddle(List<String> elements) {
    int middleIndex = elements.size() / 2;
    Optional<String> result = Optional.empty();

    for (int i = middleIndex; i < elements.size(); i++) {
        String element = elements.get(i);
        if (isSpecial(element)) {
            result = Optional.of(element);
            break;
        }
    }

    if (result.isPresent()) {
        return result;
    }

    for (int i = 0; i < middleIndex; i++) {
        String element = elements.get(i);
        if (isSpecial(element)) {
            result = Optional.of(element);
            break;
        }
    }

    return result;
}

private static boolean isSpecial(String element) {
    return element.equals("I'm special");
}
user_57714460
Related