Unfolded Iterator

FlattentNestedIterator

public class FlattentNestedIterator<T>  implements Iterator<T>{
    private Stack<Iterator<Nested<T>>> stack = new Stack<>();
    private T current;
    private boolean hasNext;
    public FlattentNestedIterator(Iterator<Nested<T>>  nestedIterator) {
        if(nestedIterator == null){
            throw new IllegalArgumentException("Invalid Iterator");
        }
        stack.push(nestedIterator);
        advance();
    }

    @Override
    public boolean hasNext() {
        return hasNext;
    }

    private void advance(){
        while(!stack.isEmpty()){
            Iterator<Nested<T>> top = stack.peek();
            if(top==null || !top.hasNext()){
                stack.pop();
            }else{
                Nested<T> n = top.next();
                if (n.isObject()) {
                    current = n.getObject();
                    hasNext = true;
                    return;
                } else {
                    stack.push(n.getIterator());
                }
            }

        }
        hasNext =false;
    }

    @Override
    public T next() {
        if(!hasNext()){
            throw new NoSuchElementException();
        }
        T result = current;
        advance();
        return result;
    }
}

Nested

public class Nested<T> {
    private T object;
    private Iterator<Nested<T>> iterator;
    private boolean isObject;

    private Nested(T object, Iterator<Nested<T>> iterator, boolean isObject) {
        this.object = object;
        this.iterator = iterator;
        this.isObject = isObject;
    }

    public T getObject() {
        return object;
    }

    public Iterator<Nested<T>> getIterator() {
        return iterator;
    }

    public boolean isObject() {
        return isObject;
    }

    public static class NestedBuiler<T>{
        public Nested<T> withObject(T object){
            return new Nested<>(object, null, true);
        }

        public Nested<T> withIterator(Iterator<Nested<T>> iterator){
            return new Nested<>(null, iterator, false);
        }
    }
}