next();
        }
      }
      current = null;
    }

    private void checkForConcurrentModification() {
      if (modCount != expectedModCount) {
        throw new ConcurrentModificationException();
      }
    }

    @Override
    public boolean hasNext() {
      checkForConcurrentModification();
      return next != null;
    }

    @CanIgnoreReturnValue
    @Override

    @ParametricNullness
    abstract T result(int entryIndex);

    private void checkForConcurrentModification() {
      if (backingMap.modCount != expectedModCount) {
        throw new ConcurrentModificationException();
      }
    }

    @Override
    public boolean hasNext() {
      checkForConcurrentModification();
      return entryIndex >= 0;
    }

    @Override
    @ParametricNullness

      @Override
      public boolean hasNext() {
        return currentIndex >= 0;
      }

      @Override
      @ParametricNullness
      public E next() {
        checkForConcurrentModification();
        if (!hasNext()) {
          throw new NoSuchElementException();
        }
        indexToRemove = currentIndex;
        E result = element(currentIndex);

      @Override
      public boolean hasNext() {
        return currentIndex >= 0;
      }

      @Override
      @ParametricNullness
      public E next() {
        checkForConcurrentModification();
        if (!hasNext()) {
          throw new NoSuchElementException();
        }
        indexToRemove = currentIndex;
        E result = element(currentIndex);

      return currentIndex >= 0;
    }

    @ParametricNullness
    abstract T getOutput(int entry);

    @Override
    @ParametricNullness
    public T next() {
      checkForConcurrentModification();
      if (!hasNext()) {
        throw new NoSuchElementException();
      }
      indexToRemove = currentIndex;
      T result = getOutput(currentIndex);

      return currentIndex >= 0;
    }

    @ParametricNullness
    abstract T getOutput(int entry);

    @Override
    @ParametricNullness
    public T next() {
      checkForConcurrentModification();
      if (!hasNext()) {
        throw new NoSuchElementException();
      }
      indexToRemove = currentIndex;
      T result = getOutput(currentIndex);