ResizableQueue

import java.util.Iterator;

class Queue<T> implements Iterable<T> {

  private class QueueIterator implements Iterator<T> {

    private int curr = head;

    public boolean hasNext() {
      return curr != tail;
    }
    public void remove() {
      throw new UnsupportedOperationException();
    }
    public T next() {
      return data[curr++];
    }
  }
  private T[] data;
  private int head, tail;
  public Iterator<T> iterator() {
    return new QueueIterator();
  }


  public Queue() {
    data = (T[]) new Object[2];
    head = 0;
    tail = 0;
  }

  public int size() {
    return tail - head;
  }

  public boolean isEmpty() {
    return size() == 0;
  }

  private void realign() {
    java.util.Arrays.sort(data, new java.util.Comparator<Object>() {
      public int compare(Object o1, Object o2) {
        if (o1 == null) return 1;
        else if(o2 == null) return -1;
        else return 0;
      }
    });
    tail -= head;
    head = 0;
  }

  private void resize() {
    if (tail == data.length && size() != data.length) {
      realign();
    }
    if (data.length == size()) {
      int newLength = data.length * 2;
      T[] newData = (T[]) new Object[newLength];
      System.arraycopy(data, 0, newData, 0, data.length);
      data = newData;
    } else if (size() == data.length / 4 && size() != 0) {
      int newLength = data.length / 2;
      T[] newData = (T[]) new Object[newLength];
      System.arraycopy(data, 0, newData, 0, data.length);
      data = newData;
    }
  }

  public void enqueue(T item) {
    if (item == null) { throw new NullPointerException(); }
    resize();
    data[tail] = item;
    tail++;
    return;
  }

  public T dequeue() {
    if (isEmpty()) { throw new java.util.NoSuchElementException(); }
    T res = data[head];
    data[head] = null;
    head++;
    if (head == tail) {
      head = tail = 0;
    }
    return res;
  }
}