public PriorityQueue(int initialCapacity,
             Comparator<? super E> comparator) {
    // Note: This restriction of at least one is not actually needed,
    // but continues for 1.5 compatibility
    if (initialCapacity < 1)
      throw new IllegalArgumentException();
    this.queue = new Object[initialCapacity];
    this.comparator = comparator;
  }

 public boolean offer(E e) {
    if (e == null)
      throw new NullPointerException();
    //记录了队列被修改的次数
    modCount++;
    int i = size;
    if (i >= queue.length)
      //扩容
      grow(i + 1);
    //增加元素个数
    size = i + 1;
    if (i == 0) 
      //第一次添加元素，直接放到第0个位置即可
      queue[0] = e;
    else
      //需要将元素放到最后，再做上滤操作
      siftUp(i, e);
    return true;
  }

 private void grow(int minCapacity) {
    int oldCapacity = queue.length;
    // Double size if small; else grow by 50%
    int newCapacity = oldCapacity + ((oldCapacity < 64) ?
                     (oldCapacity + 2) :
                     (oldCapacity >> 1));
    // overflow-conscious code
    if (newCapacity - MAX_ARRAY_SIZE > 0)
      newCapacity = hugeCapacity(minCapacity);
    //元素拷贝
    queue = Arrays.copyOf(queue, newCapacity);

private static int hugeCapacity(int minCapacity) {
    if (minCapacity < 0) // overflow
      throw new OutOfMemoryError();
    return (minCapacity > MAX_ARRAY_SIZE) ?
      Integer.MAX_VALUE :
      MAX_ARRAY_SIZE;
  }

private void siftUpUsingComparator(int k, E x) {
    while (k > 0) {
      //计算父亲节点的下标
      int parent = (k - 1) >>> 1;
      Object e = queue[parent];
      //与父节点进行比较
      if (comparator.compare(x, (E) e) >= 0)
        break;
      queue[k] = e;
      k = parent;
    }
    queue[k] = x;
  }

public E poll() {
    if (size == 0)
      return null;
    int s = --size;
    //自增变量，代表队列修改次数
    modCount++;
    E result = (E) queue[0];
    E x = (E) queue[s];
    queue[s] = null;
    if (s != 0)
      siftDown(0, x);
    return result;
  }

 private void siftDownUsingComparator(int k, E x) {
 

    int half = size >>> 1;
    //这里k必须有孩子，故叶节点需要比较
    while (k < half) {
      //以下几行代码到较小的那个儿子,用变量c表示
      int child = (k << 1) + 1;
      //这里假设左儿子比较小
      Object c = queue[child];
      int right = child + 1;
      //左右儿子比较，如果右儿子小则将c赋值为右儿子
      if (right < size &&
        comparator.compare((E) c, (E) queue[right]) > 0)
        c = queue[child = right];
      //如果x比小儿子还小，说明k就是正确位置
      if (comparator.compare(x, (E) c) <= 0)
        break;
      queue[k] = c;
      k = child;
    }
    queue[k] = x;
  }