Read priorityQueen source code

//Look at the constructor first.
public PriorityQueue() {
        this(DEFAULT_INITIAL_CAPACITY, null);
    }

public PriorityQueue(int initialCapacity,
                         Comparator<? super E> comparator) {
       
        if (initialCapacity < 1)
            throw new IllegalArgumentException();
        this.queue = new Object[initialCapacity];
        this.comparator = comparator;
    }

 public PriorityQueue(Collection<? extends E> c) {
        if (c instanceof SortedSet<?>) {
	   //The incoming is SortedSet
            SortedSet<? extends E> ss = (SortedSet<? extends E>) c;
            this.comparator = (Comparator<? super E>) ss.comparator();
            initElementsFromCollection(ss);
        }
	/ / Determine whether it is a subclass of PriorityQueue
        else if (c instanceof PriorityQueue<?>) {
            PriorityQueue<? extends E> pq = (PriorityQueue<? extends E>) c;
            this.comparator = (Comparator<? super E>) pq.comparator();
            initFromPriorityQueue(pq);
        }
        else {
            this.comparator = null;
            initFromCollection(c);
        }
    }


private void initElementsFromCollection(Collection<? extends E> c) {
        //Because the SortedSet has a natural order, copy the array directly
        Object[] a = c.toArray();
        // If c.toArray incorrectly doesn't return Object[], copy it.可能会出BUG。
        if (a.getClass() != Object[].class)
	  //recreate an array
            a = Arrays.copyOf(a, a.length, Object[].class);
        int len = a.length;
	//not understand
        if (len == 1 || this.comparator != null)
            for (int i = 0; i < len; i++)
                if (a[i] == null)
                    throw new NullPointerException();
        this.queue = a;
        this.size = a.length;
    }


private void initFromPriorityQueue(PriorityQueue<? extends E> c) {
        //The actual type is PriorityQueue
        if (c.getClass() == PriorityQueue.class) {
            this.queue = c.toArray();
            this.size = c.size();
        } else {

            initFromCollection(c);
        }
    }

 private void initFromCollection(Collection<? extends E> c) {
        initElementsFromCollection(c);
        heapify();
    }

 private void heapify() {
        for (int i = (size >>> 1) - 1; i >= 0; i--)
            siftDown(i, (E) queue[i]);
    }


 private void siftDown(int k, E x) {
        if (comparator != null)
            siftDownUsingComparator(k, x);
        else
            siftDownComparable(k, x);
    }

//sort with comparator
private void siftDownUsingComparator(int k, E x) {
        int half = size >>> 1;
        while (k < half) {
            int child = (k << 1) + 1;
            Object c = queue[child];
            int right = child + 1;
            if (right < size &&
                comparator.compare((E) c, (E) queue[right]) > 0)
                c = queue[child = right];
            if (comparator.compare(x, (E) c) <= 0)
                break;
            queue[k] = c;
            k = child;
        }
        queue[k] = x;
    }

//sort without comparator
private void siftDownComparable(int k, E x) {
        Comparable<? super E> key = (Comparable<? super E>)x;
        int half = size >>> 1;        // loop while a non-leaf
        while (k < half) {
            int child = (k << 1) + 1; // assume left child is least
            Object c = queue[child];
            int right = child + 1;
            if (right < size &&
                ((Comparable<? super E>) c).compareTo((E) queue[right]) > 0)
                c = queue[child = right];
            if (key.compareTo((E) c) <= 0)
                break;
            queue[k] = c;
            k = child;
        }
        queue[k] = key;
    }


//add method to add elements
public boolean add(E e) {
        return offer(e);
    }

public boolean offer(E e) {
        if (e == null)
            throw new NullPointerException();
	// add one to the number of modifications
        modCount++;
        int i = size;
        if (i >= queue.length)
	//If the current capacity is not enough, expand the capacity
            grow(i + 1);
        size = i + 1;
	// insert the first element
        if (i == 0)
            queue[0] = and;
        else
	// Insert the element at the appropriate position. (move element up)
            siftUp(i, e);
        return true;
    }

 private void siftUp(int k, E x) {
        if (comparator != null)
            siftUpUsingComparator(k, x);
        else
            siftUpComparable(k, x);
    }
 private void siftUpComparable(int k, E x) {
        Comparable<? super E> key = (Comparable<? super E>) x;
        while (k > 0) {
            int parent = (k - 1) >>> 1;
            Object e = queue[parent];
            if (key.compareTo((E) e) >= 0)
                break;
            queue[k] = e;
            k = parent;
        }
        queue[k] = key;
    }

 private void siftUpUsingComparator(int k, E x) {
        while (k > 0) {
            int parent = (k - 1) >>> 1;
            Object e = queue[parent];
            if (comparator.compare(x, (E) e) >= 0)
                break;
            queue[k] = e;
            k = parent;
        }
        queue[k] = x;
    }
private void grow(int minCapacity) {
        int oldCapacity = queue.length;
        // If it is less than 64, the expansion is 2 times +2, otherwise the expansion is 50%.
        int newCapacity = oldCapacity + ((oldCapacity < 64) ?
                                         (oldCapacity + 2) :
                                         (oldCapacity >> 1));
        // exceeds the maximum value of the array
        if (newCapacity - MAX_ARRAY_SIZE > 0)
            newCapacity = hugeCapacity(minCapacity);
        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 siftUp(int k, E x) {
        if (comparator != null)
            siftUpUsingComparator(k, x);
        else
            siftUpComparable(k, x);
    }

  private void siftUpUsingComparator(int k, E x) {
        while (k > 0) {
            int parent = (k - 1) >>> 1;
            Object e = queue[parent];
            if (comparator.compare(x, (E) e) >= 0)
                break;
            queue[k] = e;
            k = parent;
        }
        queue[k] = x;
    }

//Get the head element but don't remove it and return null if the queue is empty
 public E peek() {
        if (size == 0)
            return null;
        return (E) queue[0];
    }


//Remove the head element and return null if the queue is empty (move down)
 public E poll() {
        if (size == 0)
            return null;
        int s = --size;
        modCount++;
        E result = (E) queue[0];
        E x = (E) queue[s];
        queue[s] = null;
        if (s != 0)
            siftDown (0, x);
        return result;
    }



// delete element
public boolean remove(Object o) {
        int i = indexOf(o);
        if (i == -1)
            return false;
        else {
            removeAt(i);
            return true;
        }
    }

private int indexOf(Object o) {
        if (o != null) {
            for (int i = 0; i < size; i++)
                if (o.equals(queue[i]))
                    return i;
        }
        return -1;
    }

  private E removeAt(int i) {
        assert i >= 0 && i < size;
        modCount++;
        int s = --size;
        if (s == i) // removed last element
            queue[i] = null;
        else {
            E moved = (E) queue[s];
            queue[s] = null;
            siftDown(i, moved);
            if (queue[i] == moved) {
                siftUp(i, moved);
                if (queue[i] != moved)
                    return moved;
            }
        }
        return null;
    }

// clear all elements
public void clear() {
        modCount++;
        for (int i = 0; i < size; i++)
            queue[i] = null;
        size = 0;
    }

//returns the array element of the specified type
 public <T> T[] toArray(T[] a) {
       //If the incoming array length is less than the queue length, directly create a new queue and load all the elements in the queue
        if (a.length < size)
            // Make a new array of a's runtime type, but my contents:
            return (T[]) Arrays.copyOf(queue, size, a.getClass());
	 // Greater than direct copy
        System.arraycopy(queue, 0, a, 0, size);
        if (a.length > size)
            a[size] = null;
        return a;
    }


/**Summary: PriorityQueen is implemented using a binary heap. As for what is the binary heap, please refer to this blog. http://www.cnblogs.com/tstd/p/5125949.html.
*/