java源码-PriorityQueue

开篇

PriorityQueue是具备了小根堆性质的数据结构也就是优先队列PriorityQueue，内部实现是一个堆排序的数据结构。
PriorityQueue的逻辑结构是一棵完全二叉树，存储结构其实是一个数组。逻辑结构层次遍历的结果刚好是一个数组，如下图。

PriorityQueue类图

PriorityQueue类变量和构造函数

PriorityQueue的类变量当中包含存储元素的数组queue和用于排序的比较器comparator。
PriorityQueue的构造函数参数中包含存储数组的大小initialCapacity和比较器comparator。
PriorityQueue的构造函数针对传入为Collection的对象的时候会依据是否已经排好序进行初始化，针对无序的集合通过heapify进行堆序的构建。

public class PriorityQueue<E> extends AbstractQueue<E> implements java.io.Serializable { private static final long serialVersionUID = -7720805057305804111L; // 初始化容量大小 private static final int DEFAULT_INITIAL_CAPACITY = 11; // 数据存储数组 transient Object[] queue; // non-private to simplify nested class access private int size = 0; // 容器比较器 private final Comparator<? super E> comparator; transient int modCount = 0; // non-private to simplify nested class access public PriorityQueue() { this(DEFAULT_INITIAL_CAPACITY, null); } public PriorityQueue(int initialCapacity) { this(initialCapacity, null); } public PriorityQueue(Comparator<? super E> comparator) { this(DEFAULT_INITIAL_CAPACITY, comparator); } 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 PriorityQueue(Collection<? extends E> c) { if (c instanceof SortedSet<?>) { SortedSet<? extends E> ss = (SortedSet<? extends E>) c; this.comparator = (Comparator<? super E>) ss.comparator(); initElementsFromCollection(ss); } 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); } } @SuppressWarnings("unchecked") public PriorityQueue(PriorityQueue<? extends E> c) { this.comparator = (Comparator<? super E>) c.comparator(); initFromPriorityQueue(c); } @SuppressWarnings("unchecked") public PriorityQueue(SortedSet<? extends E> c) { this.comparator = (Comparator<? super E>) c.comparator(); initElementsFromCollection(c); } private void initFromPriorityQueue(PriorityQueue<? extends E> c) { if (c.getClass() == PriorityQueue.class) { this.queue = c.toArray(); this.size = c.size(); } else { initFromCollection(c); } } private void initElementsFromCollection(Collection<? extends E> c) { Object[] a = c.toArray(); // If c.toArray incorrectly doesn't return Object[], copy it. if (a.getClass() != Object[].class) a = Arrays.copyOf(a, a.length, Object[].class); int len = a.length; 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 initFromCollection(Collection<? extends E> c) { initElementsFromCollection(c); heapify(); } } 

PriorityQueue的add操作

PriorityQueue的add操作主要是在offer()函数当中，整体执行逻辑如下：
判断数组大小并通过grow进行扩容：

• 如果数组为为空则第一个加入元素就直接添加。
• 如果数组不为空则添加元素到末尾并通过siftUp上浮元素到合适的位置。
• 上浮的过程就是堆排序的过程的，通过和父节点进行比较继而上浮。

 public boolean add(E e) { return offer(e); } public boolean offer(E e) { if (e == null) throw new NullPointerException(); modCount++; //这里原来size大小就是新增元素后的最后一个元素下标 int i = size; if (i >= queue.length) grow(i + 1); size = i + 1; if (i == 0) queue[0] = e; else 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); queue = Arrays.copyOf(queue, newCapacity); } private void siftUp(int k, E x) { if (comparator != null) siftUpUsingComparator(k, x); else siftUpComparable(k, x); } @SuppressWarnings("unchecked") 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; } @SuppressWarnings("unchecked") 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 siftDown(int k, E x) { if (comparator != null) siftDownUsingComparator(k, x); else siftDownComparable(k, x); } 

PriorityQueue的remove操作

PriorityQueue的remove操作主要过程有两个过程：

• 通过遍历数组找到对应的下标index。
• 将最后一个元素和index节点的左右子节点进行比较然后下沉到合适的位置。
• 最后一个元素设置为null即可。

 private int indexOf(Object o) { if (o != null) { for (int i = 0; i < size; i++) if (o.equals(queue[i])) return i; } return -1; } public boolean remove(Object o) { int i = indexOf(o); if (i == -1) return false; else { removeAt(i); return true; } } 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; } private void siftDown(int k, E x) { if (comparator != null) siftDownUsingComparator(k, x); else siftDownComparable(k, x); } @SuppressWarnings("unchecked") 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; } @SuppressWarnings("unchecked") 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; } 

PriorityQueue的heapify操作

PriorityQueue的堆构造过程递归的执行从（n/2-1）至0递归进行构建，这个过程可以参考java源码-PriorityBlockingQueue的构建过程。
。

 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); } @SuppressWarnings("unchecked") 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; } @SuppressWarnings("unchecked") 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; }