jdk1.8 PriorityQueue

PriorityQueue

PriorityQueue底层使用的是最小堆

重要属性

private final Comparator<? super E> comparator;
private static final int DEFAULT_INITIAL_CAPACITY = 11;
transient Object[] queue; // non-private to simplify nested class access
private int size = 0;
           

构造函数

构造函数使用的参数主要有两个:初始容量和排序规则

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;
}
           

add

public boolean add(E e) {
    return offer(e);
}
public boolean offer(E e) {
    if (e == null)
        throw new NullPointerException();
    modCount++;
    int i = size;
    // 判断是否需要扩容
    if (i >= queue.length)
        grow(i + 1);
    size = i + 1;
    if (i == 0)
        queue[0] = e;
    else
        siftUp(i, e);
    return true;
}
           

下面看一下堆中调整方法siftUp

private void siftUp(int k, E x) {
	// 根据是否使用比较器，执行不同的方法，流程差不多
    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) {
    	// 获取当前节点的父节点
        int parent = (k - 1) >>> 1;
        Object e = queue[parent];
        // 如果当前节点比父节点大，那么不用再向上调整了
        // 找到插入位置了
        if (key.compareTo((E) e) >= 0)
            break;
        // 否则需要交换孩子节点和父节点
        queue[k] = e;
        k = parent;
    }
    queue[k] = key;
}
```
## 扩容
```java
private void grow(int minCapacity) {
    int oldCapacity = queue.length;
    // Double size if small; else grow by 50%
    // 如果当前容量小于64，那么容量加倍，否则容量增加一半
    int newCapacity = oldCapacity + ((oldCapacity < 64) ?
                                     (oldCapacity + 2) :
                                     (oldCapacity >> 1));
    // overflow-conscious code
    if (newCapacity - MAX_ARRAY_SIZE > 0)
        newCapacity = hugeCapacity(minCapacity);
	// 简单地进行数组复制
    queue = Arrays.copyOf(queue, newCapacity);
}
```
## poll
不能直接通过迭代器访问队列的元素，因为迭代器将会顺序遍历底层数组，而该数组中的元素并不是有序的
```java
public E poll() {
    if (size == 0)
        return null;
    int s = --size;
    modCount++;
    // 将当前最小堆的堆顶元素作为返回值
    E result = (E) queue[0];
    E x = (E) queue[s];
    queue[s] = null;
    // 向下调整，实际上就是将最后一个元素放到堆顶的位置，然后向下调整
    if (s != 0)
        siftDown(0, x);
    return result;
}
```
```java
private void siftDown(int k, E x) {
    if (comparator != null)
        siftDownUsingComparator(k, x);
    else
        siftDownComparable(k, x);
}
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) {
    	// 获取当前的位置的左孩子节点
        int child = (k << 1) + 1; // assume left child is least
        Object c = queue[child];
        // 获取当前位置的右孩子
        // 将两个孩子节点中的较小值和父节点进行交换
        int right = child + 1;
        if (right < size &&
            ((Comparable<? super E>) c).compareTo((E) queue[right]) > 0)
            c = queue[child = right];
		// 如果两个孩子都比父节点大那么停止向下调整
        if (key.compareTo((E) c) <= 0)
            break;
        queue[k] = c;
        k = child;
    }
    queue[k] = key;
}
```