java源码分析-优先队列

- 日理万妓 2022-08-10 09:38 150阅读 0赞

愿她好！

优先队列PriorityQueue

## 1.类结构 ##

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2.优先队列分析

主要是怎么确定优先级

首先我们来看下add(E)方法

public boolean add(E e) {
            return offer(e);
        }

public boolean offer(E e) {
            if (e == null)
                throw new NullPointerException();
            modCount++;
            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 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;
        }

从上述代码可以看出：第一，add方法实际上是调用offer方法；第二，加入的元素不能为空，都在抛出空指向异常，这类问题在有序的集合中都会存在；第三，若队列满了，则采用grow方法扩容；第四，通过siftUp来重整队列，如果没有从外部传入比较器，采用siftUpUsingComparator方法来重整；否则，利用传入的比较器以及siftUpComparable来重整队列。

怎么比较优先级的？以自带的比较器为例子

关键代码在于parent的确定：int parent=(k-1)>>>1; 不断地找到中值，并利用比较器比较，直到比较器判断为true

java中有三种移位运算符

<<      :     左移运算符，num << 1,相当于num乘以2

>>      :     右移运算符，num >> 1,相当于num除以2

>>>    :     无符号右移，忽略符号位，空位都以0补齐

无符号右移的规则只记住一点：忽略了符号位扩展，0补最高位  无符号右移运算符>>> 只是对32位和64位的值有意义

扩容

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);
        }

分为两种情况：如果原有容量小于64，扩容方式为加倍再加2；否则，每次增加原有容量的一半

最大容量

private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;

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

数组可能保留一些特殊字段

查找

public E peek() {
            if (size == 0)
                return null;
            return (E) queue[0];
        }

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 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;
        }

poll方法说明：第一，首先判断size是否为0；其次，弹出下标为0的元素，然后将下标为size-1的值放入下标为0的地方，并且重新排序

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) {
                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;
        }
    
        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;
        }

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