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。


java源码-PriorityQueue_第1张图片
PriorityQueue


PriorityQueue类图

java源码-PriorityQueue_第2张图片
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 extends AbstractQueue
    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 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 comparator) {
        this(DEFAULT_INITIAL_CAPACITY, comparator);
    }

    public PriorityQueue(int initialCapacity,
                         Comparator 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 c) {
        if (c instanceof SortedSet) {
            SortedSet ss = (SortedSet) c;
            this.comparator = (Comparator) ss.comparator();
            initElementsFromCollection(ss);
        }
        else if (c instanceof PriorityQueue) {
            PriorityQueue pq = (PriorityQueue) c;
            this.comparator = (Comparator) pq.comparator();
            initFromPriorityQueue(pq);
        }
        else {
            this.comparator = null;
            initFromCollection(c);
        }
    }

    @SuppressWarnings("unchecked")
    public PriorityQueue(PriorityQueue c) {
        this.comparator = (Comparator) c.comparator();
        initFromPriorityQueue(c);
    }

    @SuppressWarnings("unchecked")
    public PriorityQueue(SortedSet c) {
        this.comparator = (Comparator) c.comparator();
        initElementsFromCollection(c);
    }





    private void initFromPriorityQueue(PriorityQueue c) {
        if (c.getClass() == PriorityQueue.class) {
            this.queue = c.toArray();
            this.size = c.size();
        } else {
            initFromCollection(c);
        }
    }

    private void initElementsFromCollection(Collection 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 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 key = (Comparable) 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 key = (Comparable)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) 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 key = (Comparable)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) 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;
    }

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