ArrayDeque部分源码剖析

Posted 2022-12-09 rotk2015

笔者所用JDK为 RedHat 的 OpenJDK-8u282-win64 版。

1. ArrayDeque，数组实现的双端队列。核心思想是维护头尾指针，将数组变为循环数组（头指针指向头元素，尾指针指向尾元素后的空位）。无法按索引取值（因为没有固定的索引）。不允许插入 Null。用作栈时，比Stack快；用作队列时，比LinkedList快。缺点是：线程不安全，空间浪费。

2. 扩容机制：双倍扩容，数组长度永远是2的幂（最小8，好处是扩容、取模时只需位运算即可）。

3. 构造器函数，无参时，创建长度为16的数组；有参时，调用allocateElements()。

   public ArrayDeque() 
       elements = new Object[16];
   
   public ArrayDeque(int numElements) 
       allocateElements(numElements);
   
   public ArrayDeque(Collection<? extends E> c) 
       allocateElements(c.size());
       addAll(c);
   
   

4. allocateElements()，按输入int形参，调用calculateSize()，分配长度为2的幂次的数组。

   /**
    * Allocates empty array to hold the given number of elements.
    *
    * @param numElements  the number of elements to hold
    */
   private void allocateElements(int numElements) 
       elements = new Object[calculateSize(numElements)];
   
   

5. calculateSize()，若 numElements 小于8，分配8；否则，分配大于 numElements 的最小2幂次（若 numElements 已为2幂次，翻倍），最多为2^30。源码使用位运算实现，通过不断移位2的幂次再或，将 numElements 的最高位 1 向后传播。

   private static final int MIN_INITIAL_CAPACITY = 8;
   private static int calculateSize(int numElements) 
       int initialCapacity = MIN_INITIAL_CAPACITY;
       // Find the best power of two to hold elements.
       // Tests "<=" because arrays aren't kept full.
       if (numElements >= initialCapacity) 
           initialCapacity = numElements;
           initialCapacity |= (initialCapacity >>>  1);
           initialCapacity |= (initialCapacity >>>  2);
           initialCapacity |= (initialCapacity >>>  4);
           initialCapacity |= (initialCapacity >>>  8);
           initialCapacity |= (initialCapacity >>> 16);
           initialCapacity++;
   
           if (initialCapacity < 0)   // Too many elements, must back off
               initialCapacity >>>= 1;// Good luck allocating 2 ^ 30 elements
       
       return initialCapacity;
   
   

6. addFirst()，位运算实现取模避免 head 越界（说穿了就是在其等于-1时转换为1），若插入后数组存满（head==tail），调用 doubleCapacity() 扩容，数组长翻倍。

   那要是插入前就满了呢？该情况不会发生。首先，构造函数保证数组必有空余；其次，每次插入后都会进行判断，及时扩容。

   /**
    * Inserts the specified element at the front of this deque.
    *
    * @param e the element to add
    * @throws NullPointerException if the specified element is null
    */
   public void addFirst(E e) 
       if (e == null)
           throw new NullPointerException();
       elements[head = (head - 1) & (elements.length - 1)] = e;
       if (head == tail)
           doubleCapacity();
   
   

7. doubleCapacity()，调用 native 方法 System.arraycopy()，先复制 head 右半段，再复制左半段。

   /**
    * Doubles the capacity of this deque.  Call only when full, i.e.,
    * when head and tail have wrapped around to become equal.
    */
   private void doubleCapacity() 
       assert head == tail;
       int p = head;
       int n = elements.length;
       int r = n - p; // number of elements to the right of p
       int newCapacity = n << 1;
       if (newCapacity < 0)
           throw new IllegalStateException("Sorry, deque too big");
       Object[] a = new Object[newCapacity];
       System.arraycopy(elements, p, a, 0, r);
       System.arraycopy(elements, 0, a, r, p);
       elements = a;
       head = 0;
       tail = n;
   
   

8. pollFirst()，从头部移出一个元素，若为null，返回null。

   public E pollFirst() 
       int h = head;
       @SuppressWarnings("unchecked")
       E result = (E) elements[h];
       // Element is null if deque empty
       if (result == null)
           return null;
       elements[h] = null;     // Must null out slot
       head = (h + 1) & (elements.length - 1);
       return result;