将String添加到String数组的开头 [英] Add String to beginning of String array

问题描述

是否可以在不迭代整个数组的情况下将字符串添加到String数组的开头。

Is it possible to add a string to beginning of String array without iterating the entire array.

推荐答案

唯一的方法是这样做是为了维护一个环形缓冲区。即你有一个计数器，它记住开始的位置，你移动它而不是移动数组中的所有条目。这只能起作用，因为你重新定义了开始的含义。

The only way to do this is to maintain a ring buffer. i.e. you have a counter which remembers where the start is and you move that instead of moving all the entries in the array. This only works because you re-define what the "start" means.

参见ArrayDeque 有三个字段

   86       /**
   87        * The array in which the elements of the deque are stored.
   88        * The capacity of the deque is the length of this array, which is
   89        * always a power of two. The array is never allowed to become
   90        * full, except transiently within an addX method where it is
   91        * resized (see doubleCapacity) immediately upon becoming full,
   92        * thus avoiding head and tail wrapping around to equal each
   93        * other.  We also guarantee that all array cells not holding
   94        * deque elements are always null.
   95        */
   96       private transient E[] elements;
   97   
   98       /**
   99        * The index of the element at the head of the deque (which is the
  100        * element that would be removed by remove() or pop()); or an
  101        * arbitrary number equal to tail if the deque is empty.
  102        */
  103       private transient int head;
  104   
  105       /**
  106        * The index at which the next element would be added to the tail
  107        * of the deque (via addLast(E), add(E), or push(E)).
  108        */
  109       private transient int tail;

因此添加到开头是这样的

So adding to the start works like this

  224       public void addFirst(E e) {
  225           if (e == null)
  226               throw new NullPointerException();
  227           elements[head = (head - 1) & (elements.length - 1)] = e;
  228           if (head == tail)
  229               doubleCapacity();
  230       }


  312       /**
  313        * @throws NoSuchElementException {@inheritDoc}
  314        */
  315       public E getFirst() {
  316           E x = elements[head];
  317           if (x == null)
  318               throw new NoSuchElementException();
  319           return x;
  320       }

注意：它会移动头部而不是将所有元素移动到数组中。

Note: it moves the head rather than shifting all the elements down the array.

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