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