概要
- 類繼承關係
java.lang.Object
java.util.AbstractCollection<E>
java.util.ArrayDeque<E>
- 定義
public class ArrayDeque<E>
extends AbstractCollection<E>
implements Deque<E>, Cloneable, Serializable
-
要點
- 對
Deque
接口的實現 - 可調整大小
- 非線程安全
- 作爲棧比
Stack
快,作爲隊列比LinkedList
快。 - 除了
remove
相關的幾個操作是線性時間,其它大部分操作均攤時間都是常數級。
實現
- 基本數據結構
private transient E[] elements;
private transient int head;
private transient int tail;
數據用數組保存,head,tail
指向隊列頭尾。
- allocateElements
static int allocateElements(int numElements) {
int initialCapacity = 8;
// 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;
}
就像註釋裏說的那樣,這個函數的目的是找到一個剛好大於 numbElements
的數,它是2的n次冪。這是通過移位及或運算的方式實現。
例如這個數爲二進制的 abcdefg
,a
是最高的不爲0的位,即 a
爲 1
,那麼第一次運算後,b
所在位一定爲 1
,第二次運算後,cd
所在位一定爲 1
,依此類推。最後全爲1,再加1進位。
- doubleCapacity
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 = (E[])a;
head = 0;
tail = n;
}
展示瞭如何對數組進行擴充,由於這是一個雙端隊列,head
可能在隊列中間,隊列現在呈現環狀。隊列容量增加後,不能簡單地直接複製過去,而應該把 head
指向的元素放在新隊列數組的 0
號位置,其它元素依次向後排列。注意兩次System.arraycopy
分別複製了後一半,和前一半的元素。
- addFirst
public void addFirst(E e) {
if (e == null)
throw new NullPointerException();
elements[head = (head - 1) & (elements.length - 1)] = e;
if (head == tail)
doubleCapacity();
}
首先,參數不能爲空,其次, 注意 addFirst
如何巧妙地把 head
的位置後退一位,如果 head
爲0的話,&
會使得 head
變成數組最後一個位置。與循環隊列概念一致。
- addLast
public void addLast(E e) {
if (e == null)
throw new NullPointerException();
elements[tail] = e;
if ( (tail = (tail + 1) & (elements.length - 1)) == head)
doubleCapacity();
}
注意新 tail
是如何計算的。通過位運算,比通過 %
顯得高大上多了。
- removeFirstOccurrence
public boolean removeFirstOccurrence(Object o) {
if (o == null)
return false;
int mask = elements.length - 1;
int i = head;
E x;
while ( (x = elements[i]) != null) {
if (o.equals(x)) {
delete(i);
return true;
}
i = (i + 1) & mask;
}
return false;
}
初看起來,當隊列滿時,並且找不到元素時,似乎會永遠循環下去,但是實際上,隊列永遠不會滿,所以一定會遇到 null
元素。不會滿的原因可以看下之前的 addFirst
,當隊列將要滿的時候,就會自動擴充。任何一個操作之後,tail
指向的位置都爲空。
- delete
private void checkInvariants() {
assert elements[tail] == null;
assert head == tail ? elements[head] == null :
(elements[head] != null &&
elements[(tail - 1) & (elements.length - 1)] != null);
assert elements[(head - 1) & (elements.length - 1)] == null;
}
private boolean delete(int i) {
checkInvariants();
final E[] elements = this.elements;
final int mask = elements.length - 1;
final int h = head;
final int t = tail;
final int front = (i - h) & mask;
final int back = (t - i) & mask;
// Invariant: head <= i < tail mod circularity
if (front >= ((t - h) & mask))
throw new ConcurrentModificationException();
// Optimize for least element motion
if (front < back) {
if (h <= i) {
System.arraycopy(elements, h, elements, h + 1, front);
} else { // Wrap around
System.arraycopy(elements, 0, elements, 1, i);
elements[0] = elements[mask];
System.arraycopy(elements, h, elements, h + 1, mask - h);
}
elements[h] = null;
head = (h + 1) & mask;
return false;
} else {
if (i < t) { // Copy the null tail as well
System.arraycopy(elements, i + 1, elements, i, back);
tail = t - 1;
} else { // Wrap around
System.arraycopy(elements, i + 1, elements, i, mask - i);
elements[mask] = elements[0];
System.arraycopy(elements, 1, elements, 0, t);
tail = (t - 1) & mask;
}
return true;
}
}
首先注意 checkInvariants
的使用,它保證了執行函數前應該滿足的條件,這裏是一個前斷言。front
和 back
分別代表了要刪除的結點距離雙端隊列頭和尾的距離。通過比較 front
和 back
的大小,決定如何移動元素,使得移動次數最少。
如果front<back
,還需要區分 h
<= i
,決定移動哪一部分。注意這是一個成環的雙端隊列,畫一下位置就明白這些操作了。主要是保證數據是連續的,head/tail
仍然保證其語義。
- iterator
Deque
有兩個 iterator
,分別從隊列頭向後遍歷,從隊列尾向前遍歷。
private class DeqIterator implements Iterator<E> {
/**
* Index of element to be returned by subsequent call to next.
*/
private int cursor = head;
/**
* Tail recorded at construction (also in remove), to stop
* iterator and also to check for comodification.
*/
private int fence = tail;
/**
* Index of element returned by most recent call to next.
* Reset to -1 if element is deleted by a call to remove.
*/
private int lastRet = -1;
public boolean hasNext() {
return cursor != fence;
}
public E next() {
if (cursor == fence)
throw new NoSuchElementException();
E result = elements[cursor];
// This check doesn't catch all possible comodifications,
// but does catch the ones that corrupt traversal
if (tail != fence || result == null)
throw new ConcurrentModificationException();
lastRet = cursor;
cursor = (cursor + 1) & (elements.length - 1);
return result;
}
public void remove() {
if (lastRet < 0)
throw new IllegalStateException();
if (delete(lastRet)) { // if left-shifted, undo increment in next()
cursor = (cursor - 1) & (elements.length - 1);
fence = tail;
}
lastRet = -1;
}
}
這裏沒有什麼特殊的地方,就是通過 cursor
不斷指向下一個位置,一直到達尾部。
private class DescendingIterator implements Iterator<E> {
/*
* This class is nearly a mirror-image of DeqIterator, using
* tail instead of head for initial cursor, and head instead of
* tail for fence.
*/
private int cursor = tail;
private int fence = head;
private int lastRet = -1;
public boolean hasNext() {
return cursor != fence;
}
public E next() {
if (cursor == fence)
throw new NoSuchElementException();
cursor = (cursor - 1) & (elements.length - 1);
E result = elements[cursor];
if (head != fence || result == null)
throw new ConcurrentModificationException();
lastRet = cursor;
return result;
}
public void remove() {
if (lastRet < 0)
throw new IllegalStateException();
if (!delete(lastRet)) {
cursor = (cursor + 1) & (elements.length - 1);
fence = head;
}
lastRet = -1;
}
}
這是從後向前的版本。
- clear
public void clear() {
int h = head;
int t = tail;
if (h != t) { // clear all cells
head = tail = 0;
int i = h;
int mask = elements.length - 1;
do {
elements[i] = null;
i = (i + 1) & mask;
} while (i != t);
}
}
clear
操作並不是直接將 head/tail
置0就好了,還需要把每個元素置爲 null
。
- toArray
public <T> T[] toArray(T[] a) {
int size = size();
if (a.length < size)
a = (T[])java.lang.reflect.Array.newInstance(
a.getClass().getComponentType(), size);
copyElements(a);
if (a.length > size)
a[size] = null;
return a;
}
使用反射機制,生成與泛型一致的數組,再把現有元素複製過去。
如果對代碼有更多見解,可以在這個頁面添加註釋: rtfcode-ArrayDeque