queue接口特点：可以模拟队列行为，即“先进先出”。
接口结构
queue接口继承了Collection接口，并增加了一些新方法
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16public interface extends Collection{
boolean add(E e);
//将元素插入队列，如果失败返回false
boolean offer(E e);
//移除并返回队列中的第一个元素，队列为空时，抛异常
E remove();
//移除并返回队列中的第一个元素，队列为空时，返回null
E poll();
//返回队列中第一个元素，但不移除，队列为空时，抛异常
E element();
//返回队列中第一个元素，但不移除，对列为空时，返回null
E peek();
}
抽象类AbstractQueue
queue接口中，add与offer、remove与poll、element与peek，功能一致，只是对异常情况的处理不同。AbstractQueue源码中可以看出这些方法处理异常的方式。
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49public abstract class AbstractQueue
extends AbstractCollection
implements {
protected AbstractQueue() {
}
//add底层调用offer，当offer失败返回false时，add方法抛出异常
public boolean add(E e) {
if (offer(e))
return true;
else
throw new IllegalStateException("Queue full");
}
//remove底层调用poll，当poll返回null时，remove方法抛出异常
public E remove() {
E x = poll();
if (x != null)
return x;
else
throw new NoSuchElementException();
}
//element底层调用peek，当peek返回null时，element方法抛出异常
public E element() {
E x = peek();
if (x != null)
return x;
else
throw new NoSuchElementException();
}
//clear方法循环调用poll，直到返回为null
public void clear() {
while (poll() != null)
;
}
public boolean addAll(Collection extends E> c) {
if (c == null)
throw new NullPointerException();
if (c == this)
throw new IllegalArgumentException();
boolean modified = false;
for (E e : c)
if (add(e))
modified = true;
return modified;
}
}
Deque接口
Deque是双端队列，底层由循环数组实现，其继承了Queue接口，并扩展了新特性。
Deque重写了Queue的全部方法，Stack、Collection的部分方法，并增加了对首尾元素处理的相关方法。
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65public interface Deque extends {
void addFirst(E e);
void addLast(E e);
boolean offerFirst(E e);
boolean offerLast(E e);
E removeFirst();
E removeLast();
E pollFirst();
E pollLast();
E getFirst();
E getLast();
E peekFirst();
E peekLast();
boolean removeFirstOccurrence(Object o);
boolean removeLastOccurrence(Object o);
// *** Queue methods ***
boolean add(E e);
boolean offer(E e);
E remove();
E poll();
E element();
E peek();
// *** Stack methods ***
void push(E e);
E pop();
// *** Collection methods ***
boolean remove(Object o);
boolean contains(Object o);
public int size();
Iterator iterator();
Iterator descendingIterator();
}
ArrayDeque
ArrayDeque不可以存取null元素，因为会根据某个位置是否为null来判断元素是否存在。
当作为栈使用时，性能比stack好，作为队列使用时，性能比linkedlist好
类定义
继承了AbstractCollection，实现了Deque接口
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2public class ArrayDeque extends AbstractCollection
implements Deque, Cloneable, Serializable
重要的成员变量1
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4// 第一个元素的索引
private transient int head;
// 下个要添加元素的位置，为末尾元素的索引 + 1
private transient int tail;
构造方法1
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14//默认数组大小16
public ArrayDeque() {
elements = new Object[16];
}
//传入数组大小
public ArrayDeque(int numElements) {
//调整实际数组大小为大于传入值的最小的2的幂次
allocateElements(numElements);
}
//直接传入集合
public ArrayDeque(Collection extends E> c) {
allocateElements(c.size());
addAll(c);
}
调整数组大小1
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17private void allocateElements(int numElements) {
int initialCapacity = MIN_INITIAL_CAPACITY;
// 找到大于需要长度的最小的2的幂整数。
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
}
elements = (E[]) new Object[initialCapacity];
}
add1
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34public void addFirst(E e) {
if (e == null)
throw new NullPointerException();
//因为elements.length是2的幂次，(head - 1) & (elements.length - 1)相当于求模操作
//head-1只可能为-1，-1&(elements.length - 1)=(elements.length - 1)
elements[head = (head - 1) & (elements.length - 1)] = e;
if (head == tail)
doubleCapacity();
}
public void addLast(E e) {
if (e == null)
throw new NullPointerException();
elements[tail] = e;
//与head操作类似，为了处理tail=length-1的情况
if ( (tail = (tail + 1) & (elements.length - 1)) == head)
doubleCapacity();
}
public boolean add(E e) {
addLast(e);
return true;
}
public boolean offerFirst(E e) {
addFirst(e);
return true;
}
public boolean offerLast(E e) {
addLast(e);
return true;
}
remove1
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40public E removeFirst() {
E x = pollFirst();
if (x == null)
throw new NoSuchElementException();
return x;
}
public E removeLast() {
E x = pollLast();
if (x == null)
throw new NoSuchElementException();
return x;
}
public E pollFirst() {
int h = head;
("unchecked")
E result = (E) elements[h];
// Element is null if deque empty
if (result == null)
return null;
//删除头元素
elements[h] = null; // Must null out slot
//更改head下标，处理越界情况让head=0
head = (h + 1) & (elements.length - 1);
return result;
}
public E pollLast() {
//获取tail下标，处理tail=0的特殊情况，移除元素后tail=elements.length-1
int t = (tail - 1) & (elements.length - 1);
("unchecked")
E result = (E) elements[t];
if (result == null)
return null;
elements[t] = null;
//tail指向下个要添加元素的索引
tail = t;
return result;
}
删除指定元素
需要遍历数组，时间复杂度较高
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33public boolean removeFirstOccurrence(Object o) {
if (o == null)
return false;
int mask = elements.length - 1;
int i = head;
Object x;
while ( (x = elements[i]) != null) {
if (o.equals(x)) {
delete(i);
return true;
}
i = (i + 1) & mask;
}
return false;
}
public boolean removeLastOccurrence(Object o) {
if (o == null)
return false;
int mask = elements.length - 1;
//末尾元素的索引
int i = (tail - 1) & mask;
Object x;
while ( (x = elements[i]) != null) {
if (o.equals(x)) {
delete(i);
return true;
}
//从尾到头遍历
i = (i - 1) & mask;
}
return false;
}
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44private boolean delete(int i) {
//检查有效性
checkInvariants();
final Object[] elements = this.elements;
final int mask = elements.length - 1;
final int h = head;
final int t = tail;
//i前面的元素
final int front = (i - h) & mask;
//i后面的元素
final int back = (t - i) & mask;
//i不在t和h之间
if (front >= ((t - h) & mask))
throw new ConcurrentModificationException();
// Optimize for least element motion
if (front < back) {
// head X X i X X X X tail，head-i的元素大于i-tail元素个数
if (h <= i) {
System.arraycopy(elements, h, elements, h + 1, front);
} else { // Wrap around
// i X X X X tail X X head
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 {
// head X X X X i X X tail
if (i < t) { // Copy the null tail as well
System.arraycopy(elements, i + 1, elements, i, back);
tail = t - 1;
} else { // Wrap around
// tail X X head X X X X i
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;
}
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10private void checkInvariants() {
//tail没有元素
assert elements[tail] == null;
//head和tail位置重合，队列为空，否则head、tail-1位置有元素
assert head == tail ? elements[head] == null :
(elements[head] != null &&
elements[(tail - 1) & (elements.length - 1)] != null);
//head-1的位置没有元素。
assert elements[(head - 1) & (elements.length - 1)] == null;
}
扩容1
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18private void doubleCapacity() {
//tail和head重合的时候进行扩容，此时tail在head左侧
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];
//先复制head到element数组末尾的元素
System.arraycopy(elements, p, a, 0, r);
//在复制0到tail之间的元素
System.arraycopy(elements, 0, a, r, p);
elements = a;
head = 0;
tail = n;
}
标签：head,elements,java,int,queue,tail,源码,return,null
来源： https://www.cnblogs.com/petewell/p/11588764.html
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