Java进阶--ArrayDeque双端队列完全解析

ArrayDeque的基本用法

public class DequeDemo {

public static void main(String[] args) {
int []arr={1,2,3,4,5,6,7,8};
ArrayDeque<Integer> aDeque=new ArrayDeque<Integer>();
for(int i=0;i<arr.length;i++)
{
if((i&1)==0)
//如果i是偶数从队头加入，否则从队尾加入
aDeque.addFirst(arr[i]);
else
aDeque.addLast(arr[i]);

}
while(!aDeque.isEmpty())
{
aDeque.pollFirst();
aDeque.pollLast();//此处不严谨但是由于arr的元素恰好是偶数个，本例中也不会出现问题。
}

}

}

ArrayDeque

addFirst(arr[i])

addLast(arr[i])

pollFirst()

pollLast()

addFirst(arr[i])

pollFirst()

addLast(arr[i])

pollLast()

public class DequeDemo {

public static void main(String[] args) {
int []arr={1,2,3,4,5,6,7,8};
ArrayDeque<Integer> aDeque=new ArrayDeque<Integer>();
//从双端队列的头部加元素
for(int i=0;i<arr.length;i++)
aDeque.addFirst(arr[i]);
//从双端队列的尾部删元素
while(!aDeque.isEmpty())
aDeque.pollLast();
}

}

addFirst(arr[i])

pollLast()

pollLast()

addFirst(arr[i])

pollLast()

addLast(arr[i])

pollFirst()

深入了解ArrayDeque

构造函数

public ArrayDeque() {
elements = (E[]) new Object[16];
//默认构造函数新建了一个大小为16的数组。
}

public ArrayDeque(int numElements) {
allocateElements(numElements);
}

private void allocateElements(int numElements) {
int initialCapacity = 8;

if (numElements >= initialCapacity) {
initialCapacity = numElements;
initialCapacity |= (initialCapacity >>>  1);
initialCapacity |= (initialCapacity >>>  2);
initialCapacity |= (initialCapacity >>>  4);
initialCapacity |= (initialCapacity >>>  8);
initialCapacity |= (initialCapacity >>> 16);
initialCapacity++;

if (initialCapacity < 0)
initialCapacity >>>= 1;// 最大分配2 ^ 30 空间
}
elements = (E[]) new Object[initialCapacity];
}

public ArrayDeque(Collection<? extends E> c) {
allocateElements(c.size());
//分配数组
addAll(c);
// 将元素加入到 ArrayDeque中
}

public boolean addAll(Collection<? extends E> c) {
boolean modified = false;
for (E e : c)
if (add(e))
modified = true;
return modified;
//将元素全部加到ArrayDeque中。其中add 方法调用了addLast 后面会对addLast源码进行分析
}

public boolean add(E e) {
addLast(e);
return true;
}

ArrayDeque(Collection<? extends E> c)

public class DequeDemo {
public static void main(String[] args) {
Integer []arr={1,2,3,4,5,6,7,8};
List<Integer> list=Arrays.asList(arr);
//初始化的时候传入一个List的容器。
ArrayDeque<Integer> aDeque=new ArrayDeque<Integer>(list);

while(!aDeque.isEmpty())
System.out.print(aDeque.pollLast()+" ");

}

}

addLast

addList

addFirst

public void addFirst(E e) {
if (e == null)
throw new NullPointerException();
//如果加入的元素为null 抛出空指针异常。
elements[head = (head - 1) & (elements.length - 1)] = e;
//head值先减1，然后将元素放到head位置
if (head == tail)
doubleCapacity();
//如果head == tail 将数组大小扩大一倍
}

addLast

public void addLast(E e) {
if (e == null)
throw new NullPointerException();
elements[tail] = e;
//将e的值放到tail位置，然后tail+1，如果数组满了扩容一倍。
if ( (tail = (tail + 1) & (elements.length - 1)) == head)
doubleCapacity();
}

pollFirst

public E pollFirst() {
int h = head;
E result = elements[h]; //将elements[h]赋值给result。
if (result == null)
return null;//如果result 为null 直接返回
elements[h] = null;     // 如果result不为null 将elements[h] 赋值为null
head = (h + 1) & (elements.length - 1);// head+1.
return result;
}

pollFirst()

public class DequeDemo {
public static void main(String[] args) {
Integer []arr={1,2,3,4,5,6,7,8};
List<Integer> list=Arrays.asList(arr);
ArrayDeque<Integer> aDeque=new ArrayDeque<Integer>(list);
for(int i=0;i<3;i++)
aDeque.addFirst(arr[i]);
//往双端队列中添加元素
while(!aDeque.isEmpty())
aDeque.pollFirst();
//使用pollFirst()删除。
}

}

pollFirst()

pollFirst()

pollFirst()

pollFirst()

pollFirst()

pollLast

public E pollLast() {
int t = (tail - 1) & (elements.length - 1);
E result = elements[t];
//tail 先减1然后获取tail中的内容。
if (result == null)
return null;//如果result 为null 直接返回
elements[t] = null;//否则将t位置的元素清空再返回result
tail = t;
return result;
}

pollFirst（）

pollLast()

public class DequeDemo {
public static void main(String[] args) {
Integer []arr={1,2,3,4,5,6,7,8};
List<Integer> list=Arrays.asList(arr);
ArrayDeque<Integer> aDeque=new ArrayDeque<Integer>(list);
for(int i=0;i<3;i++)
aDeque.addFirst(arr[i]);
//添加元素
while(!aDeque.isEmpty())
aDeque.pollLast();
//删除元素
}

}

pollLast()

pollLast()

pollLast()

pollLast()

pollLast()

removeFirst()

public E removeFirst() {
E x = pollFirst();
//调用pollFirst()
if (x == null)
throw new NoSuchElementException();
//当pollFirst()返回null抛出null 元素异常。
return x;
}

removeFirst()

pollFirst()

pollFirst()

removeFirst()

removeLast()

public E removeLast() {
E x = pollLast();
//调用 pollLast() 当pollLast()返回null抛出null 元素异常。
if (x == null)
throw new NoSuchElementException();
return x;
}

removeLast()

pollLast()

pollLast()

removeLast()

add(E e)

public boolean add(E e) {
//调用 addLast(e); 添加成功返回 true。
addLast(e);
return true;
}

remove()

public E remove() {
return removeFirst();
}