基础数据机构之ArrayDeque队列源码分析

1.ArrayDeque双端队列，jdk中util包中无ArrayQueue的类实现，在com.sun.jmx.remote.internal中才有ArrayQueue队列的实现。

  ArrayDeque双端队列方法有，队列头部插入addFirst，头部移除pollFirst，尾部插入addLast，尾部移除元素pollLast

  

public class ArrayDeque<E> extends AbstractCollection<E>
                           implements Deque<E>, Cloneable, Serializable
{
    /**
     * The array in which the elements of the deque are stored.
     * The capacity of the deque is the length of this array, which is
     * always a power of two. The array is never allowed to become
     * full, except transiently within an addX method where it is
     * resized (see doubleCapacity) immediately upon becoming full,			队列永远不会满，达到限制后会自动按照双倍容量扩容
     * thus avoiding head and tail wrapping around to equal each
     * other.  We also guarantee that all array cells not holding                       队列中每个元素不能为null
     * deque elements are always null.
     */
    transient Object[] elements; // non-private to simplify nested class access	//队列数据

    /**
     * The index of the element at the head of the deque (which is the
     * element that would be removed by remove() or pop()); or an
     * arbitrary number equal to tail if the deque is empty.
     */
    transient int head;														//队列头部指针

    /**
     * The index at which the next element would be added to the tail
     * of the deque (via addLast(E), add(E), or push(E)).
     */
    transient int tail;														//队列尾部指针

    /**
     * The minimum capacity that we'll use for a newly created deque.
     * Must be a power of 2.
     */
    private static final int MIN_INITIAL_CAPACITY = 8;						//队列最小容量8

双端队列头部入队列

/**
     * Inserts the specified element at the front of this deque.
     *
     * @param e the element to add
     * @throws NullPointerException if the specified element is null
     */
    public void addFirst(E e) {								//双端队列头部插入元素
        if (e == null)									//       head           tail 
            throw new NullPointerException();						//0   0   0   0   0   0   0   0   0   0   0   0   0	
        elements[head = (head - 1) & (elements.length - 1)] = e;	//头部添加元素,head向左移动
        if (head == tail)						//双端队列已满，两倍容量扩容
            doubleCapacity();
    }

尾部入队列

/**
     * Inserts the specified element at the end of this deque.
     *
     * <p>This method is equivalent to {@link #add}.
     *
     * @param e the element to add
     * @throws NullPointerException if the specified element is null
     */
    public void addLast(E e) {										//双端队列尾部追加元素
        if (e == null)
            throw new NullPointerException();
        elements[tail] = e;											//tail
        if ( (tail = (tail + 1) & (elements.length - 1)) == head)	//tail尾指针向后移动
            doubleCapacity();										//两倍元素扩容
    }

头部出队列

 public E pollFirst() {								//双端队列头部出队列
        final Object[] elements = this.elements;		
        final int h = head;								
        @SuppressWarnings("unchecked")
        E result = (E) elements[h];						//读取队列head头部元素
        // Element is null if deque empty
        if (result != null) {
            elements[h] = null; // Must null out slot
            head = (h + 1) & (elements.length - 1);		//队列头部指针head向后移动，有效元素个数减1
        }
        return result;
    }

尾部出队列

 public E pollLast() {								//双端队列尾部出队列
        final Object[] elements = this.elements;
        final int t = (tail - 1) & (elements.length - 1);//队列尾部指针tail向左移动，有效元素个数-1
        @SuppressWarnings("unchecked")
        E result = (E) elements[t];						//读取队列尾部数据
        if (result != null) {							//获取到了该元素
            elements[t] = null;							//该队列元素置null
            tail = t;									//tail指针向左移动
        }
        return result;
    }

读取队列头部元素

/**
     * @throws NoSuchElementException {@inheritDoc}
     */
    public E getFirst() {							    //读取双端队列队列头部元素
        @SuppressWarnings("unchecked")
        E result = (E) elements[head];				  
        if (result == null)
            throw new NoSuchElementException();
        return result;
    }

读取队列尾部元素

/**
     * @throws NoSuchElementException {@inheritDoc}
     */
    public E getLast() {								//读取双端队列尾部元素
        @SuppressWarnings("unchecked")				
        E result = (E) elements[(tail - 1) & (elements.length - 1)];	//尾部元素
        if (result == null)
            throw new NoSuchElementException();
        return result;
    }