读ArrayDeque源码

//一个双端队列 比stack和LinkedList效率高
//先看构造函数

public ArrayDeque() {
        elements = (E[]) new Object[16];
    }

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

private void allocateElements(int numElements) {
        int initialCapacity = MIN_INITIAL_CAPACITY;
        
        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];
    }


public ArrayDeque(Collection<? extends E> c) {
        allocateElements(c.size());
        addAll(c);
    }

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

//在队尾添加元素
 public void addLast(E e) {
        if (e == null)
            throw new NullPointerException();
        elements[tail] = e;
	
        if ( (tail = (tail + 1) & (elements.length - 1)) == head)
	//扩容
            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];
	//把head之前的元素复制到a数组
        System.arraycopy(elements, p, a, 0, r);
	//复制head之后的元素到a数组
        System.arraycopy(elements, 0, a, r, p);
        elements = (E[])a;
        head = 0;
        tail = n;
    }

//在队首添加元素
public void addFirst(E e) {
        if (e == null)
            throw new NullPointerException();
        elements[head = (head - 1) & (elements.length - 1)] = e;
        if (head == tail)
            doubleCapacity();
    }

public boolean offerFirst(E e) {
        addFirst(e);
        return true;
    }


//压入队列的头部
public void push(E e) {
        addFirst(e);
    }

public boolean offer(E e) {
        return offerLast(e);
    }

//获取第一个
public E getFirst() {
        E x = elements[head];
        if (x == null)
            throw new NoSuchElementException();
        return x;
    }

//获取最后一个元素
 public E getLast() {
        E x = elements[(tail - 1) & (elements.length - 1)];
        if (x == null)
            throw new NoSuchElementException();
        return x;
    }

public E peekFirst() {
        return elements[head]; 
    }

   public E peekLast() {
        return elements[(tail - 1) & (elements.length - 1)];
    }


//从开头获取元素并移除
public E pollFirst() {
        int h = head;
        E result = elements[h]; // Element is null if deque empty
        if (result == null)
            return null;
        elements[h] = null;     // Must null out slot
        head = (h + 1) & (elements.length - 1);
        return result;
    }

//从队尾获取元素并移除
public E pollLast() {
        int t = (tail - 1) & (elements.length - 1);
        E result = elements[t];
        if (result == null)
            return null;
        elements[t] = null;
        tail = t;
        return result;
    }

public 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 remove() {
        return removeFirst();
    }

//从队列头部遍历删除元素第一次出现的位置
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;
    }

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;
        }
    }

//从后遍历删除元素
public boolean removeLastOccurrence(Object o) {
        if (o == null)
            return false;
        int mask = elements.length - 1;
        int i = (tail - 1) & mask;
        E x;
        while ( (x = elements[i]) != null) {
            if (o.equals(x)) {
                delete(i);
                return true;
            }
            i = (i - 1) & mask;
        }
        return false;
    }

 public E element() {
        return getFirst();
    }

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

//返回队列长度
 public int size() {
        return (tail - head) & (elements.length - 1);
    }

public boolean isEmpty() {
        return head == tail;
    }

//是否包含某个元素
public boolean contains(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))
                return true;
            i = (i + 1) & mask;
        }
        return false;
    }

//清空队列
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);
        }
    }