arrayqueue源码_thinkphp源码分析

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愉快地聊一聊ArrayDeque的特点吧~（以下都是基于jdk1.8）

ArrayDeque的继承树如下图：

（1）双端队列，可从两端添加、删除元素。作为队列使用时，性能优于LinkedList。作为栈使用时，性能优于Stack。

（2）底层使用可变数组Object[] elements， 数组容量按需增长

（3）不能存储null

（4）支持双向迭代器遍历

（5）线程不安全

（6）fail-fast机制。

（7）最小数组容量MIN_INITIAL_CAPACITY = 8。Must be a power of 2

（8）默认数组初始容量是16

（9）调用指定初始容量的构造函数，并不会按照指定值分配容量。

（10）先添加，再判断是否需要扩容

这里只取部分源码进行分析：指定初始容量的构造函数、扩容机制，以及主要方法。

好了，先把类中定义的变量熟悉一下：

	 /** * 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, * thus avoiding head and tail wrapping around to equal each * other. We also guarantee that all array cells not holding * deque elements are always null. */
    transient Object[] elements; // non-private to simplify nested class access

    /** * 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;

    /** * 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;

    /** * 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;

（1）指定初始容量的构造函数：

找到该构造函数，从此入手：

    /** * Constructs an empty array deque with an initial capacity * sufficient to hold the specified number of elements. * * @param numElements lower bound on initial capacity of the deque */
    public ArrayDeque(int numElements) { 
   
        allocateElements(numElements);
    }

再看allocateElements方法：

    /** * Allocates empty array to hold the given number of elements. * * @param numElements the number of elements to hold */
    private void allocateElements(int numElements) { 
   
        int initialCapacity = MIN_INITIAL_CAPACITY;
        // 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
        }
        elements = new Object[initialCapacity];
    }

拿给定元素数量与数组最小容量8做比较，因为此集合不允许数组变满（添加元素的方法中，数组容量一满就立刻扩容），所以当给定元素数量>=数组最小容量8时，会进行一系列的无符号右移运算、或运算，以便找到能够容纳给定元素的最佳的2的幂次方。

这个最佳的2的幂次方就是调用该构造函数后底层为我们分配的数组容量。

（2）扩容机制：

找到扩容的核心方法：

    /** * Doubles the capacity of this deque. Call only when full, i.e., * when head and tail have wrapped around to become equal. */
    private void doubleCapacity() { 
   
    	//断言 判断head与tail指针是否相等
        assert head == tail;
        int p = head;
        int n = elements.length;
        int r = n - p; // number of elements to the right of p
        //左移1位，相当于*2操作，只是<<效率要高于*运算。
        int newCapacity = n << 1;
        if (newCapacity < 0)
            throw new IllegalStateException("Sorry, deque too big");
        //扩容，实际上是定义了一个指定容量的数组，将elements数组中的元素复制到新数组a中。
        Object[] a = new Object[newCapacity];
        System.arraycopy(elements, p, a, 0, r);
        System.arraycopy(elements, 0, a, r, p);
        //重新设置head和tail的指针
        elements = a;
        head = 0;
        tail = n;
    }

（3）主要方法：

添加元素：

    // The main insertion and extraction methods are addFirst,
    // addLast, pollFirst, pollLast. The other methods are defined in
    // terms of these.

    /** * 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) { 
   
        if (e == null)
            throw new NullPointerException();
        //此运算可以快速定位到要插入的位置，实际上是从数组最右侧开始插入的，head是递减的
        elements[head = (head - 1) & (elements.length - 1)] = e;
        //head与tail重叠时，开始扩容
        if (head == tail)
            doubleCapacity();
    }
    
     /** * 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) { 
   
        if (e == null)
            throw new NullPointerException();
        //tail初始值是0，指向待插入元素的位置，tail是递增的
        elements[tail] = e;
        //先插入元素，再判断是否需要扩容。
        //tail + 1 & (elements.length - 1 )用于定位下一个待插入元素的位置。
        //如果tail与head重叠，数组容量已满，
        if ( (tail = (tail + 1) & (elements.length - 1)) == head)
            doubleCapacity();
    }
     
     /** * Inserts the specified element at the front of this deque. * * @param e the element to add * @return {@code true} (as specified by {@link Deque#offerFirst}) * @throws NullPointerException if the specified element is null */
    public boolean offerFirst(E e) { 
   
        addFirst(e);
        return true;
    }

    /** * Inserts the specified element at the end of this deque. * * @param e the element to add * @return {@code true} (as specified by {@link Deque#offerLast}) * @throws NullPointerException if the specified element is null */
    public boolean offerLast(E e) { 
   
        addLast(e);
        return true;
    }

删除首尾元素：

    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 pollFirst() { 
   
        int h = head;
        @SuppressWarnings("unchecked")
        E result = (E) 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;
    }

    public E pollLast() { 
   
        int t = (tail - 1) & (elements.length - 1);
        @SuppressWarnings("unchecked")
        E result = (E) elements[t];
        if (result == null)
            return null;
        elements[t] = null;
        tail = t;
        return result;
    }

删除指定元素：

	/** * Removes the first occurrence of the specified element in this * deque (when traversing the deque from head to tail). * If the deque does not contain the element, it is unchanged. * More formally, removes the first element {@code e} such that * {@code o.equals(e)} (if such an element exists). * Returns {@code true} if this deque contained the specified element * (or equivalently, if this deque changed as a result of the call). * * @param o element to be removed from this deque, if present * @return {@code true} if the deque contained the specified element */
public 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;
}
/** * Removes the last occurrence of the specified element in this * deque (when traversing the deque from head to tail). * If the deque does not contain the element, it is unchanged. * More formally, removes the last element {@code e} such that * {@code o.equals(e)} (if such an element exists). * Returns {@code true} if this deque contained the specified element * (or equivalently, if this deque changed as a result of the call). * * @param o element to be removed from this deque, if present * @return {@code true} if the deque contained the specified element */
public boolean removeLastOccurrence(Object o) { 

if (o == null)
return false;
int mask = elements.length - 1;
int i = (tail - 1) & mask;
Object x;
while ( (x = elements[i]) != null) { 

if (o.equals(x)) { 

delete(i);
return true;
}
i = (i - 1) & mask;
}
return false;
}
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;
}
/** * Removes the element at the specified position in the elements array, * adjusting head and tail as necessary. This can result in motion of * elements backwards or forwards in the array. * * <p>This method is called delete rather than remove to emphasize * that its semantics differ from those of {@link List#remove(int)}. * * @return true if elements moved backwards */
private boolean delete(int i) { 

checkInvariants();
final Object[] 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;
}
}

暂时先写到这里了~