ArrayList源码分析(一)--扩容

ArrayList即动态数组，能自动扩容，常用于当作某一对象的容器，ArrayList有较快的访问速度，而插入和删除操作则逊于LinkedList。ArrayList不是线程安全的，多线程下可以使用Vector类或者使用Collections.synchronizedList(list) 返回一个线程安全的ArrayList。

此文章简单探讨ArrayList在Java8中的实现，其实就是翻译一下注释（ps:其实读读自带的注释比看任何文章都好，所以本文不删除原注释）

下面是ArryaLIst 中的构造器实现：

/**
     * Default initial capacity.  
     *默认容量为10
     */
    private static final int DEFAULT_CAPACITY = 10;
  
    /**
     * Shared empty array instance used for empty instances.
     *空的对象数组
     */
    private static final Object[] EMPTY_ELEMENTDATA = {};
  
    /**
     * Shared empty array instance used for default sized empty instances. We
     * distinguish this from EMPTY_ELEMENTDATA to know how much to inflate when
     * first element is added.
     *默认的空对象数组，如果创建ArrayList时使用空的构造器，则使用此对象数组
     */
    private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {};
  
    /**
     * The array buffer into which the elements of the ArrayList are stored.
     * The capacity of the ArrayList is the length of this array buffer. Any
     * empty ArrayList with elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA
     * will be expanded to DEFAULT_CAPACITY when the first element is added.
     *该数组用于保存数据
     *用transient声明一个实例变量，当对象存储时，它的值不需要维持。即不可序列化
     */
    transient Object[] elementData; // non-private to simplify nested class access
  
    /**
     * The size of the ArrayList (the number of elements it contains).
     *ArrayList 大小（元素个数）
     * @serial
     */
    private int size;
  
     /**
     * Constructs an empty list with the specified initial capacity.
     *
     * @param  initialCapacity  the initial capacity of the list
     * @throws IllegalArgumentException if the specified initial capacity
     *         is negative
     *参数指定初始大小，如果为负数抛出异常
     */
  public ArrayList(int initialCapacity) {
        if (initialCapacity > 0) {
            this.elementData = new Object[initialCapacity];
        } else if (initialCapacity == 0) {
            this.elementData = EMPTY_ELEMENTDATA;
        } else {
            throw new IllegalArgumentException("Illegal Capacity: "+
                                               initialCapacity);
        }
    }
  
    /**
     * Constructs an empty list with an initial capacity of ten.
     *看代码是直接赋给默认空数组，可这里写初始容量为10？
     *猜测应该是注释没跟上新的实现，ArrayList有一个ensureCapacityInternal方法
     *当增加元素时，elementData等于DEFAULTCAPACITY_EMPTY_ELEMENTDATA就会
     *把最小容量设为默认容量10
     *如果错误麻烦大家指出
     */
    public ArrayList() {
        this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA;
    }
  
    /**
     * Constructs a list containing the elements of the specified
     * collection, in the order they are returned by the collection's
     * iterator.
     *
     * @param c the collection whose elements are to be placed into this list
     * @throws NullPointerException if the specified collection is null
     *将一个Collection实现类的对象转换为一个ArrayList，首先将调用toArray()
     *方法返回object[]，然后用copyOf方法将object[]里的值复制一份给elementData
     */
    public ArrayList(Collection<? extends E> c) {
        elementData = c.toArray();
        if ((size = elementData.length) != 0) {
            // c.toArray might (incorrectly) not return Object[] (see 6260652)
            if (elementData.getClass() != Object[].class)
                elementData = Arrays.copyOf(elementData, size, Object[].class);
        } else {
            // replace with empty array.
            this.elementData = EMPTY_ELEMENTDATA;
        }
    }

扩容相关方法，这是ArrayList实现动态数组的核心：

/**
     * Increases the capacity of this <tt>ArrayList</tt> instance, if
     * necessary, to ensure that it can hold at least the number of elements
     * specified by the minimum capacity argument.
     * @param   minCapacity   the desired minimum capacity
     *注意，这个方法是公开的，如果对ArrayList要装的对象数目有所预期
     *利用这个方法对数组进行扩容，可以提高效率
     */
    public void ensureCapacity(int minCapacity) {
        int minExpand = (elementData != DEFAULTCAPACITY_EMPTY_ELEMENTDATA)
            // any size if not default element table
            ? 0
            // larger than default for default empty table. It's already
            // supposed to be at default size.
            : DEFAULT_CAPACITY;
  
        if (minCapacity > minExpand) {
            ensureExplicitCapacity(minCapacity);
        }
    }
  
    /**
    *这个方法竟然没有注释，不过逻辑很简单
    *传进来的最小容量没默认大时，最小容量为10
    *然后调用ensureExplicitCapacity判断是否需要扩容
    */
    private void ensureCapacityInternal(int minCapacity) {
        if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) {
            minCapacity = Math.max(DEFAULT_CAPACITY, minCapacity);
        }
  
        ensureExplicitCapacity(minCapacity);
    }
     
    /**
    *也没有注释，不过逻辑很简单
    *所需的最小容量比elementData的长度大
    *需要扩容，调用grow方法
    */
    private void ensureExplicitCapacity(int minCapacity) {
        modCount++;
  
        // overflow-conscious code
        if (minCapacity - elementData.length > 0)
            grow(minCapacity);
    }
  
    /**
     * The maximum size of array to allocate.
     * Some VMs reserve some header words in an array.
     * Attempts to allocate larger arrays may result in
     * OutOfMemoryError: Requested array size exceeds VM limit
     *数组大小最大为最大int值-8，这是为了考虑溢出
     */
    private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;
  
    /**
     * Increases the capacity to ensure that it can hold at least the
     * number of elements specified by the minimum capacity argument.
     *
     * @param minCapacity the desired minimum capacity
     *>>为右移符号，右移即/2，所以首次扩容大小为原来的1.5倍
     *如果扩容后空间还不够，则把所需最小容量设为容量大小
     *如果所需容量大于最大值MAX_ARRAY_SIZE，进入hugeCapacity方法
     */
    private void grow(int minCapacity) {
        // overflow-conscious code
        int oldCapacity = elementData.length;
        int newCapacity = oldCapacity + (oldCapacity >> 1);
        if (newCapacity - minCapacity < 0)
            newCapacity = minCapacity;
        if (newCapacity - MAX_ARRAY_SIZE > 0)
            newCapacity = hugeCapacity(minCapacity);
        // minCapacity is usually close to size, so this is a win:
        elementData = Arrays.copyOf(elementData, newCapacity);
    }
  
   /**
    *这样设计应该是因为担心溢出，溢出后直接抛出异常
    * 没有溢出再设为最大的int值
    *不过如此大的数据量真有人用ArrayList存吗。。。。
    */
    private static int hugeCapacity(int minCapacity) {
        if (minCapacity < 0) // overflow
            throw new OutOfMemoryError();
        return (minCapacity > MAX_ARRAY_SIZE) ?
            Integer.MAX_VALUE :
            MAX_ARRAY_SIZE;
}

鉴于篇幅，将文章分篇， ArrayList的方法分析在第二篇，内部类分析在第三篇

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