ArrayList源码分析

1. 下面是ArrayList的继承体系结构

　　从上图可以看出ArrayList的继承体系：

1. 继承AbstractList,实现List,它是一个数组队列，提供了相关的添加、删除、修改、遍历等功能

2. 实现RandomAccess接口,实现快速随机访问：通过元素的序号快速获取元素对象

3. 实现Cloneable接口, 重写了clone(), 可以进行克隆(浅拷贝)

4. 实现Serialiazable接口, 可以实现序列化

2. ArrayList的数据结构

2.1  重要的全局变量

    /**
     * Default initial capacity.
     * 在初次进行元素添加的时候初始化的数组长度
     */
    private static final int DEFAULT_CAPACITY = 10;

    /**
     * Shared empty array instance used for empty instances.
     * 默认的一个空数组
     */
    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.
     * 使用默认构造器创建ArrayList的对象时默认使用的一个空数组
     */
    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.
     * ArrayList的底层数组
     */
    transient Object[] elementData; // non-private to simplify nested class access

    /**
     * The size of the ArrayList (the number of elements it contains).
     * ArrayList中已存在的元素个数
     * @serial
     */
    private int size;

　　
   /**
    * 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
    * ArrayList的允许的最大元素个数
    */
    private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;

2.2 构造器

    /**
     * Constructs an empty list with the specified initial capacity.
     * 创建一个指定容量的空列表，如果传入的参数为零，这使用一个默认已经创建好的空列表
     * 如果参数小于0，这会抛出异常
     * @param  initialCapacity  the initial capacity of the list
     * @throws IllegalArgumentException if the specified initial capacity
     *         is negative
     */
    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.
     * 默认的构造器，初始化容量为0，在第一个元素新增时
     * 使用DEFAULT_CAPACITY（10）完成有容量的初始化
     */
    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.
     * 接受一个Collection对象直接转换为ArrayList
     * @param c the collection whose elements are to be placed into this list
     * @throws NullPointerException if the specified collection is null
     */
    public ArrayList(Collection<? extends E> c) {

        //底层的动态数组
        elementData = c.toArray();

        //如果存在元素，且数组中的元素不是Object类型的，则进行转换并赋给ArrayList的底层数组
        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 {  //创建一个空容量的数组
            // replace with empty array.
            this.elementData = EMPTY_ELEMENTDATA;
        }
    }