手撕ArrayList

1.源码结构

 Resizable-array implementation of the <tt>List</tt> interface.  Implements
 all optional list operations, and permits all elements, including
 <tt>null</tt>.  In addition to implementing the <tt>List</tt> interface,
 this class provides methods to manipulate the size of the array that is
 used internally to store the list.  (This class is roughly equivalent to
 <tt>Vector</tt>, except that it is unsynchronized.)

2.重图解的角度分析ArrayList ，重点为扩容机制，初始化机制

API的使用并非使用的重点，我所要掌握的是从ArrayList的源码感受其数据结构的使用理解

①属性部分

    private static final long serialVersionUID = 8683452581122892189L;

    /**
     * Default initial capacity.
     */
    private static final int DEFAULT_CAPACITY = 10;

    /**
     * Shared empty array instance used for empty instances.工具而已
     */
    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.工具而已，就是为了与上面的区分在第一次添加数据时如何确定扩容机制
     */
    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.
     */
    transient Object[] elementData; // non-private to simplify nested class access  底层实际存储数据的数据结构即为数组

    /**
     * The size of the ArrayList (the number of elements it contains).
     *
     * @serial
     */
    private int size;

②初始化部分

 /**
     * 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
     */
    public ArrayList(int initialCapacity) {
        if (initialCapacity > 0) {
            this.elementData = new Object[initialCapacity];//传入初始容量
        } else if (initialCapacity == 0) {
            this.elementData = EMPTY_ELEMENTDATA;//与默认初始空数组作区分使用，这种虽然是空数组，但是这是传入的初始容量为0，有着本质区别
        } else {
            throw new IllegalArgumentException("Illegal Capacity: "+
                                               initialCapacity);
        }
    }

    /**
     * Constructs an empty list with an initial capacity of ten.
     */
    public ArrayList() {
        this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA;//默认一般都是这种，不需要传入初始容量
    }

    /**
     * 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
     */
    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;//这个与传入初始容量为0的一样
        }
    }

③添加到扩容机制 

来自为知笔记(Wiz)