理解JAVA集合：ArrayList-优快云博客

本文详细解析了Java中ArrayList的工作原理，包括其内部结构、构造函数、自动扩容机制等关键特性。通过对源码的深入分析，帮助读者理解ArrayList如何高效地进行元素的添加与存储。

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一、ArrayList

ArrayList的继承关系：

变量

DEFAULT_CAPCITY : 默认容量
EMPTY_ELEMENTDATA: 空元素数据
DEFAULTCAPACITY_EMPTY_ELEMENTDATA：默认容量下空元素数据
elementData：元素数据素组，用于保存数据
size: 记录列表大小

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

    /**
     * Shared empty array instance used for empty instances.
     */
    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.
     */
    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

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

构造函数：

默认构造函数：初始化数组大小为10的数组。
带初始化数组大小的构造函数：初始化大小为指定大小的数组。
Copy一个 Collection的List作为一个初始化数据的构造函数,这里有个判断elementData.getClass() != Object[].class，前面的toArray 方法有可能不会返回Object[]，如果不是返回的Object[]则使用Arrays.copyOf进行拷贝。

   /**
     * Constructs an empty list with an initial capacity of ten.
     */
    public ArrayList() {
        this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA;
    }
   /**
     * 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];
        } else if (initialCapacity == 0) {
            this.elementData = EMPTY_ELEMENTDATA;
        } else {
            throw new IllegalArgumentException("Illegal Capacity: "+initialCapacity);
        }
    }

   /**
     * 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;
        }
    }

自动扩容

arrayList是自动扩容，初始化默认大小为10，在add或者addAll的时候进行自动扩容。
int newCapacity = oldCapacity + (oldCapacity >> 1);   
扩容的大小为原来的1.5倍,此处的oldCapacity右移动一位等于 oldCapacity除以2，另外还判断了如果扩容之后的大小没有minCapacity大，则取minCapacity的大小，如果超过了最大大小则取Integer的最大值Integer.MAX_VALUE，最后使用Arrays.copyOf 将原来的复制一份到新的大小数组里面这里Arrays.copyOf 调用的是
System.arraycopy(Object src, //源数组
                 int srcPos, //源数组要复制的起始位置
                 Object dest,//目的数组
                 int destPos,//目的数组放置的起始位置
                 int length);//复制的长度
这个方法可以复制自身，ArrayList使用到了这个方法实现添加和删除，每次在添加的时候都使用方法ensureCapacityInternal进行扩容。

   /**
     * 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
     */
    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);
    }
   //扩容调用的方法 
   private void ensureCapacityInternal(int minCapacity) {
        if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) {
            minCapacity = Math.max(DEFAULT_CAPACITY, minCapacity);
        }
        ensureExplicitCapacity(minCapacity);
    }

    private void ensureExplicitCapacity(int minCapacity) {
        modCount++;
        // overflow-conscious code
        if (minCapacity - elementData.length > 0)
            grow(minCapacity);
    }

转载于:https://my.oschina.net/xiaofli/blog/1526920