ArrayList源码解析

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    /**
     * 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.
     */
    // Android-note: Also accessed from java.util.Collections
    transient Object[] elementData; // non-private to simplify nested class access

ArrayList 的数据结构是数组。

    /**
     * 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 an empty list with an initial capacity of ten.
     */
    public ArrayList() {
        this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA;
    }

从构造函数可以分析出来，默认构造函数会创建一个空的对象数组。携参构造函数会创建指定长度的对象数组。长度小于0会抛出IllegalArgumentException。
二、Add方法及ArrayList扩容

   /**
     * Appends the specified element to the end of this list.
     *
     * @param e element to be appended to this list
     * @return <tt>true</tt> (as specified by {@link Collection#add})
     */
    public boolean add(E e) {
        //确保内部容量
        ensureCapacityInternal(size + 1);  // Increments modCount!!
        elementData[size++] = e;
        return true;
    }
    //确保容量
    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);
    }

当我们Add一个数据时，ArrayList确保内部容量是否符合。核心算法在grow方法中

    /**
     * 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) {
        // 获取老容量
        int oldCapacity = elementData.length;
        //扩容之前容量的1/2
        int newCapacity = oldCapacity + (oldCapacity >> 1);
        //如果扩容后容量小于最小容量，数组的容量等于最小容量
        if (newCapacity - minCapacity < 0)
            newCapacity = minCapacity;
        if (newCapacity - MAX_ARRAY_SIZE > 0)
            //数组最大容量为2^31-8     private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;
            newCapacity = hugeCapacity(minCapacity);
            //拷贝原数据到新数组
        elementData = Arrays.copyOf(elementData, newCapacity);
    }

ArrayList 每次新增时都会判断是否扩容，产生扩容的条件是所需最小容量是否大于当前数组的长度。如果大于就扩容当前数组的1/2. 如果扩容之后的大小依然小于所需最小容量，那么数组就扩容。