ArrayList源码分析

ArrayList是容量可以改变的非线程安全集合。内部实现使用数组进行存储（需要一块内存连续的空间），集合扩容时会创建更大的数组空间，把原有数据复制到新的数组中。

ArrayList支持对元素的快速随机访问，但是插入与删除速度通常很慢（会涉及元素的移动，时间复杂度为O(n)）。

public class ArrayList<E> extends AbstractList<E>
        implements List<E>, RandomAccess, Cloneable, java.io.Serializable
{
    private static final long serialVersionUID = 8683452581122892189L;

    /**
     * Default initial capacity. 默认初始容量为10
     */
    private static final int DEFAULT_CAPACITY = 10; 

    /**
     * Shared empty array instance used for empty instances.　　创建ArrayList对象时默认内部数组为空
     */
    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). ArrayList集合包含的元素数量
     *
     * @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
     */
    public ArrayList(int initialCapacity) {
　　　　// 若给定的初始容量大小合法，则创建一个给定大小的数组
        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;
    }
}

添加元素方法：

    /**
     * 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) {
        ensureCapacityInternal(size + 1);  // Increments modCount!!
        elementData[size++] = e;
        return true;
    }

    /**
     * Inserts the specified element at the specified position in this
     * list. Shifts the element currently at that position (if any) and
     * any subsequent elements to the right (adds one to their indices).
     *
     * @param index index at which the specified element is to be inserted
     * @param element element to be inserted
     * @throws IndexOutOfBoundsException {@inheritDoc}
     */
    public void add(int index, E element) {
        rangeCheckForAdd(index);

        ensureCapacityInternal(size + 1);  // Increments modCount!!
        System.arraycopy(elementData, index, elementData, index + 1,
                         size - index);
        elementData[index] = element;
        size++;
    }

ensureCapacityInternal：确保容量满足添加条件

 private static int calculateCapacity(Object[] elementData, int minCapacity) {
        if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) {
            return Math.max(DEFAULT_CAPACITY, minCapacity);
        }
        return minCapacity;
    }

    private void ensureCapacityInternal(int minCapacity) {
        ensureExplicitCapacity(calculateCapacity(elementData, minCapacity));
    }

    private void ensureExplicitCapacity(int minCapacity) {
        modCount++;

        // overflow-conscious code
　　　　　// 如果添加后的数组长度大于现在的数组容量，那么扩容

        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
     */
    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
     */
    private void grow(int minCapacity) {
        // overflow-conscious code
        int oldCapacity = elementData.length;  旧的数组长度
        int newCapacity = oldCapacity + (oldCapacity >> 1);　　新的数组长度为旧的数组长度的1.5倍
        if (newCapacity - minCapacity < 0)　　// 正数带符号右移的值肯定是正值，所以oldCapacity + (oldCapacity >> 1)的结果可能超过int可以表示的最大值，反而可能比参数
            newCapacity = minCapacity;　　　　minCapacity更小，则返回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);
    }

    private static int hugeCapacity(int minCapacity) {
        if (minCapacity < 0) // overflow
            throw new OutOfMemoryError();
        return (minCapacity > MAX_ARRAY_SIZE) ?
            Integer.MAX_VALUE :
            MAX_ARRAY_SIZE;
    }

ArrayList是线程不安全的，要使用线程安全的List：

List<?> list = Collections.synchronizedList(new LinkedList<?>());

　　但是无论是读取还是写入，Collections.synchronizedList都会进行加锁，当我们并发级别特别高，线程之间在任何操作上都会进行等待，因此在某些场景中它不是最好的选择。在很多的场景中，我们的读取操作可能远远大于写入操作，为了将读取的性能发挥到极致，提供了CopyOnWriteArrayList类，该类在使用过程中，读读之间不互斥并且更厉害的是读写也不互斥。