源码分析之ArrayList（未完待续，写到了274行）

先看

public class ArrayList<E> extends AbstractList<E>
        implements List<E>, RandomAccess, Cloneable, java.io.Serializable

继承自AbstractList，实现了4个接口，List,RandomAccess,Cloneable,java.io.Serializable.

List:这是一个序列，可以有重复元素。

RandomAccess：可以随机快速访问的标记。

Cloneable: 它实现了clone方法，可以合法地调用object.clone()方法。

java.io.Serializable：支持序列化。

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

默认初始大小为10。

    /**
     * Shared empty array instance used for empty instances.
     */
    private static final Object[] EMPTY_ELEMENTDATA = {};

为空的实例所共享的空数组实例

    /**
     * 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 = {};

用于默认大小的空实例的共享空数组实例。 将此与EMPTY_ELEMENTDATA区分开来，以了解添加第一个元素时要膨胀多少。

    /**
     * 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

可见ArrayList内部是用数组实现的。trasient 修饰符，这个属性不会被序列化。当第一个元素添加到空的ArrayList来的时候，这个数组会扩展到默认大小，也就是10.

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

ArrayList的大小。也就是这个容器中元素的个数。

    /**
     * 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);
        }
    }

这是ArrayList的一个带参构造方法，可以看到，初始容量不能为负，否则会主动抛出异常。这里，如果初始容量大于0，新建了一个Object数组并赋给了elementData。如果初始容量等于0，直接将EMPTY_ELEMENTDATA赋值给elementData。换句话说，在jdk1.8中，所有以带初始容量入参的构造方法创建的且传入的初始容量为0的ArrayList变量，指向的是同一个空的对象数组常量。

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

无参构造方法。会将DEFAULTCAPACITY_EMPTY_ELEMENTDATA赋值给elementData.

    /**
     * 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;
        }
    }

一个带参的构造方法，可以传入E的子类组成的集合。elementData直接被赋值c.toArray()。如果这时它长度为0，

会被替换成内部的EMPTY_ELEMENTDATA；如果长度不为0，进一步判断它的class是否为Object[].class，

如果不是，则会用Arrays.copyOf()方法对其按照Object[]的class进行一次拷贝。

PS:see 6260652指的是以前这个地方有bug,具体可以看：https://bugs.java.com/bugdatabase/view_bug.do?bug_id=6260652

    /**
     * Trims the capacity of this <tt>ArrayList</tt> instance to be the
     * list's current size.  An application can use this operation to minimize
     * the storage of an <tt>ArrayList</tt> instance.
     */
    public void trimToSize() {
        modCount++;
        if (size < elementData.length) {
            elementData = (size == 0)
              ? EMPTY_ELEMENTDATA
              : Arrays.copyOf(elementData, size);
        }
    }

调整ArrayList的容量为当前列表的大小。这个方法可以用来最小化ArrayList实例所占用的空间。

    /**
     * Increases the capacity of this <tt>ArrayList</tt> instance, if
     * necessary, to ensure that it can hold at least the number of elements
     * specified by the minimum capacity argument.
     *
     * @param   minCapacity   the desired minimum capacity
     */
    public void ensureCapacity(int minCapacity) {
        int minExpand = (elementData != DEFAULTCAPACITY_EMPTY_ELEMENTDATA)
            // any size if not default element table
            ? 0
            // larger than default for default empty table. It's already
            // supposed to be at default size.
            : DEFAULT_CAPACITY;

        if (minCapacity > minExpand) {
            ensureExplicitCapacity(minCapacity);
        }
    }

增加ArrayList 实例的容量，如有必要，保证它至少可以容纳minCapacity(最小容量)个元素。minExpand(最小扩展)在elementData为空数据的时候为默认容量10，否则为0。当入参最小容量大于这个最小扩展时，会调用ensureExplicitCapacity()方法，并传入minCapacity。

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

计算容量，如果elementData为空数据，返回初始容量10和minCapacity中的大者，否则直接返回minCapacity。

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

内部的保证容量的方法，先调calculateCapacity()计算容量，再调ensureExplicitCapacity()。

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

        // overflow-conscious code
        if (minCapacity - elementData.length > 0)
            grow(minCapacity);
    }

保证精确的容量，成员变量modCount自增一次，当最小容量大于elementData的长度时，调用grow()，并传入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;

这个私有常量表明数组最大的长度可以为Integer的最大值减8，如果尝试分配更大的数组会引发内存溢出。

    /**
     * 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);
    }

grow()方法上面ensureExplicitCapacity()方法有调用。旧容量就是elementData的长度，新容量是老容量加老容量左移1位，就是1.5倍的样子。再看如果新容量小于最小容量，就取最小容量。如果新容量大于最大数组大小，就调hugeCapacity()计算一下。最后，将elementData按照上面得出的最小容量，进行一次拷贝。

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

大容量计算，如果minCapacity小于0，直接抛内存溢出error；再比较它与最大数组大小，小于则取整型最大值，超过了则取最大数组大小。

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