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。

1     /**
2      * Default initial capacity.
3      */
4     private static final int DEFAULT_CAPACITY = 10;

默认初始大小为10。

1     /**
2      * Shared empty array instance used for empty instances.
3      */
4     private static final Object[] EMPTY_ELEMENTDATA = {};

为空的实例所共享的空数组实例

1     /**
2      * Shared empty array instance used for default sized empty instances. We
3      * distinguish this from EMPTY_ELEMENTDATA to know how much to inflate when
4      * first element is added.
5      */
6     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。

1     /**
2      * The array buffer into which the elements of the ArrayList are stored.
3      * The capacity of the ArrayList is the length of this array buffer. Any
4      * empty ArrayList with elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA
5      * will be expanded to DEFAULT_CAPACITY when the first element is added.
6      */
7     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.

1     /**
2      * The size of the ArrayList (the number of elements it contains).
3      *
4      * @serial
5      */
6     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。

1     /**
2      * Constructs an empty list with an initial capacity of ten.
3      */
4     public ArrayList() {
5         this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA;
6     }

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

 1     /**
 2      * Constructs a list containing the elements of the specified
 3      * collection, in the order they are returned by the collection's
 4      * iterator.
 5      *
 6      * @param c the collection whose elements are to be placed into this list
 7      * @throws NullPointerException if the specified collection is null
 8      */
 9     public ArrayList(Collection<? extends E> c) {
10         elementData = c.toArray();
11         if ((size = elementData.length) != 0) {
12             // c.toArray might (incorrectly) not return Object[] (see 6260652)
13             if (elementData.getClass() != Object[].class)
14                 elementData = Arrays.copyOf(elementData, size, Object[].class);
15         } else {
16             // replace with empty array.
17             this.elementData = EMPTY_ELEMENTDATA;
18         }
19     }

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

 1     /**
 2      * Trims the capacity of this <tt>ArrayList</tt> instance to be the
 3      * list's current size.  An application can use this operation to minimize
 4      * the storage of an <tt>ArrayList</tt> instance.
 5      */
 6     public void trimToSize() {
 7         modCount++;
 8         if (size < elementData.length) {
 9             elementData = (size == 0)
10               ? EMPTY_ELEMENTDATA
11               : Arrays.copyOf(elementData, size);
12         }
13     }

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。

 1     /**
 2      * Increases the capacity of this <tt>ArrayList</tt> instance, if
 3      * necessary, to ensure that it can hold at least the number of elements
 4      * specified by the minimum capacity argument.
 5      *
 6      * @param   minCapacity   the desired minimum capacity
 7      */
 8     public void ensureCapacity(int minCapacity) {
 9         int minExpand = (elementData != DEFAULTCAPACITY_EMPTY_ELEMENTDATA)
10             // any size if not default element table
11             ? 0
12             // larger than default for default empty table. It's already
13             // supposed to be at default size.
14             : DEFAULT_CAPACITY;
15 
16         if (minCapacity > minExpand) {
17             ensureExplicitCapacity(minCapacity);
18         }
19     }

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。

1     private static int calculateCapacity(Object[] elementData, int minCapacity) {
2         if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) {
3             return Math.max(DEFAULT_CAPACITY, minCapacity);
4         }
5         return minCapacity;
6     }

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。

1     private void ensureCapacityInternal(int minCapacity) {
2         ensureExplicitCapacity(calculateCapacity(elementData, minCapacity));
3     }

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

1     private void ensureExplicitCapacity(int minCapacity) {
2         modCount++;
3 
4         // overflow-conscious code
5         if (minCapacity - elementData.length > 0)
6             grow(minCapacity);
7     }

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。

 1     /**
 2      * Increases the capacity to ensure that it can hold at least the
 3      * number of elements specified by the minimum capacity argument.
 4      *
 5      * @param minCapacity the desired minimum capacity
 6      */
 7     private void grow(int minCapacity) {
 8         // overflow-conscious code
 9         int oldCapacity = elementData.length;
10         int newCapacity = oldCapacity + (oldCapacity >> 1);
11         if (newCapacity - minCapacity < 0)
12             newCapacity = minCapacity;
13         if (newCapacity - MAX_ARRAY_SIZE > 0)
14             newCapacity = hugeCapacity(minCapacity);
15         // minCapacity is usually close to size, so this is a win:
16         elementData = Arrays.copyOf(elementData, newCapacity);
17     }

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。

1     private static int hugeCapacity(int minCapacity) {
2         if (minCapacity < 0) // overflow
3             throw new OutOfMemoryError();
4         return (minCapacity > MAX_ARRAY_SIZE) ?
5             Integer.MAX_VALUE :
6             MAX_ARRAY_SIZE;
7     }

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。