JDK1.8源码分析：ArrayList

<>数据结构

*
ArrayList相对于数组Array只是提供了动态拓容的功能，在内部也是使用一个数组来存储数据的。

*
ArrayList也不是线程安全的，如果需要线程安全，则需要使用Collections.synchronziedList来进行包装，如：List list
= Collections.synchronizedList(new ArrayList(…));
/** * 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
*

元素个数size维护：size记录了当前list的内部数组存放了多少个元素，所以可以通过size方法来通过O(1)时间复杂度返回size，而不需要每次都对数组进行一次遍历计算。主要是在添加和删除元素时，执行size++或size–来同步更新：
/** * 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!! // 递增size elementData[size++] = e; return true; }
/** * Removes the element at the specified position in this list. * Shifts any
subsequent elements to the left (subtracts one from their * indices). * *
@param index the index of the element to be removed * @return the element that
was removed from the list * @throws IndexOutOfBoundsException {@inheritDoc} */
public E remove(int index) { rangeCheck(index); modCount++; E oldValue =
elementData(index); int numMoved = size - index - 1; if (numMoved > 0) System.
arraycopy(elementData, index+1, elementData, index, numMoved); // 递减size
elementData[--size] = null; // clear to let GC do its work return oldValue; }
<>拓容规则

*

内部数组拓容规则：每次在调用add添加元素时，如果内部数组大小满了，则会进行拓容。拓容规则为：初始大小为10，之后每次拓容为当前数组容量的1.5倍，实现源码如下：主要在grow方法实现。
/** * 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; // 默认拓容为原来的1.5倍 int newCapacity = oldCapacity +
(oldCapacity >> 1); // 如果原来的容量加一比原来的1.5倍大，则使用该加一的版本 if (newCapacity -
minCapacity< 0) newCapacity = minCapacity; // 最大不超过Integer.MAX_VALUE - 8 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; }
*

拓容在确定好新的容量之后，会通过Arrays.copyOf方法来新建一个该新容量的数组，然后在Arrays.copyOf内部通过System.arraycopy方法将旧数组的内容拷贝到新数组，然后返回该新数组作为内部数组。其中System.arraycopy为本地native方法，性能较高。
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: // 数组拓容 elementData = Arrays.copyOf(
elementData, newCapacity); } // Arrays.copyOf的实现： public static <T> T[] copyOf(T
[] original, int newLength) { return (T[]) copyOf(original, newLength, original.
getClass()); } public static <T,U> T[] copyOf(U[] original, int newLength, Class
<? extends T[]> newType) { @SuppressWarnings("unchecked") T[] copy = ((Object)
newType== (Object)Object[].class) ? (T[]) new Object[newLength] : (T[]) Array.
newInstance(newType.getComponentType(), newLength); System.arraycopy(original, 0
, copy, 0, Math.min(original.length, newLength)); return copy; }
<>子集SubList

*

子集合SubList：SubList为List列表的一个子集，在底层与List列表公用一个内部数组来存储数据，所以对suList返回的子列表进行操作也会对主列表List产生影响。如下：set方法其实是对主列表的内部数组elementData进行操作的。
private class SubList extends AbstractList<E> implements RandomAccess { private
final AbstractList<E> parent; private final int parentOffset; private final int
offset; int size; SubList(AbstractList<E> parent, int offset, int fromIndex, int
toIndex) { this.parent = parent; this.parentOffset = fromIndex; this.offset =
offset+ fromIndex; this.size = toIndex - fromIndex; this.modCount = ArrayList.
this.modCount; } public E set(int index, E e) { rangeCheck(index);
checkForComodification(); E oldValue = ArrayList.this.elementData(offset + index
); ArrayList.this.elementData[offset + index] = e; return oldValue; } ... }
<>迭代器

*

迭代器iterator：迭代器也是fail-fast的，注意判断fail-fast的条件是在当遍历到的数组下标i大于当前内部数组的size或者数组容量capacity时，才抛出异常。所以在使用迭代器的时候，不能依赖数组的位置关系，否则可能会出错，如在遍历第5个位置之前，其他线程在1到5之间插入了一个元素，则第5个其实是产生该迭代器的时候的第4个。

*
如果需要依赖位置关系，即线程安全方面的考虑，则需要使用Collections.synchronizedList来包装成线程安全的List。
public Iterator<E> iterator() { return new Itr(); } /** * An optimized version
of AbstractList.Itr */ private class Itr implements Iterator<E> { int cursor;
// index of next element to return int lastRet = -1; // index of last element
returned; -1 if no such int expectedModCount = modCount; public boolean hasNext(
) { return cursor != size; } @SuppressWarnings("unchecked") public E next() {
checkForComodification(); int i = cursor; // i 大于数组大小 if (i >= size) throw new
NoSuchElementException(); Object[] elementData = ArrayList.this.elementData; //
i 大于数组容量 if (i >= elementData.length) throw new ConcurrentModificationException(
); cursor = i + 1; return (E) elementData[lastRet = i]; } ... }

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