ArrayList源码解读—Java8版本（上）

/**
 * Resizable-array implementation of the <tt>List</tt> interface.  Implements
 * all optional list operations, and permits all elements, including
 * <tt>null</tt>.  In addition to implementing the <tt>List</tt> interface,
 * this class provides methods to manipulate the size of the array that is
 * used internally to store the list.  (This class is roughly equivalent to
 * <tt>Vector</tt>, except that it is unsynchronized.)
 *
 * <p>The <tt>size</tt>, <tt>isEmpty</tt>, <tt>get</tt>, <tt>set</tt>,
 * <tt>iterator</tt>, and <tt>listIterator</tt> operations run in constant
 * time.  The <tt>add</tt> operation runs in <i>amortized constant time</i>,
 * that is, adding n elements requires O(n) time.  All of the other operations
 * run in linear time (roughly speaking).  The constant factor is low compared
 * to that for the <tt>LinkedList</tt> implementation.
 *
 * <p>Each <tt>ArrayList</tt> instance has a <i>capacity</i>.  The capacity is
 * the size of the array used to store the elements in the list.  It is always
 * at least as large as the list size.  As elements are added to an ArrayList,
 * its capacity grows automatically.  The details of the growth policy are not
 * specified beyond the fact that adding an element has constant amortized
 * time cost.
 *
 * <p>An application can increase the capacity of an <tt>ArrayList</tt> instance
 * before adding a large number of elements using the <tt>ensureCapacity</tt>
 * operation.  This may reduce the amount of incremental reallocation.
 *
 * <p><strong>Note that this implementation is not synchronized.</strong>
 * If multiple threads access an <tt>ArrayList</tt> instance concurrently,
 * and at least one of the threads modifies the list structurally, it
 * <i>must</i> be synchronized externally.  (A structural modification is
 * any operation that adds or deletes one or more elements, or explicitly
 * resizes the backing array; merely setting the value of an element is not
 * a structural modification.)  This is typically accomplished by
 * synchronizing on some object that naturally encapsulates the list.
 *
 * If no such object exists, the list should be "wrapped" using the
 * {@link Collections#synchronizedList Collections.synchronizedList}
 * method.  This is best done at creation time, to prevent accidental
 * unsynchronized access to the list:<pre>
 *   List list = Collections.synchronizedList(new ArrayList(...));</pre>
 *
 * <p><a name="fail-fast">
 * The iterators returned by this class's {@link #iterator() iterator} and
 * {@link #listIterator(int) listIterator} methods are <em>fail-fast</em>:</a>
 * if the list is structurally modified at any time after the iterator is
 * created, in any way except through the iterator's own
 * {@link ListIterator#remove() remove} or
 * {@link ListIterator#add(Object) add} methods, the iterator will throw a
 * {@link ConcurrentModificationException}.  Thus, in the face of
 * concurrent modification, the iterator fails quickly and cleanly, rather
 * than risking arbitrary, non-deterministic behavior at an undetermined
 * time in the future.
 *
 * <p>Note that the fail-fast behavior of an iterator cannot be guaranteed
 * as it is, generally speaking, impossible to make any hard guarantees in the
 * presence of unsynchronized concurrent modification.  Fail-fast iterators
 * throw {@code ConcurrentModificationException} on a best-effort basis.
 * Therefore, it would be wrong to write a program that depended on this
 * exception for its correctness:  <i>the fail-fast behavior of iterators
 * should be used only to detect bugs.</i>
 *
 * <p>This class is a member of the
 * <a href="{@docRoot}/../technotes/guides/collections/index.html">
 * Java Collections Framework</a>.
 *
 * @author  Josh Bloch
 * @author  Neal Gafter
 * @see     Collection
 * @see     List
 * @see     LinkedList
 * @see     Vector
 * @since   1.2
 */
public class ArrayList<E> extends AbstractList<E>
        implements List<E>, RandomAccess, Cloneable, java.io.Serializable
{
...
}

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

/**
 * 初始化默认容量。
 */
private static final int DEFAULT_CAPACITY = 10;
/**
 * 指定该ArrayList容量为0时，返回该空数组。
 */
private static final Object[] EMPTY_ELEMENTDATA = {};
/**
 * 当调用无参构造方法，返回的是该数组。刚创建一个ArrayList 时，其内数据量为0。
 * 它与EMPTY_ELEMENTDATA的区别就是：该数组是默认返回的，而后者是在用户指定容量为0时返回。
 */
private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {};
/**
 * 保存添加到ArrayList中的元素。
 * ArrayList的容量就是该数组的长度。
 * 该值为DEFAULTCAPACITY_EMPTY_ELEMENTDATA 时，当第一次添加元素进入ArrayList中时，数组将扩容值DEFAULT_CAPACITY。
 * 被标记为transient，在对象被序列化的时候不会被序列化。
 */
transient Object[] elementData; // non-private to simplify nested class access
/**
 * ArrayList的实际大小（数组包含的元素个数）。
 * @serial
 */
private int size;