java源码学习-----ArrayList

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* ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
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package java.util;

/**
* 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>:
* 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
*/
//看看它的继承关系，父类AbstractList<E>  接口List<E>
public class ArrayList<E> extends AbstractList<E>
implements List<E>, RandomAccess, Cloneable, java.io.Serializable
{
private static final long serialVersionUID = 8683452581122892189L;

/**
* The array buffer into which the elements of the ArrayList are stored.
* The capacity of the ArrayList is the length of this array buffer.
*/
private transient Object[] elementData; //声明为transient，表示不会被持久化。ArrayList的数据持久化的时候不就丢了？

/**
* The size of the ArrayList (the number of elements it contains).
*
* @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) { //都没有给size赋值的
super();  //触发父类的初始化AbstractList
if (initialCapacity < 0)  //不可传入小于零的initialCapacity
throw new IllegalArgumentException("Illegal Capacity: "+
initialCapacity);
this.elementData = new Object[initialCapacity];  //构造大小为initialCapacity的数组
}

/**
* Constructs an empty list with an initial capacity of ten.
*/
public ArrayList() { //默认的initialCapacity为10
this(10);
}

/**
* 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(); //直接toArray就赋值了
size = elementData.length;
// c.toArray might (incorrectly) not return Object[] (see 6260652)
if (elementData.getClass() != Object[].class) //toArray（）失手的时候
elementData = Arrays.copyOf(elementData, size, Object[].class); //调用Arrays.copyOf
}

/**
* 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++; //这个是父类的变量
int oldCapacity = elementData.length;
if (size < oldCapacity) {
elementData = Arrays.copyOf(elementData, size);  //难道size才是真正大小，而elementData有可能存在不属于的数据
}
}

/**

*如果有需要的话，增加ArrayList的容量。确保ArrayList至少可以容纳minCapacity个元素

* 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) {
if (minCapacity > 0)
ensureCapacityInternal(minCapacity); //看ensureCapacityInternal做了什么工作
}

private void ensureCapacityInternal(int minCapacity) {
modCount++; //第二次modeCount++了
// overflow-conscious code（可能会越界的代码？）
if (minCapacity - elementData.length > 0) //如果minCapacity不够大，就不做什么工作
grow(minCapacity);//继续看grow做了什么：扩大了elementData 的容量
}

/**
* 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;
int newCapacity = oldCapacity + (oldCapacity >> 1); //oldCapacity>>1 -- oldCapacity/2 取整
if (newCapacity - minCapacity < 0)
newCapacity = minCapacity; //如果minCappacity比newCapacity要大，就取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); //也就是说elementData有可能是没有填满的
}

private static int hugeCapacity(int minCapacity) { //minCapacity小于0，不行；太大，必须缩小
if (minCapacity < 0) // overflow 怎么
throw new OutOfMemoryError();
return (minCapacity > MAX_ARRAY_SIZE) ?
Integer.MAX_VALUE :
MAX_ARRAY_SIZE;
}

/**
* Returns the number of elements in this list.
*
* @return the number of elements in this list
*/
public int size() {
return size; //直接返回size 也就是说获得size的时候，是不用循环的。对么？
}

/**
* Returns <tt>true</tt> if this list contains no elements.
*
* @return <tt>true</tt> if this list contains no elements
*/
public boolean isEmpty() { // 这个跟String里面挺像
return size == 0;
}

/**
* Returns <tt>true</tt> if this list contains the specified element.
* More formally, returns <tt>true</tt> if and only if this list contains
* at least one element <tt>e</tt> such that
* <tt>(o==null ? e==null : o.equals(e))</tt>.
*
* @param o element whose presence in this list is to be tested
* @return <tt>true</tt> if this list contains the specified element
*/
public boolean contains(Object o) { //也是通过访问indexOf
return indexOf(o) >= 0;
}

/**
* Returns the index of the first occurrence of the specified element
* in this list, or -1 if this list does not contain the element.
* More formally, returns the lowest index <tt>i</tt> such that
* <tt>(o==null ? get(i)==null : o.equals(get(i)))</tt>,
* or -1 if there is no such index.
*/
public int indexOf(Object o) {
if (o == null) { //参数还可以为null的
for (int i = 0; i < size; i++)
if (elementData[i]==null) //==
return i;
} else {
for (int i = 0; i < size; i++)
if (o.equals(elementData[i])) //equals,这时可以确定o一定不是null的
return i;
}
return -1;
}

/**
* Returns the index of the last occurrence of the specified element
* in this list, or -1 if this list does not contain the element.
* More formally, returns the highest index <tt>i</tt> such that
* <tt>(o==null ? get(i)==null : o.equals(get(i)))</tt>,
* or -1 if there is no such index.
*/
public int lastIndexOf(Object o) {  //倒过来查找
if (o == null) {  //1.为null的情况
for (int i = size-1; i >= 0; i--)
if (elementData[i]==null)
return i;
} else {  //2不为null的情况
for (int i = size-1; i >= 0; i--)
if (o.equals(elementData[i]))
return i;
}
return -1;
}

/**浅拷贝（The elements themselves are not copied）
* Returns a shallow copy of this <tt>ArrayList</tt> instance.  (The
* elements themselves are not copied.)
*
* @return a clone of this <tt>ArrayList</tt> instance
*/

//对clone得到的结果进行修改不会影响到原生的ArrayList(ArrayList<String> 改变其中的字符串)

//该方法返回的是Object。如果作为ArrayList使用，需要强制转换；
public Object clone() {
try {
@SuppressWarnings("unchecked")
ArrayList<E> v = (ArrayList<E>) super.clone(); //这个到native那里去了
v.elementData = Arrays.copyOf(elementData, size); //最后会走System.arraycopy
v.modCount = 0;//怎么没有给size赋值
return v;
} catch (CloneNotSupportedException e) {
// this shouldn't happen, since we are Cloneable
throw new InternalError();
}
}

/**
* Returns an array containing all of the elements in this list
* in proper sequence (from first to last element).
*返回的object数组跟原来的ArrayList是分离的，使用返回的结果，不受源的影响
* <p>The returned array will be "safe" in that no references to it are
* maintained by this list.  (In other words, this method must allocate
* a new array).  The caller is thus free to modify the returned array.
*    这个方法时array-based和collection-based直接的桥梁
* <p>This method acts as bridge between array-based and collection-based
* APIs.
*
* @return an array containing all of the elements in this list in
*         proper sequence
*/
public Object[] toArray() {
return Arrays.copyOf(elementData, size); //跟clone 走的差不多
}

/**
* Returns an array containing all of the elements in this list in proper
* sequence (from first to last element); the runtime type of the returned
* array is that of the specified array.  If the list fits in the
* specified array, it is returned therein.  Otherwise, a new array is
* allocated with the runtime type of the specified array and the size of
* this list.
*
* <p>If the list fits in the specified array with room to spare
* (i.e., the array has more elements than the list), the element in
* the array immediately following the end of the collection is set to
* <tt>null</tt>.  

*( 把最后一个设为null，为了让调用者方便知道list的长度，如果调用者知道list不包含任何null元素。不知这个list是源还是目的数组

*  This is useful in determining the length of the
* list <i>only</i> if the caller knows that the list does not contain
* any null elements.)
*
* @param a the array into which the elements of the list are to
*          be stored, if it is big enough; otherwise, a new array of the
*          same runtime type is allocated for this purpose.
* @return an array containing the elements of the list
* @throws ArrayStoreException if the runtime type of the specified array
*         is not a supertype of the runtime type of every element in
*         this list
* @throws NullPointerException if the specified array is null
*/
@SuppressWarnings("unchecked")
public <T> T[] toArray(T[] a) {
if (a.length < size) //如果目的数组比源的容量小
// Make a new array of a's runtime type, but my contents:
return (T[]) Arrays.copyOf(elementData, size, a.getClass());//生成一个新的数组，同时它的内容为源，而类型为目的

System.arraycopy(elementData, 0, a, 0, size);  //如果足够容纳，就直接赋值到目的数组
if (a.length > size)
a[size] = null; //用来做标记？
return a;
}

// Positional Access Operations

@SuppressWarnings("unchecked")
E elementData(int index) { //包访问权限的方法
return (E) elementData[index];
}

/**
* Returns the element at the specified position in this list.
*
* @param  index index of the element to return
* @return the element at the specified position in this list
* @throws IndexOutOfBoundsException {@inheritDoc}
*/
public E get(int index) {
rangeCheck(index);

return elementData(index);  //直接通过数组读取；不知为什么要分成两步，多出个包访问权限的方法
}

/**
* Replaces the element at the specified position in this list with
* the specified element.
*
* @param index index of the element to replace
* @param element element to be stored at the specified position
* @return the element previously at the specified position
* @throws IndexOutOfBoundsException {@inheritDoc}
*/
public E set(int index, E element) {
rangeCheck(index);

E oldValue = elementData(index);
elementData[index] = element;//直接对数组赋值
return oldValue;  //返回旧数据
}

/**
* 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!!   //如果有必要，需要扩大空间
elementData[size++] = e;//给数组赋值
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); //index开始的数据往后挪动一位
elementData[index] = element; //赋值
size++;//更新size
}

/**
* 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, //index+1 开始的往前移一位
numMoved);
elementData[--size] = null; // Let gc do its work 促使提前回收资源？如果需要remove的是最后一位，就直接赋值为null了

return oldValue;
}

/**
* Removes the first occurrence of the specified element from this list,
* if it is present.  If the list does not contain the element, it is
* unchanged.  More formally, removes the element with the lowest index
* <tt>i</tt> such that
* <tt>(o==null ? get(i)==null : o.equals(get(i)))</tt>
* (if such an element exists).  Returns <tt>true</tt> if this list
* contained the specified element (or equivalently, if this list
* changed as a result of the call).
*
* @param o element to be removed from this list, if present
* @return <tt>true</tt> if this list contained the specified element
*/
public boolean remove(Object o) { //循环查找，然后移动数据
if (o == null) { //为null
for (int index = 0; index < size; index++)
if (elementData[index] == null) {
fastRemove(index); //跟remove(int index)做的工作一样的呀，把数组数据统一向前移位
return true;
}
} else { //不为null
for (int index = 0; index < size; index++)
if (o.equals(elementData[index])) {
fastRemove(index);
return true;
}
}
return false;
}

/*
* Private remove method that skips bounds checking and does not
* return the value removed.
*/
private void fastRemove(int index) {  //跟remove(int index)一样的工作
modCount++;
int numMoved = size - index - 1;
if (numMoved > 0)
System.arraycopy(elementData, index+1, elementData, index,
numMoved);
elementData[--size] = null; // Let gc do its work
}

/**
* Removes all of the elements from this list.  The list will
* be empty after this call returns.
*/
public void clear() {
modCount++;

// Let gc do its work
for (int i = 0; i < size; i++)
elementData[i] = null;  //每位赋值为null

size = 0;  //设size为0
}

/**
* Appends all of the elements in the specified collection to the end of
* this list, in the order that they are returned by the
* specified collection's Iterator.  The behavior of this operation is
* undefined if the specified collection is modified while the operation
* is in progress.  (This implies that the behavior of this call is
* undefined if the specified collection is this list, and this
* list is nonempty.)
*
* @param c collection containing elements to be added to this list
* @return <tt>true</tt> if this list changed as a result of the call
* @throws NullPointerException if the specified collection is null
*/
public boolean addAll(Collection<? extends E> c) {
Object[] a = c.toArray();//先转成数组
int numNew = a.length;
ensureCapacityInternal(size + numNew);  // Increments modCount
System.arraycopy(a, 0, elementData, size, numNew);//然后使用System.arraycopy
size += numNew;
return numNew != 0;
}

/**
* Inserts all of the elements in the specified collection into this
* list, starting at the specified position.  Shifts the element
* currently at that position (if any) and any subsequent elements to
* the right (increases their indices).  The new elements will appear
* in the list in the order that they are returned by the
* specified collection's iterator.
*
* @param index index at which to insert the first element from the
*              specified collection
* @param c collection containing elements to be added to this list
* @return <tt>true</tt> if this list changed as a result of the call
* @throws IndexOutOfBoundsException {@inheritDoc}
* @throws NullPointerException if the specified collection is null
*/
public boolean addAll(int index, Collection<? extends E> c) { //index表示要添加进来的collection在原来ArrayList的起始位置
rangeCheckForAdd(index);//这里，如果index比原来的ArrayList的size(是可以等于size的）还要大，会FC  IndexOutOfBoundsException

Object[] a = c.toArray();
int numNew = a.length;
ensureCapacityInternal(size + numNew);  // Increments modCount

int numMoved = size - index;
if (numMoved > 0)//大于零才需要。等于零就直接放到后面好了
System.arraycopy(elementData, index, elementData, index + numNew, //先把原来的数据移位
numMoved);

System.arraycopy(a, 0, elementData, index, numNew); //这个把需要插入的数据插入进去
size += numNew;
return numNew != 0;
}

/**
* Removes from this list all of the elements whose index is between
* {@code fromIndex}, inclusive, and {@code toIndex}, exclusive.
* Shifts any succeeding elements to the left (reduces their index).
* This call shortens the list by {@code (toIndex - fromIndex)} elements.
* (If {@code toIndex==fromIndex}, this operation has no effect.)
*
* @throws IndexOutOfBoundsException if {@code fromIndex} or
*         {@code toIndex} is out of range
*         ({@code fromIndex < 0 ||
*          fromIndex >= size() ||
*          toIndex > size() ||
*          toIndex < fromIndex})
*/
protected void removeRange(int fromIndex, int toIndex) {//把从[fromIndex,toIndex)的数据移除
modCount++;
int numMoved = size - toIndex;
System.arraycopy(elementData, toIndex, elementData, fromIndex,  //移动数据
numMoved);

// Let gc do its work
int newSize = size - (toIndex-fromIndex); //计算新size
while (size != newSize)
elementData[--size] = null;  //把相应的位置设为null，注意这里size自减了，同时执行了更新size的动作
}

/**
* Checks if the given index is in range.  If not, throws an appropriate
* runtime exception.  This method does *not* check if the index is
* negative: It is always used immediately prior to an array access,
* which throws an ArrayIndexOutOfBoundsException if index is negative.
*/
private void rangeCheck(int index) {
if (index >= size)  //这里等于size也会抛异常（说明index是可以小于0的）
throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
}

/**
* A version of rangeCheck used by add and addAll.
*/
private void rangeCheckForAdd(int index) {
if (index > size || index < 0)  //这里可以等于size，但是不容许小于零
throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
}

/**
* Constructs an IndexOutOfBoundsException detail message.
* Of the many possible refactorings of the error handling code,
* this "outlining" performs best with both server and client VMs.
*/
private String outOfBoundsMsg(int index) {
return "Index: "+index+", Size: "+size;
}

/**
* Removes from this list all of its elements that are contained in the
* specified collection.
*
* @param c collection containing elements to be removed from this list
* @return {@code true} if this list changed as a result of the call
* @throws ClassCastException if the class of an element of this list
*         is incompatible with the specified collection
* (<a href="Collection.html#optional-restrictions">optional</a>)
* @throws NullPointerException if this list contains a null element and the
*         specified collection does not permit null elements
* (<a href="Collection.html#optional-restrictions">optional</a>),
*         or if the specified collection is null
* @see Collection#contains(Object)
*/
public boolean removeAll(Collection<?> c) {//移除所有属于参数Collection里面的元素
return batchRemove(c, false);  //参数为false
}

/**
* Retains only the elements in this list that are contained in the
* specified collection.  In other words, removes from this list all
* of its elements that are not contained in the specified collection.
*
* @param c collection containing elements to be retained in this list
* @return {@code true} if this list changed as a result of the call
* @throws ClassCastException if the class of an element of this list
*         is incompatible with the specified collection
* (<a href="Collection.html#optional-restrictions">optional</a>)
* @throws NullPointerException if this list contains a null element and the
*         specified collection does not permit null elements
* (<a href="Collection.html#optional-restrictions">optional</a>),
*         or if the specified collection is null
* @see Collection#contains(Object)
*/
public boolean retainAll(Collection<?> c) {  //只留下属于参数Collection里面的元素
return batchRemove(c, true);  //参数为true
}

private boolean batchRemove(Collection<?> c, boolean complement) {
final Object[] elementData = this.elementData;
int r = 0, w = 0;
boolean modified = false;
try {
for (; r < size; r++) //怪不得是遍历原来的ArrayList而不是参数Collection.  只用了一个elementData算不算是节约了空间？
if (c.contains(elementData[r]) == complement)  //1.complement为false，那么elementData[w]赋值的为属于原来ArrayList但是不属于c的元素
elementData[w++] = elementData[r];         //2.complement为true，那么 elementData[w]赋值的为属于原来ArrayList同时也属于c的元素
} finally {
// Preserve behavioral compatibility with AbstractCollection,
// even if c.contains() throws.
if (r != size) {//可是r怎么可能不等于size，除非中间抛异常了
System.arraycopy(elementData, r,
elementData, w,
size - r);
w += size - r;
}
if (w != size) {
for (int i = w; i < size; i++)
elementData[i] = null; //elementData 里面的废数据清空，为了早日gc
modCount += size - w;
size = w;
modified = true;//表示有过改变
}
}
return modified;
}

/**
* Save the state of the <tt>ArrayList</tt> instance to a stream (that
* is, serialize it).序列化？
*
* @serialData The length of the array backing the <tt>ArrayList</tt>
*             instance is emitted (int), followed by all of its elements
*             (each an <tt>Object</tt>) in the proper order.
*/
private void writeObject(java.io.ObjectOutputStream s)
throws java.io.IOException{
// Write out element count, and any hidden stuff
int expectedModCount = modCount;
s.defaultWriteObject();  //目前没看懂，需要这一层干嘛的初始化环境？

// Write out array length
s.writeInt(elementData.length);  //length，而不是size。

// Write out all elements in the proper order.
for (int i=0; i<size; i++) //这里使用的是size
s.writeObject(elementData[i]); //写数据

if (modCount != expectedModCount) { //表示在序列化的过程中有对ArrayList改变过
throw new ConcurrentModificationException();
}

}

/**
* Reconstitute the <tt>ArrayList</tt> instance from a stream (that is,
* deserialize it).
*/
private void readObject(java.io.ObjectInputStream s)
throws java.io.IOException, ClassNotFoundException { //反序列化
// Read in size, and any hidden stuff
s.defaultReadObject();  //read

// Read in array length and allocate array
int arrayLength = s.readInt();
Object[] a = elementData = new Object[arrayLength]; //elementData被初始化

// Read in all elements in the proper order.
for (int i=0; i<size; i++)
a[i] = s.readObject(); //readObject
}

/**
* Returns a list iterator over the elements in this list (in proper
* sequence), starting at the specified position in the list.
* The specified index indicates the first element that would be
* returned by an initial call to {@link ListIterator#next next}.
* An initial call to {@link ListIterator#previous previous} would
* return the element with the specified index minus one.
*
* <p>The returned list iterator is <a href="#fail-fast"><i>fail-fast</i></a>.
*
* @throws IndexOutOfBoundsException {@inheritDoc}
*/
public ListIterator<E> listIterator(int index) {
if (index < 0 || index > size)
throw new IndexOutOfBoundsException("Index: "+index);
return new ListItr(index);
}

/**
* Returns a list iterator over the elements in this list (in proper
* sequence).
*
* <p>The returned list iterator is <a href="#fail-fast"><i>fail-fast</i></a>.
*
* @see #listIterator(int)
*/
public ListIterator<E> listIterator() {
return new ListItr(0);
}

/**
* Returns an iterator over the elements in this list in proper sequence.
*
* <p>The returned iterator is <a href="#fail-fast"><i>fail-fast</i></a>.
*
* @return an iterator over the elements in this list in proper sequence
*/
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;
if (i >= size)
throw new NoSuchElementException();
Object[] elementData = ArrayList.this.elementData;
if (i >= elementData.length)
throw new ConcurrentModificationException();
cursor = i + 1;
return (E) elementData[lastRet = i];
}

public void remove() {
if (lastRet < 0)
throw new IllegalStateException();
checkForComodification();

try {
ArrayList.this.remove(lastRet);
cursor = lastRet;
lastRet = -1;
expectedModCount = modCount;
} catch (IndexOutOfBoundsException ex) {
throw new ConcurrentModificationException();
}
}

final void checkForComodification() {
if (modCount != expectedModCount)
throw new ConcurrentModificationException();
}
}

/**
* An optimized version of AbstractList.ListItr
*/
private class ListItr extends Itr implements ListIterator<E> {
ListItr(int index) {
super();
cursor = index;
}

public boolean hasPrevious() {
return cursor != 0;
}

public int nextIndex() {
return cursor;
}

public int previousIndex() {
return cursor - 1;
}

@SuppressWarnings("unchecked")
public E previous() {
checkForComodification();
int i = cursor - 1;
if (i < 0)
throw new NoSuchElementException();
Object[] elementData = ArrayList.this.elementData;
if (i >= elementData.length)
throw new ConcurrentModificationException();
cursor = i;
return (E) elementData[lastRet = i];
}

public void set(E e) {
if (lastRet < 0)
throw new IllegalStateException();
checkForComodification();

try {
ArrayList.this.set(lastRet, e);
} catch (IndexOutOfBoundsException ex) {
throw new ConcurrentModificationException();
}
}

public void add(E e) {
checkForComodification();

try {
int i = cursor;
ArrayList.this.add(i, e);
cursor = i + 1;
lastRet = -1;
expectedModCount = modCount;
} catch (IndexOutOfBoundsException ex) {
throw new ConcurrentModificationException();
}
}
}

/**
* Returns a view of the portion of this list between the specified
* {@code fromIndex}, inclusive, and {@code toIndex}, exclusive.  (If
* {@code fromIndex} and {@code toIndex} are equal, the returned list is
* empty.)  The returned list is backed by this list, so non-structural
* changes in the returned list are reflected in this list, and vice-versa.
* The returned list supports all of the optional list operations.
*
* <p>This method eliminates the need for explicit range operations (of
* the sort that commonly exist for arrays).  Any operation that expects
* a list can be used as a range operation by passing a subList view
* instead of a whole list.  For example, the following idiom
* removes a range of elements from a list:
* <pre>
*      list.subList(from, to).clear();
* </pre>
* Similar idioms may be constructed for {@link #indexOf(Object)} and
* {@link #lastIndexOf(Object)}, and all of the algorithms in the
* {@link Collections} class can be applied to a subList.
*
* <p>The semantics of the list returned by this method become undefined if
* the backing list (i.e., this list) is <i>structurally modified</i> in
* any way other than via the returned list.  (Structural modifications are
* those that change the size of this list, or otherwise perturb it in such
* a fashion that iterations in progress may yield incorrect results.)
*
* @throws IndexOutOfBoundsException {@inheritDoc}
* @throws IllegalArgumentException {@inheritDoc}
*/
public List<E> subList(int fromIndex, int toIndex) {
subListRangeCheck(fromIndex, toIndex, size);
return new SubList(this, 0, fromIndex, toIndex);
}

static void subListRangeCheck(int fromIndex, int toIndex, int size) {
if (fromIndex < 0)
throw new IndexOutOfBoundsException("fromIndex = " + fromIndex);
if (toIndex > size)
throw new IndexOutOfBoundsException("toIndex = " + toIndex);
if (fromIndex > toIndex)
throw new IllegalArgumentException("fromIndex(" + fromIndex +
") > toIndex(" + toIndex + ")");
}

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;
}

public E get(int index) {
rangeCheck(index);
checkForComodification();
return ArrayList.this.elementData(offset + index);
}

public int size() {
checkForComodification();
return this.size;
}

public void add(int index, E e) {
rangeCheckForAdd(index);
checkForComodification();
parent.add(parentOffset + index, e);
this.modCount = parent.modCount;
this.size++;
}

public E remove(int index) {
rangeCheck(index);
checkForComodification();
E result = parent.remove(parentOffset + index);
this.modCount = parent.modCount;
this.size--;
return result;
}

protected void removeRange(int fromIndex, int toIndex) {
checkForComodification();
parent.removeRange(parentOffset + fromIndex,
parentOffset + toIndex);
this.modCount = parent.modCount;
this.size -= toIndex - fromIndex;
}

public boolean addAll(Collection<? extends E> c) {
return addAll(this.size, c);
}

public boolean addAll(int index, Collection<? extends E> c) {
rangeCheckForAdd(index);
int cSize = c.size();
if (cSize==0)
return false;

checkForComodification();
parent.addAll(parentOffset + index, c);
this.modCount = parent.modCount;
this.size += cSize;
return true;
}

public Iterator<E> iterator() {
return listIterator();
}

public ListIterator<E> listIterator(final int index) {
checkForComodification();
rangeCheckForAdd(index);
final int offset = this.offset;

return new ListIterator<E>() {
int cursor = index;
int lastRet = -1;
int expectedModCount = ArrayList.this.modCount;

public boolean hasNext() {
return cursor != SubList.this.size;
}

@SuppressWarnings("unchecked")
public E next() {
checkForComodification();
int i = cursor;
if (i >= SubList.this.size)
throw new NoSuchElementException();
Object[] elementData = ArrayList.this.elementData;
if (offset + i >= elementData.length)
throw new ConcurrentModificationException();
cursor = i + 1;
return (E) elementData[offset + (lastRet = i)];
}

public boolean hasPrevious() {
return cursor != 0;
}

@SuppressWarnings("unchecked")
public E previous() {
checkForComodification();
int i = cursor - 1;
if (i < 0)
throw new NoSuchElementException();
Object[] elementData = ArrayList.this.elementData;
if (offset + i >= elementData.length)
throw new ConcurrentModificationException();
cursor = i;
return (E) elementData[offset + (lastRet = i)];
}

public int nextIndex() {
return cursor;
}

public int previousIndex() {
return cursor - 1;
}

public void remove() {
if (lastRet < 0)
throw new IllegalStateException();
checkForComodification();

try {
SubList.this.remove(lastRet);
cursor = lastRet;
lastRet = -1;
expectedModCount = ArrayList.this.modCount;
} catch (IndexOutOfBoundsException ex) {
throw new ConcurrentModificationException();
}
}

public void set(E e) {
if (lastRet < 0)
throw new IllegalStateException();
checkForComodification();

try {
ArrayList.this.set(offset + lastRet, e);
} catch (IndexOutOfBoundsException ex) {
throw new ConcurrentModificationException();
}
}

public void add(E e) {
checkForComodification();

try {
int i = cursor;
SubList.this.add(i, e);
cursor = i + 1;
lastRet = -1;
expectedModCount = ArrayList.this.modCount;
} catch (IndexOutOfBoundsException ex) {
throw new ConcurrentModificationException();
}
}

final void checkForComodification() {
if (expectedModCount != ArrayList.this.modCount)
throw new ConcurrentModificationException();
}
};
}

public List<E> subList(int fromIndex, int toIndex) {
subListRangeCheck(fromIndex, toIndex, size);
return new SubList(this, offset, fromIndex, toIndex);
}

private void rangeCheck(int index) {
if (index < 0 || index >= this.size)
throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
}

private void rangeCheckForAdd(int index) {
if (index < 0 || index > this.size)
throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
}

private String outOfBoundsMsg(int index) {
return "Index: "+index+", Size: "+this.size;
}

private void checkForComodification() {
if (ArrayList.this.modCount != this.modCount)
throw new ConcurrentModificationException();
}
}
}