1、前言
ArrayList 是一个数组队列，相当于动态数组。与Java中的数组相比，它的容量能动态增长。它继承于AbstractList，实现了List, RandomAccess, Cloneable, java.io.Serializable这些接口。
ArrayList 继承了AbstractList，实现了List。它是一个数组队列，提供了相关的添加、删除、修改、遍历等功能。
ArrayList实现了RandmoAccess接口，即提供了随机访问功能。RandmoAccess是java中用来被List实现，为List提供快速访问功能的。在ArrayList中，我们即可以通过元素的序号快速获取元素对象；这就是快速随机访问。
ArrayList 实现了Cloneable接口，即覆盖了函数clone()，能被克隆。
- ArrayList 实现java.io.Serializable接口，这意味着ArrayList支持序列化，能通过序列化去传输。
简单地来说，ArrayList是对数组进行快速操作的一系列API。
ArrayList中的操作不是线程安全的。所以，建议在单线程中才使用ArrayList，而在多线程中可以选择Vector或者CopyOnWriteArrayList。
2、简单使用
ArrayList<Integer> list = new ArrayList<Integer>();
for(int i=0;i< 10; i++ ){
//给数组增加10个Int元素
list.add(i);
}
System.out.println("数组是否包含3："+list.contains(3));
System.out.println("数组元素的数量："+list.size());
System.out.println("数组的第三个元素："+list.get(3));
//移除第三个元素
list.remove(3);
System.out.println("数组是否包含23："+list.contains(3));
System.out.println("数组元素的数量："+list.size());
System.out.println("数组的第三个元素："+list.get(3));
list.clear();
System.out.println("数组元素的数量："+list.size());结果：
数组是否包含3：true
数组元素的数量：10
数组的第三个元素：3
数组是否包含23：false
数组元素的数量：9
数组的第三个元素：4
数组元素的数量：03、源码简单分析
3.1 构造方法
ArrayList的构造方法有3个。
前面说到ArrayList是对数组进行快速操作的一系列API。而ArrayList的构造方法就是对这个默认数组elementData的初始化。
- 默认的构造方法，并为内置的elementData初始化一个DEFAULTCAPACITY_EMPTY_ELEMENTDATA的空数组。
/**
* 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 = {};
/**
* Constructs an empty list with an initial capacity of ten.
*/
public ArrayList() {
this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA;
}- 第二个构造方法是通过定义长度，为内置的elementData初始化，长度为initialCapacity，长度不能为负数。
/**
* 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);
}
}- 根据另外一个数据集合，为内置的elementData初始化，把集合中的数据拷贝到elementData中。
/**
* 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;
}
}3.2 增加元素
为数据增加元素的方法有四个：
- 将指定的元素添加到此列表的尾部。
/**
* 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!!
elementData[size++] = e;
return true;
}代码比较简单，就是先调用ensureCapacityInternal，通过Arrays类的copyOf方法把旧数组拷贝到一个长度比旧数组大1的新数组中去（其实我们看来就是旧数组的长度加1，但我们都知道数组不是变长的，所有只能使用拷贝创建新数组的方式来实现变长的功能），然后再把新的元素添加到数组最后一个位置。
当然，我们看到grow方法中，可能不是把数组的容量增大。旧数组的长度和新数组的长度是一样的，数组本身尾部的一些位置都是空闲的，因为ArrayList作了移除元素的操作。
private void ensureCapacityInternal(int minCapacity) {
if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) {
minCapacity = Math.max(DEFAULT_CAPACITY, minCapacity);
}
ensureExplicitCapacity(minCapacity);
}
private void ensureExplicitCapacity(int minCapacity) {
modCount++;
// overflow-conscious code
if (minCapacity - elementData.length > 0)
grow(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;
/**
* 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);
}- 将指定的元素插入此列表中的指定位置。
/**
* 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);
elementData[index] = element;
size++;
}首先判断index位置的合法性：
private void rangeCheckForAdd(int index) {
if (index < 0 || index > this.size)
throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
}然后和前一个方法一样，在ensureCapacityInternal方法里面，使用数组的长度加1。
这里再通过System.arraycopy对数组再一次进行拷贝。与Arrays.copyOf不同的是，System.arraycopy只是把index位置及其后面的元素，拷贝到数组的index+1及其后面的位置中，也就是把index及其后面的元素全部后移一位。
最后，把新的元素放到数组的index位置。
public static void arraycopy(Object src, int srcPos, Object dest, int destPos, int length)
参数
src -- 这是源数组.
srcPos -- 这是源数组中的起始位置。
dest -- 这是目标数组。
destPos -- 这是目标数据中的起始位置。
length -- 这是一个要复制的数组元素的数目。- 按照指定集合的迭代器所返回的元素顺序，将该集合中的所有元素添加到此列表的尾部。
/**
* 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();
int numNew = a.length;
ensureCapacityInternal(size + numNew); // Increments modCount
System.arraycopy(a, 0, elementData, size, numNew);
size += numNew;
return numNew != 0;
}理解了前面两个方法后，这个方法就比较容易了。先获取增加进来的集合的数量numNew，使用数组的长度加numNew ，再通过System.arraycopy把集合中所以的元素增加到数组后面的位置中。
- 从指定的位置开始，将指定集合中的所有元素插入到此列表中。
/**
* 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) {
rangeCheckForAdd(index);
Object[] a = c.toArray();
int numNew = a.length;
ensureCapacityInternal(size + numNew); // Increments modCount
int numMoved = size - index;
if (numMoved > 0)
System.arraycopy(elementData, index, elementData, index + numNew,
numMoved);
System.arraycopy(a, 0, elementData, index, numNew);
size += numNew;
return numNew != 0;
}和前面那些方法基本原理是一样的，只是过程中需要判断插入的位置，如果插入的位置刚好在原数组的尾部，那我们直接添加到后面就好。如果不在尾部，那就把原数组index位置及后面的元素，通过System.arraycopy，往后移numNew位，再把集合中的元素添加到index及后面的位置即可。
注意：所有增加元素的方法中，都会使ArrayList的size属性发生变化。
3.3 替换元素
用指定的元素替代此列表中指定位置上的元素。
/**
* 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;
return oldValue;
}很简单，先通过rangeCheck判断index这个位置是否是合法的。然后把新的元素放到index位置，返回旧元素。
private void rangeCheck(int index) {
if (index < 0 || index >= this.size)
throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
}3.4 移除元素
移除元素的方法有6个：
- 移除此列表中指定位置上的元素。
/**
* 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);
elementData[--size] = null; // clear to let GC do its work
return oldValue;
}在判断index的合法性后，判断index的位置，如果index是最后一个元素，直接把此位置的元素位置设置为空。如果不是，则把这个元素之后的元素都往前移一位，然后把最后一个位置设置为空。
- 移除此列表中首次出现的指定元素（如果存在）。
/**
* 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) {
if (o == null) {
for (int index = 0; index < size; index++)
if (elementData[index] == null) {
fastRemove(index);
return true;
}
} else {
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) {
modCount++;
int numMoved = size - index - 1;
if (numMoved > 0)
System.arraycopy(elementData, index+1, elementData, index,
numMoved);
elementData[--size] = null; // clear to let GC do its work
}在这个移除方法中，最要是为了找出要移除元素出现的第一个位置（假如存在）。然后再使用fastRemove将这个位置的元素移除掉。
这里的fastRemove(int index)方法与前面的remove(int index)基本相同，只是少了检查index的合法性及返回值而已。
- 移除列表中索引在 fromIndex（包括）和 toIndex（不包括）之间的所有元素。
/**
* 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) {
modCount++;
int numMoved = size - toIndex;
System.arraycopy(elementData, toIndex, elementData, fromIndex,
numMoved);
// clear to let GC do its work
int newSize = size - (toIndex-fromIndex);
for (int i = newSize; i < size; i++) {
elementData[i] = null;
}
size = newSize;
}这里很简单，把toIndex位置及其后的元素，移到fromIndex及其后面的位置上，再后面的位置全部清空。
- 移除此列表中包含着指定集合里的元素（如果存在）
/**
* 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) {
Objects.requireNonNull(c);
return batchRemove(c, false);
}- 移除此列表中除指定集合里的元素之外的元素
/**
* 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) {
Objects.requireNonNull(c);
return batchRemove(c, true);
}
上面两个方法都很简单，都是先判断集合参数是否为空，然后调用batchRemove来作数据的移除。
batchRemove方法的complement参数用于判断是保留集合元素或删除集合元素。
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++)
if (c.contains(elementData[r]) == complement)
elementData[w++] = elementData[r];
} finally {
// Preserve behavioral compatibility with AbstractCollection,
// even if c.contains() throws.
if (r != size) {
System.arraycopy(elementData, r,
elementData, w,
size - r);
w += size - r;
}
if (w != size) {
// clear to let GC do its work
for (int i = w; i < size; i++)
elementData[i] = null;
modCount += size - w;
size = w;
modified = true;
}
}
return modified;
}- 移除此列表中的所有元素。
/**
* Removes all of the elements from this list. The list will
* be empty after this call returns.
*/
public void clear() {
modCount++;
// clear to let GC do its work
for (int i = 0; i < size; i++)
elementData[i] = null;
size = 0;
}这方法就更简单了，就是把数组所有位置都设置为空。
注意：所有移除元素的方法中，都会使ArrayList的size属性发生变化。
3.5 获取元素
获取ArrayList元素的方法也有两种：
- 返回此列表中指定位置上的元素。
// Positional Access Operations
@SuppressWarnings("unchecked")
E elementData(int index) {
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);
}检查完index的合法性后，直接根据index坐标返回内置数组位置的元素。
- 通过Iterator接口实现来获取元素。
这种方式常用于列表元素的遍历。
/**
* 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();
}iterator方法会创建一个Iterator接口实现对象。
ArrrayList内置的Iterator接口实现(如下)，是通过游标cursor的方式来确定列表的访问的当前位置，然后通过cursor作为数组下标来获取或者移除列表元素。
/**
* 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();
}
}
@Override
@SuppressWarnings("unchecked")
public void forEachRemaining(Consumer<? super E> consumer) {
Objects.requireNonNull(consumer);
final int size = ArrayList.this.size;
int i = cursor;
if (i >= size) {
return;
}
final Object[] elementData = ArrayList.this.elementData;
if (i >= elementData.length) {
throw new ConcurrentModificationException();
}
while (i != size && modCount == expectedModCount) {
consumer.accept((E) elementData[i++]);
}
// update once at end of iteration to reduce heap write traffic
cursor = i;
lastRet = i - 1;
checkForComodification();
}
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();
}
}
}3.6 其它方法
- 返回此列表中首次出现的指定元素的索引
/**
* 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) {
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]))
return i;
}
return -1;
}如果没有提定元素则返回-1。
- 如果此列表中包含指定的元素
/**
* 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) {
return indexOf(o) >= 0;
}如果找得到此列表中首次出现的指定元素的索引，说明是包含指定元素。
- 返回此列表中的元素数。
/**
* Returns the number of elements in this list.
*
* @return the number of elements in this list
*/
public int size() {
return size;
}我们在前面增加和移除列表元素的时候都会使用size发生变化。ArrayList是通过size来记录元素的个数的。
为什么我们不是返回内置数组elementData的长度呢？很明显，ArrayList做移除操作的时候，只是把最后位置设置为空，则不是把长度减短。
- 列表中是否有元素
/**
* Returns <tt>true</tt> if this list contains no elements.
*
* @return <tt>true</tt> if this list contains no elements
*/
public boolean isEmpty() {
return size == 0;
}3.7 关于RandomAccess接口
List 实现所使用的标记接口，用来表明其支持快速（通常是固定时间）随机访问。此接口的主要目的是允许一般的算法更改其行为，从而在将其应用到随机或连续访问列表时能提供良好的性能。
将操作随机访问列表的最佳算法（如 ArrayList）应用到连续访问列表（如 LinkedList）时，可产生二次项的行为。如果将某个算法应用到连续访问列表，那么在应用可能提供较差性能的算法前，鼓励使用一般的列表算法检查给定列表是否为此接口的一个 instanceof，如果需要保证可接受的性能，还可以更改其行为。
现在已经认识到，随机和连续访问之间的区别通常是模糊的。例如，如果列表很大时，某些 List 实现提供渐进的线性访问时间，但实际上是固定的访问时间。这样的 List 实现通常应该实现此接口。实际经验证明，如果是下列情况，则 List 实现应该实现此接口，即对于典型的类实例而言，此循环：
for (int i=0, n=list.size(); i < n; i++)
list.get(i);的运行速度要快于以下循环：
for (Iterator i=list.iterator(); i.hasNext(); )
i.next();如通过Collection.shuffle()随机打乱一个顺序数组,JDK判断如果集合属于RandomAccess，则通过简单的for循环遍历数组，而不属于RandomAccess的话，则使用ListIterator来遍历。
/**
* Randomly permute the specified list using the specified source of
* randomness. All permutations occur with equal likelihood
* assuming that the source of randomness is fair.<p>
*
* This implementation traverses the list backwards, from the last element
* up to the second, repeatedly swapping a randomly selected element into
* the "current position". Elements are randomly selected from the
* portion of the list that runs from the first element to the current
* position, inclusive.<p>
*
* This method runs in linear time. If the specified list does not
* implement the {@link RandomAccess} interface and is large, this
* implementation dumps the specified list into an array before shuffling
* it, and dumps the shuffled array back into the list. This avoids the
* quadratic behavior that would result from shuffling a "sequential
* access" list in place.
*
* @param list the list to be shuffled.
* @param rnd the source of randomness to use to shuffle the list.
* @throws UnsupportedOperationException if the specified list or its
* list-iterator does not support the <tt>set</tt> operation.
*/
@SuppressWarnings({"rawtypes", "unchecked"})
public static void shuffle(List<?> list, Random rnd) {
int size = list.size();
if (size < SHUFFLE_THRESHOLD || list instanceof RandomAccess) {
for (int i=size; i>1; i--)
swap(list, i-1, rnd.nextInt(i));
} else {
Object arr[] = list.toArray();
// Shuffle array
for (int i=size; i>1; i--)
swap(arr, i-1, rnd.nextInt(i));
// Dump array back into list
// instead of using a raw type here, it's possible to capture
// the wildcard but it will require a call to a supplementary
// private method
ListIterator it = list.listIterator();
for (int i=0; i<arr.length; i++) {
it.next();
it.set(arr[i]);
}
}
}关于这个访问速度的问题，可以参考：http://blog.csdn.net/keda8997110/article/details/8635005
4、说明
- Vector的整个实现与ArrayList基本相似，只是在对一些操作性的方法上加上了synchronized关键字，使之在线程上是安全的。
- 本文中源码大部分来源于JDK1.8。