java集合框架02——ArrayList和源码分析

2019年3月10日 265次阅读来源: java集合源码分析

上一章学习了Collection的架构，并阅读了部分源码，这一章开始，我们将对Collection的具体实现进行详细学习。首先学习List。而ArrayList又是List中最为常用的，因此本章先学习ArrayList。先对ArrayList有个整体的认识，然后学习它的源码，深入剖析ArrayList。

1. ArrayList简介

首先看看ArrayList与Collection的关系：

《java集合框架02——ArrayList和源码分析》

ArrayList的继承关系如下：

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java.lang.Object
↳ java.util.AbstractCollection<E>
↳ java.util.AbstractList<E>
↳ java.util.ArrayList<E>
public class ArrayList<E> extends AbstractList<E>
implements List<E>, RandomAccess, Cloneable, java.io.Serializable {}

java.lang.Object
   ↳     java.util.AbstractCollection<E>
         ↳     java.util.AbstractList<E>
               ↳     java.util.ArrayList<E>

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

ArrayList继承了AbstractList，实现了List。它是一个数组队列，相当于动态数组。提供了相关的添加、删除、修改和遍历等功能。

ArrayList实现了RandomAccess接口，即提供了随机访问功能。RandomAccess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来快速获取元素对象，这就是快速随机访问。下文会比较List的“快速随机访问”和使用“Iterator迭代器访问”的效率。

ArrayList实现了Cloneable接口，即覆盖了函数clone()，能被克隆。

ArrayList实现了java.io.Serializable接口，这意味着ArrayList支持序列化，能通过序列化去传输。

和Vector不同，ArrayList中的操作是非线程安全的。所以建议在单线程中使用ArrayList，在多线程中选择Vector或者CopyOnWriteArrayList。

我们先总览下ArrayList的构造函数和API

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/****************** ArrayList中的构造函数 ***************/
// 默认构造函数
ArrayList()
// capacity是ArrayList的默认容量大小。当由于增加数据导致容量不足时，容量会添加上一次容量大小的一半。
ArrayList(int capacity)
// 创建一个包含collection的ArrayList
ArrayList(Collection<? extends E> collection)
/****************** ArrayList中的API ********************/
// Collection中定义的API
boolean add(E object)
boolean addAll(Collection<? extends E> collection)
void clear()
boolean contains(Object object)
boolean containsAll(Collection<?> collection)
boolean equals(Object object)
int hashCode()
boolean isEmpty()
Iterator<E> iterator()
boolean remove(Object object)
boolean removeAll(Collection<?> collection)
boolean retainAll(Collection<?> collection)
int size()
<T> T[] toArray(T[] array)
Object[] toArray()
// AbstractCollection中定义的API
void add(int location, E object)
boolean addAll(int location, Collection<? extends E> collection)
E get(int location)
int indexOf(Object object)
int lastIndexOf(Object object)
ListIterator<E> listIterator(int location)
ListIterator<E> listIterator()
E remove(int location)
E set(int location, E object)
List<E> subList(int start, int end)
// ArrayList新增的API
Object clone()
void ensureCapacity(int minimumCapacity)
void trimToSize()
void removeRange(int fromIndex, int toIndex)

/****************** ArrayList中的构造函数 ***************/
// 默认构造函数
ArrayList()

// capacity是ArrayList的默认容量大小。当由于增加数据导致容量不足时，容量会添加上一次容量大小的一半。
ArrayList(int capacity)

// 创建一个包含collection的ArrayList
ArrayList(Collection<? extends E> collection)

/****************** ArrayList中的API ********************/
// Collection中定义的API
boolean             add(E object)
boolean             addAll(Collection<? extends E> collection)
void                clear()
boolean             contains(Object object)
boolean             containsAll(Collection<?> collection)
boolean             equals(Object object)
int                 hashCode()
boolean             isEmpty()
Iterator<E>         iterator()
boolean             remove(Object object)
boolean             removeAll(Collection<?> collection)
boolean             retainAll(Collection<?> collection)
int                 size()
<T> T[]             toArray(T[] array)
Object[]            toArray()
// AbstractCollection中定义的API
void                add(int location, E object)
boolean             addAll(int location, Collection<? extends E> collection)
E                   get(int location)
int                 indexOf(Object object)
int                 lastIndexOf(Object object)
ListIterator<E>     listIterator(int location)
ListIterator<E>     listIterator()
E                   remove(int location)
E                   set(int location, E object)
List<E>             subList(int start, int end)
// ArrayList新增的API
Object               clone()
void                 ensureCapacity(int minimumCapacity)
void                 trimToSize()
void                 removeRange(int fromIndex, int toIndex)

ArrayList包含了两个重要的对象：elementData和size。

elementData是Object[]类型的数组，它保存了添加到ArrayList中的元素。实际上，elementData是一个动态数组，我们能通过ArrayList(int initialCapacity)来执行它的初始容量为initialCapacity。如果通过不含参数的构造函数来创建ArrayList，则elementData是一个空数组（后面会调整其大小）。elementData数组的大小会根据ArrayList容量的增长而动态的增长，具体见下面的源码。

size则是动态数组实际的大小。

2. ArrayList源码分析(基于JDK1.7)

下面通过分析ArrayList的源码更加深入的了解ArrayList原理。由于ArrayList是通过数组实现的，所以源码比较容易理解：

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package java.util;
public class ArrayList<E> extends AbstractList<E>
implements List<E>, RandomAccess, Cloneable, java.io.Serializable
{
//序列版本号
private static final long serialVersionUID = 8683452581122892189L;
//默认初始化容量
private static final int DEFAULT_CAPACITY = 10;
//空数组，用来实例化不带容量大小的构造函数
private static final Object[] EMPTY_ELEMENTDATA = {};
//保存ArrayList中数据的数组
private transient Object[] elementData;
//ArrayList中实际数据的数量
private int size;
/******************************** Constructor ***********************************/
//ArrayList带容量大小的构造函数
public ArrayList(int initialCapacity) {
super();
if (initialCapacity < 0)
throw new IllegalArgumentException(“Illegal Capacity: ”+
initialCapacity);
this.elementData = new Object[initialCapacity]; //新建一个数组初始化elementData
}
//不带参数的构造函数
public ArrayList() {
super();
this.elementData = EMPTY_ELEMENTDATA;//使用空数组初始化elementData
}
//用Collection来初始化ArrayList
public ArrayList(Collection<? extends E> c) {
elementData = c.toArray(); //将Collection中的内容转换成数组初始化elementData
size = elementData.length;
// c.toArray might (incorrectly) not return Object[] (see 6260652)
if (elementData.getClass() != Object[].class)
elementData = Arrays.copyOf(elementData, size, Object[].class);
}
/********************************* Array size ************************************/
//重新“修剪”数组容量大小
public void trimToSize() {
modCount++;
//当ArrayList中的元素个数小于elementData数组大小时，重新修整elementData到size大小
if (size < elementData.length) {
elementData = Arrays.copyOf(elementData, size);
}
}
//给数组扩容，该方法是提供给外界调用的，是public的，真正扩容是在下面的private方法里
public void ensureCapacity(int minCapacity) {
int minExpand = (elementData != EMPTY_ELEMENTDATA)
// any size if real element table
? 0
// larger than default for empty table. It’s already supposed to be
// at default size.
: DEFAULT_CAPACITY;
if (minCapacity > minExpand) {
ensureExplicitCapacity(minCapacity);
}
}
private void ensureCapacityInternal(int minCapacity) {
//如果是个空数组
if (elementData == EMPTY_ELEMENTDATA) {
//取minCapacity和10的较大者
minCapacity = Math.max(DEFAULT_CAPACITY, minCapacity);
}
//如果数组已经有数据了
ensureExplicitCapacity(minCapacity);
}
//确保数组容量大于ArrayList中元素个数
private void ensureExplicitCapacity(int minCapacity) {
modCount++; //将“修改统计数”+1
//如果实际数据容量大于数组容量，就给数组扩容
if (minCapacity – elementData.length > 0)
grow(minCapacity);
}
//分配的最大数组空间
private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE – 8;
//增大数组空间
private void grow(int minCapacity) {
// overflow-conscious code
int oldCapacity = elementData.length;
int newCapacity = oldCapacity + (oldCapacity >> 1); //在原来容量的基础上加上 oldCapacity/2
if (newCapacity – minCapacity < 0)
newCapacity = minCapacity; //最少保证容量和minCapacity一样
if (newCapacity – MAX_ARRAY_SIZE > 0)
newCapacity = hugeCapacity(minCapacity); //最多不能超过最大容量
// 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;
}
//返回ArrayList的实际大小
public int size() {
return size;
}
//判断ArrayList是否为空
public boolean isEmpty() {
return size == 0;
}
/****************************** Search Operations *************************/
//判断ArrayList是否包含Object o
public boolean contains(Object o) {
return indexOf(o) >= 0;
}
//正向查找，返回元素的索引值
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;
}
//反向查找，返回元素的索引值
public int lastIndexOf(Object o) {
if (o == null) {
for (int i = size-1; i >= 0; i–)
if (elementData[i]==null)
return i;
} else {
for (int i = size-1; i >= 0; i–)
if (o.equals(elementData[i]))
return i;
}
return –1;
}
/******************************* Clone *********************************/
//克隆函数
public Object clone() {
try {
@SuppressWarnings(“unchecked”)
ArrayList<E> v = (ArrayList<E>) super.clone();
//将当前ArrayList的全部元素拷贝到v中
v.elementData = Arrays.copyOf(elementData, size);
v.modCount = 0;
return v;
} catch (CloneNotSupportedException e) {
// this shouldn’t happen, since we are Cloneable
throw new InternalError();
}
}
/********************************* toArray *****************************/
/**
* 返回一个Object数组，包含ArrayList中所有的元素
* toArray()方法扮演着array-based和collection-based API之间的桥梁
*/
public Object[] toArray() {
return Arrays.copyOf(elementData, size);
}
//返回ArrayList的模板数组
@SuppressWarnings(“unchecked”)
public <T> T[] toArray(T[] a) {
//如果数组a的大小 < ArrayList的元素个数，
//则新建一个T[]数组，大小为ArrayList元素个数，并将“ArrayList”全部拷贝到新数组中。
if (a.length < size)
return (T[]) Arrays.copyOf(elementData, size, a.getClass());
//如果数组a的大小 >= ArrayList的元素个数，
//则将ArrayList全部拷贝到新数组a中。
System.arraycopy(elementData, 0, a, 0, size);
if (a.length > size)
a[size] = null;
return a;
}
/******************** Positional Access Operations ********************/
@SuppressWarnings(“unchecked”)
E elementData(int index) {
return (E) elementData[index];
}
//获取index位置的元素值
public E get(int index) {
rangeCheck(index); //首先判断index的范围是否合法
return elementData(index);
}
//将index位置的值设为element，并返回原来的值
public E set(int index, E element) {
rangeCheck(index);
E oldValue = elementData(index);
elementData[index] = element;
return oldValue;
}
//将e添加到ArrayList中
public boolean add(E e) {
ensureCapacityInternal(size + 1); // Increments modCount!!
elementData[size++] = e;
return true;
}
//将element添加到ArrayList的指定位置
public void add(int index, E element) {
rangeCheckForAdd(index);
ensureCapacityInternal(size + 1); // Increments modCount!!
//将index以及index之后的数据复制到index+1的位置往后，即从index开始向后挪了一位
System.arraycopy(elementData, index, elementData, index + 1,
size – index);
elementData[index] = element; //然后在index处插入element
size++;
}
//删除ArrayList指定位置的元素
public E remove(int index) {
rangeCheck(index);
modCount++;
E oldValue = elementData(index);
int numMoved = size – index – 1;
if (numMoved > 0)
//向左挪一位，index位置原来的数据已经被覆盖了
System.arraycopy(elementData, index+1, elementData, index,
numMoved);
//多出来的最后一位删掉
elementData[–size] = null; // clear to let GC do its work
return oldValue;
}
//删除ArrayList中指定的元素
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的快速删除与上面的public普通删除区别在于，没有进行边界判断以及不返回删除值
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
}
//清空ArrayList，将全部元素置为null
public void clear() {
modCount++;
// clear to let GC do its work
for (int i = 0; i < size; i++)
elementData[i] = null;
size = 0;
}
//将集合C中所有的元素添加到ArrayList中
public boolean addAll(Collection<? extends E> c) {
Object[] a = c.toArray();
int numNew = a.length;
ensureCapacityInternal(size + numNew); // Increments modCount
//在原来数组的后面添加c中所有的元素
System.arraycopy(a, 0, elementData, size, numNew);
size += numNew;
return numNew != 0;
}
//从index位置开始，将集合C中所欲的元素添加到ArrayList中
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)
//将index开始向后的所有数据，向后移动numNew个位置，给新插入的数据腾出空间
System.arraycopy(elementData, index, elementData, index + numNew,
numMoved);
//将集合C中的数据插到刚刚腾出的位置
System.arraycopy(a, 0, elementData, index, numNew);
size += numNew;
return numNew != 0;
}
//删除从fromIndex到toIndex之间的数据，不包括toIndex位置的数据
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;
}
//范围检测
private void rangeCheck(int index) {
if (index >= size)
throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
}
//add和addAll方法中的范围检测
private void rangeCheckForAdd(int index) {
if (index > size || index < 0)
throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
}
private String outOfBoundsMsg(int index) {
return “Index: ”+index+“, Size: ”+size;
}
//删除ArrayList中所有集合C中包含的数据
public boolean removeAll(Collection<?> c) {
return batchRemove(c, false);
}
//删除ArrayList中除了集合C中包含的数据外的其他所有数据
public boolean retainAll(Collection<?> c) {
return batchRemove(c, true);
}
//批量删除
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.
//官方的注释是为了保持和AbstractCollection的兼容性
//我的理解是上面c.contains抛出了异常，导致for循环终止，那么必然会导致r != size
//所以0-w之间是需要保留的数据，同时从w索引开始将剩下没有循环的数据(也就是从r开始的)拷贝回来，也保留
if (r != size) {
System.arraycopy(elementData, r,
elementData, w,
size – r);
w += size – r;
}
//for循环完毕，检测了所有的元素
//0-w之间保存了需要留下的数据，w开始以及后面的数据全部清空
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;
}
/***************************** Writer and Read Object *************************/
//java.io.Serializable的写入函数
//将ArrayList的“容量、所有的元素值”都写入到输出流中
private void writeObject(java.io.ObjectOutputStream s)
throws java.io.IOException{
// Write out element count, and any hidden stuff
int expectedModCount = modCount;
s.defaultWriteObject();
// Write out size as capacity for behavioural compatibility with clone()
//写入“数组的容量”，保持与clone()的兼容性
s.writeInt(size);
//写入“数组的每一个元素”
for (int i=0; i<size; i++) {
s.writeObject(elementData[i]);
}
if (modCount != expectedModCount) {
throw new ConcurrentModificationException();
}
}
//java.io.Serializable的读取函数：根据写入方式读出
private void readObject(java.io.ObjectInputStream s)
throws java.io.IOException, ClassNotFoundException {
elementData = EMPTY_ELEMENTDATA;
// Read in size, and any hidden stuff
s.defaultReadObject();
//从输入流中读取ArrayList的“容量”
s.readInt(); // ignored
if (size > 0) {
// be like clone(), allocate array based upon size not capacity
ensureCapacityInternal(size);
Object[] a = elementData;
//从输入流中将“所有元素值”读出
for (int i=0; i<size; i++) {
a[i] = s.readObject();
}
}
}
/******************************** Iterators ************************************/
/**
* 该部分的方法重写了AbstractList抽象类中Iterator部分的方法，因为ArrayList继承
* 了AbstractList，基本大同小异，只是这里针对本类的数组，思想与AbstractList一致
* 可以参照上一章Collection架构与源码分析的AbatractList部分
*/
public ListIterator<E> listIterator(int index) {
if (index < 0 || index > size)
throw new IndexOutOfBoundsException(“Index: ”+index);
return new ListItr(index);
}
public ListIterator<E> listIterator() {
return new ListItr(0);
}
public Iterator<E> iterator() {
return new 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();
}
}
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();
}
}
}
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();
}
}
}

package java.util;

public class ArrayList<E> extends AbstractList<E>
        implements List<E>, RandomAccess, Cloneable, java.io.Serializable
{
    //序列版本号
    private static final long serialVersionUID = 8683452581122892189L;

    //默认初始化容量
    private static final int DEFAULT_CAPACITY = 10;

    //空数组，用来实例化不带容量大小的构造函数
    private static final Object[] EMPTY_ELEMENTDATA = {};

    //保存ArrayList中数据的数组
    private transient Object[] elementData;

    //ArrayList中实际数据的数量
    private int size;

/******************************** Constructor ***********************************/

    //ArrayList带容量大小的构造函数
    public ArrayList(int initialCapacity) {
        super();
        if (initialCapacity < 0)
            throw new IllegalArgumentException("Illegal Capacity: "+
                                               initialCapacity);
        this.elementData = new Object[initialCapacity]; //新建一个数组初始化elementData
    }

    //不带参数的构造函数
    public ArrayList() {
        super();
        this.elementData = EMPTY_ELEMENTDATA;//使用空数组初始化elementData
    }

    //用Collection来初始化ArrayList
    public ArrayList(Collection<? extends E> c) {
        elementData = c.toArray(); //将Collection中的内容转换成数组初始化elementData
        size = elementData.length;
        // c.toArray might (incorrectly) not return Object[] (see 6260652)
        if (elementData.getClass() != Object[].class)
            elementData = Arrays.copyOf(elementData, size, Object[].class);
    }

/********************************* Array size ************************************/

    //重新“修剪”数组容量大小
    public void trimToSize() {
        modCount++;
        //当ArrayList中的元素个数小于elementData数组大小时，重新修整elementData到size大小
        if (size < elementData.length) {
            elementData = Arrays.copyOf(elementData, size);
        }
    }

    //给数组扩容，该方法是提供给外界调用的，是public的，真正扩容是在下面的private方法里
    public void ensureCapacity(int minCapacity) {
        int minExpand = (elementData != EMPTY_ELEMENTDATA)
            // any size if real element table
            ? 0
            // larger than default for empty table. It's already supposed to be
            // at default size.
            : DEFAULT_CAPACITY;

        if (minCapacity > minExpand) {
            ensureExplicitCapacity(minCapacity);
        }
    }

    private void ensureCapacityInternal(int minCapacity) {
    //如果是个空数组
        if (elementData == EMPTY_ELEMENTDATA) {
        //取minCapacity和10的较大者
            minCapacity = Math.max(DEFAULT_CAPACITY, minCapacity);
        }
    //如果数组已经有数据了
        ensureExplicitCapacity(minCapacity);
    }

    //确保数组容量大于ArrayList中元素个数
    private void ensureExplicitCapacity(int minCapacity) {
        modCount++; //将“修改统计数”+1

        //如果实际数据容量大于数组容量，就给数组扩容
        if (minCapacity - elementData.length > 0)
            grow(minCapacity);
    }

    //分配的最大数组空间
    private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;

    //增大数组空间
    private void grow(int minCapacity) {
        // overflow-conscious code
        int oldCapacity = elementData.length;
        int newCapacity = oldCapacity + (oldCapacity >> 1); //在原来容量的基础上加上 oldCapacity/2
        if (newCapacity - minCapacity < 0)
            newCapacity = minCapacity; //最少保证容量和minCapacity一样
        if (newCapacity - MAX_ARRAY_SIZE > 0)
            newCapacity = hugeCapacity(minCapacity); //最多不能超过最大容量
        // 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;
    }

    //返回ArrayList的实际大小
    public int size() {
        return size;
    }

    //判断ArrayList是否为空
    public boolean isEmpty() {
        return size == 0;
    } 

/****************************** Search Operations *************************/

    //判断ArrayList是否包含Object o
    public boolean contains(Object o) {
        return indexOf(o) >= 0;
    }

    //正向查找，返回元素的索引值
    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;
    }

    //反向查找，返回元素的索引值
    public int lastIndexOf(Object o) {
        if (o == null) {
            for (int i = size-1; i >= 0; i--)
                if (elementData[i]==null)
                    return i;
        } else {
            for (int i = size-1; i >= 0; i--)
                if (o.equals(elementData[i]))
                    return i;
        }
        return -1;
    }

/******************************* Clone *********************************/

    //克隆函数
    public Object clone() {
        try {
            @SuppressWarnings("unchecked")
                ArrayList<E> v = (ArrayList<E>) super.clone();
        //将当前ArrayList的全部元素拷贝到v中
            v.elementData = Arrays.copyOf(elementData, size);
            v.modCount = 0;
            return v;
        } catch (CloneNotSupportedException e) {
            // this shouldn't happen, since we are Cloneable
            throw new InternalError();
        }
    }

/********************************* toArray *****************************/

    /**
    * 返回一个Object数组，包含ArrayList中所有的元素
    * toArray()方法扮演着array-based和collection-based API之间的桥梁
    */
    public Object[] toArray() {
        return Arrays.copyOf(elementData, size);
    }

    //返回ArrayList的模板数组
    @SuppressWarnings("unchecked")
    public <T> T[] toArray(T[] a) {
    //如果数组a的大小 < ArrayList的元素个数，
    //则新建一个T[]数组，大小为ArrayList元素个数，并将“ArrayList”全部拷贝到新数组中。
        if (a.length < size)
            return (T[]) Arrays.copyOf(elementData, size, a.getClass());

    //如果数组a的大小 >= ArrayList的元素个数，
    //则将ArrayList全部拷贝到新数组a中。
        System.arraycopy(elementData, 0, a, 0, size);
        if (a.length > size)
            a[size] = null;
        return a;
    }

/******************** Positional Access Operations ********************/

    @SuppressWarnings("unchecked")
    E elementData(int index) {
        return (E) elementData[index];
    }

    //获取index位置的元素值
    public E get(int index) {
        rangeCheck(index); //首先判断index的范围是否合法

        return elementData(index);
    }

    //将index位置的值设为element，并返回原来的值
    public E set(int index, E element) {
        rangeCheck(index);

        E oldValue = elementData(index);
        elementData[index] = element;
        return oldValue;
    }

    //将e添加到ArrayList中
    public boolean add(E e) {
        ensureCapacityInternal(size + 1);  // Increments modCount!!
        elementData[size++] = e;
        return true;
    }

    //将element添加到ArrayList的指定位置
    public void add(int index, E element) {
        rangeCheckForAdd(index);

        ensureCapacityInternal(size + 1);  // Increments modCount!!
    //将index以及index之后的数据复制到index+1的位置往后，即从index开始向后挪了一位
        System.arraycopy(elementData, index, elementData, index + 1,
                         size - index); 
        elementData[index] = element; //然后在index处插入element
        size++;
    }

    //删除ArrayList指定位置的元素
    public E remove(int index) {
        rangeCheck(index);

        modCount++;
        E oldValue = elementData(index);

        int numMoved = size - index - 1;
        if (numMoved > 0)
        //向左挪一位，index位置原来的数据已经被覆盖了
            System.arraycopy(elementData, index+1, elementData, index,
                             numMoved);
    //多出来的最后一位删掉
        elementData[--size] = null; // clear to let GC do its work

        return oldValue;
    }

    //删除ArrayList中指定的元素
    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的快速删除与上面的public普通删除区别在于，没有进行边界判断以及不返回删除值
    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
    }

    //清空ArrayList，将全部元素置为null
    public void clear() {
        modCount++;

        // clear to let GC do its work
        for (int i = 0; i < size; i++)
            elementData[i] = null;

        size = 0;
    }

    //将集合C中所有的元素添加到ArrayList中
    public boolean addAll(Collection<? extends E> c) {
        Object[] a = c.toArray();
        int numNew = a.length;
        ensureCapacityInternal(size + numNew);  // Increments modCount
    //在原来数组的后面添加c中所有的元素
        System.arraycopy(a, 0, elementData, size, numNew);
        size += numNew;
        return numNew != 0;
    }

    //从index位置开始，将集合C中所欲的元素添加到ArrayList中
    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)
        //将index开始向后的所有数据，向后移动numNew个位置，给新插入的数据腾出空间
            System.arraycopy(elementData, index, elementData, index + numNew,
                             numMoved);
    //将集合C中的数据插到刚刚腾出的位置
        System.arraycopy(a, 0, elementData, index, numNew);
        size += numNew;
        return numNew != 0;
    }

    //删除从fromIndex到toIndex之间的数据，不包括toIndex位置的数据
    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;
    }

    //范围检测
    private void rangeCheck(int index) {
        if (index >= size)
            throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
    }

    //add和addAll方法中的范围检测
    private void rangeCheckForAdd(int index) {
        if (index > size || index < 0)
            throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
    }

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

    //删除ArrayList中所有集合C中包含的数据
    public boolean removeAll(Collection<?> c) {
        return batchRemove(c, false);
    }

    //删除ArrayList中除了集合C中包含的数据外的其他所有数据
    public boolean retainAll(Collection<?> c) {
        return batchRemove(c, true);
    }

    //批量删除
    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.
        //官方的注释是为了保持和AbstractCollection的兼容性
        //我的理解是上面c.contains抛出了异常，导致for循环终止，那么必然会导致r != size
        //所以0-w之间是需要保留的数据，同时从w索引开始将剩下没有循环的数据(也就是从r开始的)拷贝回来，也保留
            if (r != size) {
                System.arraycopy(elementData, r,
                                 elementData, w,
                                 size - r);
                w += size - r;
            }
        //for循环完毕，检测了所有的元素
        //0-w之间保存了需要留下的数据，w开始以及后面的数据全部清空
            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;
    }

/***************************** Writer and Read Object *************************/

    //java.io.Serializable的写入函数
    //将ArrayList的“容量、所有的元素值”都写入到输出流中
    private void writeObject(java.io.ObjectOutputStream s)
        throws java.io.IOException{
        // Write out element count, and any hidden stuff
        int expectedModCount = modCount;
        s.defaultWriteObject();

        // Write out size as capacity for behavioural compatibility with clone()
    //写入“数组的容量”，保持与clone()的兼容性
        s.writeInt(size);

        //写入“数组的每一个元素”
        for (int i=0; i<size; i++) {
            s.writeObject(elementData[i]);
        }

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

    //java.io.Serializable的读取函数：根据写入方式读出
    private void readObject(java.io.ObjectInputStream s)
        throws java.io.IOException, ClassNotFoundException {
        elementData = EMPTY_ELEMENTDATA;

        // Read in size, and any hidden stuff
        s.defaultReadObject();

        //从输入流中读取ArrayList的“容量”
        s.readInt(); // ignored

        if (size > 0) {
            // be like clone(), allocate array based upon size not capacity
            ensureCapacityInternal(size);

            Object[] a = elementData;
            //从输入流中将“所有元素值”读出
            for (int i=0; i<size; i++) {
                a[i] = s.readObject();
            }
        }
    }

/******************************** Iterators ************************************/

    /**
    * 该部分的方法重写了AbstractList抽象类中Iterator部分的方法，因为ArrayList继承
    * 了AbstractList，基本大同小异，只是这里针对本类的数组，思想与AbstractList一致
    * 可以参照上一章Collection架构与源码分析的AbatractList部分
    */
    public ListIterator<E> listIterator(int index) {
        if (index < 0 || index > size)
            throw new IndexOutOfBoundsException("Index: "+index);
        return new ListItr(index);
    }

    public ListIterator<E> listIterator() {
        return new ListItr(0);
    }

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

    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();
            }
        }
    }

    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();
        }
    }
}

总结一下：

1）. ArrayList实际上是通过一个数组去保存数据的，当我们构造ArrayList时，如果使用默认构造函数，最后ArrayList的默认容量大小是10。

2）. 当ArrayList容量不足以容纳全部元素时，ArrayList会自动扩张容量，新的容量 = 原始容量 + 原始容量 / 2。

3）. ArrayList的克隆函数，即是将全部元素克隆到一个数组中。

4. ArrayList实现java.io.Serializable的方式。当写入到输出流时，先写入“容量”，再依次写出“每一个元素”；当读出输入流时，先读取“容量”，再依次读取“每一个元素”。

3. ArrayList遍历方式

ArrayList支持三种遍历方式，下面我们逐个讨论：

1）. 通过迭代器遍历。即Iterator迭代器。

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Integer value = null;
Iterator it = list.iterator();
while (it.hasNext()) {
value = (Integer)it.next();
}

Integer value = null;
Iterator it = list.iterator();
while (it.hasNext()) {
    value = (Integer)it.next();
}

2）. 随机访问，通过索引值去遍历。由于ArrayList实现了RandomAccess接口，所以它支持通过索引值去随机访问元素。

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Integer value = null;
int size = list.size();
for (int i = 0; i < size; i++) {
value = (Integer)list.get(i);
}

Integer value = null;
int size = list.size();
for (int i = 0; i < size; i++) {
    value = (Integer)list.get(i);        
}

3）. 通过for循环遍历。

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Integer value = null;
for (Integer integ : list) {
value = integ;
}

Integer value = null;
for (Integer integ : list) {
    value = integ;
}

下面写了一个测试用例，比较这三种遍历方式的效率：

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import java.util.*;
/*
* @description ArrayList三种遍历方式效率的测试
* @author eson_15
*/
public class ArrayListRandomAccessTest {
public static void main(String[] args) {
List<Integer> list = new ArrayList<Integer>();
for (int i=0; i<500000; i++)
list.add(i);
isRandomAccessSupported(list);//判断是否支持RandomAccess
iteratorThroughRandomAccess(list) ;
iteratorThroughIterator(list) ;
iteratorThroughFor(list) ;
}
private static void isRandomAccessSupported(List<Integer> list) {
if (list instanceof RandomAccess) {
System.out.println(”RandomAccess implemented!”);
} else {
System.out.println(”RandomAccess not implemented!”);
}
}
public static void iteratorThroughRandomAccess(List<Integer> list) {
long startTime;
long endTime;
startTime = System.currentTimeMillis();
for (int i=0; i<list.size(); i++) {
list.get(i);
}
endTime = System.currentTimeMillis();
long interval = endTime – startTime;
System.out.println(”RandomAccess遍历时间：” + interval+“ ms”);
}
public static void iteratorThroughIterator(List<Integer> list) {
long startTime;
long endTime;
startTime = System.currentTimeMillis();
for(Iterator<Integer> it = list.iterator(); it.hasNext(); ) {
it.next();
}
endTime = System.currentTimeMillis();
long interval = endTime – startTime;
System.out.println(”Iterator遍历时间：” + interval+“ ms”);
}
@SuppressWarnings(“unused”)
public static void iteratorThroughFor(List<Integer> list) {
long startTime;
long endTime;
startTime = System.currentTimeMillis();
for(Object obj : list)
;
endTime = System.currentTimeMillis();
long interval = endTime – startTime;
System.out.println(”For循环遍历时间：” + interval+“ ms”);
}
}

import java.util.*;

/*
 * @description ArrayList三种遍历方式效率的测试
 * @author eson_15
 */
public class ArrayListRandomAccessTest {

    public static void main(String[] args) {
        List<Integer> list = new ArrayList<Integer>();
        for (int i=0; i<500000; i++)
            list.add(i);
        isRandomAccessSupported(list);//判断是否支持RandomAccess
        iteratorThroughRandomAccess(list) ;
        iteratorThroughIterator(list) ;
        iteratorThroughFor(list) ;

    }

    private static void isRandomAccessSupported(List<Integer> list) {
        if (list instanceof RandomAccess) {
            System.out.println("RandomAccess implemented!");
        } else {
            System.out.println("RandomAccess not implemented!");
        }

    }

    public static void iteratorThroughRandomAccess(List<Integer> list) {

        long startTime;
        long endTime;
        startTime = System.currentTimeMillis();
        for (int i=0; i<list.size(); i++) {
            list.get(i);
        }
        endTime = System.currentTimeMillis();
        long interval = endTime - startTime;
        System.out.println("RandomAccess遍历时间：" + interval+" ms");
    }

    public static void iteratorThroughIterator(List<Integer> list) {

        long startTime;
        long endTime;
        startTime = System.currentTimeMillis();
        for(Iterator<Integer> it = list.iterator(); it.hasNext(); ) {
            it.next();
        }
        endTime = System.currentTimeMillis();
        long interval = endTime - startTime;
        System.out.println("Iterator遍历时间：" + interval+" ms");
    }


    @SuppressWarnings("unused")
    public static void iteratorThroughFor(List<Integer> list) {

        long startTime;
        long endTime;
        startTime = System.currentTimeMillis();
        for(Object obj : list)
            ;
        endTime = System.currentTimeMillis();
        long interval = endTime - startTime;
        System.out.println("For循环遍历时间：" + interval+" ms");
    }
}

每次执行的结果会有一点点区别，在这里我统计了6次执行结果，见下表：

	RandomAccess(ms)	Iterator(ms)	For(ms)
第一次	5	8	7
第二次	4	7	7
第三次	5	8	10
第四次	5	7	6
第五次	5	8	7
第六次	5	7	6
平均	4.8	7.5	7.1

由此可见，遍历ArrayList时，使用随机访问（即通过索引号访问）效率最高，而使用迭代器的效率最低。

4. toArray()异常问题

当我们调用ArrayList中的toArray()方法时，可能会遇到”java.lang.ClassCastException”异常的情况，下面来讨论下出现的原因：

ArrayList中提供了2个toArray()方法：

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Object[] toArray()
<T> T[] toArray(T[] contents)

Object[] toArray()
<T> T[] toArray(T[] contents)

调用toArray()函数会抛出”java.lang.ClassCastException”异常，但是调用toArray(T[] contents)能正常返回T[]。toArray()会抛出异常是因为toArray()返回的是Object[]数组，将Object[]转换为其它类型（比如将Object[]转换为Integer[]）则会抛出“java.lang.ClassCastException”异常，因为java不支持向下转型。解决该问题的办法是调用<T> T[] toArray(T[] contents)，而不是Object[] toArray()。

调用<T> T[] toArray(T[] contents)返回T[]可以通过以下几种方式实现：

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// toArray(T[] contents)调用方式一
public static Integer[] vectorToArray1(ArrayList<Integer> v) {
Integer[] newText = new Integer[v.size()];
v.toArray(newText);
return newText;
}
// toArray(T[] contents)调用方式二。<span style=”color:#FF6666;”>最常用！</span>
public static Integer[] vectorToArray2(ArrayList<Integer> v) {
Integer[] newText = (Integer[])v.toArray(new Integer[v.size()]);
return newText;
}
// toArray(T[] contents)调用方式三
public static Integer[] vectorToArray3(ArrayList<Integer> v) {
Integer[] newText = new Integer[v.size()];
Integer[] newStrings = (Integer[])v.toArray(newText);
return newStrings;
}

// toArray(T[] contents)调用方式一
public static Integer[] vectorToArray1(ArrayList<Integer> v) {
    Integer[] newText = new Integer[v.size()];
    v.toArray(newText);
    return newText;
}

// toArray(T[] contents)调用方式二。<span style="color:#FF6666;">最常用！</span>
public static Integer[] vectorToArray2(ArrayList<Integer> v) {
    Integer[] newText = (Integer[])v.toArray(new Integer[v.size()]);
    return newText;
}

// toArray(T[] contents)调用方式三
public static Integer[] vectorToArray3(ArrayList<Integer> v) {
    Integer[] newText = new Integer[v.size()];
    Integer[] newStrings = (Integer[])v.toArray(newText);
    return newStrings;
}

三种方式都大同小异。

ArrayList源码就讨论这么多，如有错误，欢迎留言指正~

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    原文作者：java集合源码分析
    原文地址: https://blog.csdn.net/PORSCHE_GT3RS/article/details/78327767
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