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：

ArrayList的继承关系如下：

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

/****************** 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：

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ArrayListpublic ArrayList(Collection<? extends E> c) {elementData = c.toArray(); //将Collection中的内容转换成数组初始化elementDatasize = 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 codeint oldCapacity = elementData.length;int newCapacity = oldCapacity + (oldCapacity >> 1); //在原来容量的基础上加上 oldCapacity/2if (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) // overflowthrow 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 opublic 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 Cloneablethrow 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elementsize++;}//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 workreturn 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nullpublic void clear() {modCount++;// clear to let GC do its workfor (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 modCountint 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 workint 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 workfor (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 stuffint 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 stuffs.defaultReadObject();//从输入流中读取ArrayList的“容量”s.readInt(); // ignoredif (size > 0) {// be like clone(), allocate array based upon size not capacityensureCapacityInternal(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 returnint lastRet = -1; // index of last element returned; -1 if no suchint 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。

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。

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。

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：

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RandomAccessiteratorThroughRandomAccess(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：

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：

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：

// 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~

_____________________________________________________________________________________________________________________________________________________

—–乐于分享，共同进步！

—–更多文章请看：http://blog.csdn.net/eson_15