java集合框架02——ArrayList和源码分析
2017-10-24 11:32
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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。
![](https://img-blog.csdn.net/20160411152519031?watermark/2/text/aHR0cDovL2Jsb2cuY3Nkbi5uZXQv/font/5a6L5L2T/fontsize/400/fill/I0JBQkFCMA==/dissolve/70/gravity/Center)
ArrayList的继承关系如下:
[java] view plain copy print?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
[java] view plain copy print?/****************** 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)
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。
[java] view plain copy print?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();
}
}
}
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。
[java] view plain copy print?Integer value = null; Iterator it = list.iterator(); while (it.hasNext()) { value = (Integer)it.next(); }
[java] view plain copy print?Integer value = null; int size = list.size(); for (int i = 0; i < size; i++) { value = (Integer)list.get(i); }
[java] view plain copy print?Integer value = null; for (Integer integ : list) { value = integ; }
[java] view plain copy print?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”);
}
}
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。
ArrayList中提供了2个toArray()方法:
[java] view plain copy print?Object[] toArray() <T> T[] toArray(T[] contents)
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:
[java] view plain copy print?// 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;
}
ArrayList源码就讨论这么多,如有错误,欢迎留言指正~
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1. ArrayList简介
首先看看ArrayList与Collection的关系:ArrayList的继承关系如下:
[java] view plain copy print?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
[java] view plain copy print?/****************** 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> 24000 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:[java] view plain copy print?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; } 15d6a } 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。
[java] view plain copy print?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。
[java] view plain copy print?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。
[java] view plain copy print?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:
[java] view plain copy print?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 |
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:
[java] view plain copy print?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:
[java] view plain copy print?// 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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