Java基础之容器Vector详解

Java基础之容器Vector详解

一、前言

知识补充：Arrays.copyOf函数：

public static int[] copyOf(int[] original, int newLength) {

int[] copy = new int[newLength];

System.arraycopy(original, 0, copy, 0,

Math.min(original.length, newLength));

return copy;

}

可见copyOf()在内部新建一个数组，调用arrayCopy()将original内容复制到copy中去，并且长度为newLength。返回copy;

继续看一下System.arraycopy函数：

public static native void arraycopy(Object src, int srcPos,

Object dest, int destPos,

int length);

src - 源数组。

srcPos - 源数组中的起始位置。

dest - 目标数组。

destPos - 目标数据中的起始位置。

length - 要复制的数组元素的数量。

该方法是用了native关键字，调用的为C++编写的底层函数，可见其为JDK中的底层函数。

二、Vector简介

public class Vector

extends AbstractList

implements List, RandomAccess, Cloneable, java.io.Serializable

Vector类实现了一个可增长的对象数组，内部是以动态数组的形式来存储数据的。

Vector具有数组所具有的特性、通过索引支持随机访问、所以通过随机访问Vector中的元素效率非常高、但是执行插入、删除时效率比较低下。

继承了AbstractList,此类提供 List 接口的骨干实现，以最大限度地减少实现”随机访问”数据存储（如数组）支持的该接口所需的工作.对于连续的访问数据（如链表），应优先使用 AbstractSequentialList，而不是此类.

实现了List接口,意味着Vector元素是有序的,可以重复的,可以有null元素的集合.

实现了RandomAccess接口标识着其支持随机快速访问,实际上,我们查看RandomAccess源码可以看到,其实里面什么都没有定义.因为ArrayList底层是数组,那么随机快速访问是理所当然的,访问速度O(1).

实现了Cloneable接口,标识着可以它可以被复制.注意,ArrayList里面的clone()复制其实是浅复制

实现了Serializable 标识着集合可被序列化。

三、Vector源码

public class Vector

extends AbstractList

implements List, RandomAccess, Cloneable, java.io.Serializable

{

//保存Vector数据的数组

protected Object[] elementData;

//实际数据的数量

protected int elementCount;

//容量增长的系数

protected int capacityIncrement;

// Vector的序列版本号

private static final long serialVersionUID = -2767605614048989439L;

//指定Vector初始大小和增长系数的构造函数

public Vector(int initialCapacity, int capacityIncrement) {

super();

if (initialCapacity < 0)

throw new IllegalArgumentException("Illegal Capacity: "+

initialCapacity);

this.elementData = new Object[initialCapacity];

this.capacityIncrement = capacityIncrement;

}

//指定初始容量的构造函数

public Vector(int initialCapacity) {

this(initialCapacity, 0);

}

//Vector构造函数，默认容量为10

public Vector() {

this(10);

}

//初始化一个指定集合数据的构造函数

public Vector(Collection extends E> c) {

elementData = c.toArray();

elementCount = elementData.length;

// c.toArray might (incorrectly) not return Object[] (see 6260652)

if (elementData.getClass() != Object[].class)

elementData = Arrays.copyOf(elementData, elementCount, Object[].class);

}

//将Vector全部元素拷贝到anArray数组中

public synchronized void copyInto(Object[] anArray) {

System.arraycopy(elementData, 0, anArray, 0, elementCount);

}

//当前的数组中元素个数大于记录的元素个数时，重新赋值给当前数组所记录的元素

public synchronized void trimToSize() {

modCount++;

int oldCapacity = elementData.length;

if (elementCount < oldCapacity) {

elementData = Arrays.copyOf(elementData, elementCount);

}

}

//确定Vector的容量

public synchronized void ensureCapacity(int minCapacity) {

if (minCapacity > 0) {

// 将Vector的改变统计数+1

modCount++;

ensureCapacityHelper(minCapacity);

}

}

//确定容量的帮助函数，如果所需容量大于当前的容量时则执行扩容

private void ensureCapacityHelper(int minCapacity) {

// overflow-conscious code

if (minCapacity - elementData.length > 0)

grow(minCapacity);

}

//数组所允许的最大容量

private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;

//执行扩容函数

private void grow(int minCapacity) {

// overflow-conscious code

//记录当前容量

int oldCapacity = elementData.length;

//如果扩容系数大于0则新容量等于当前容量+扩容系数，如果扩容系数小于等于0则新容量等于当前容量的2倍

int newCapacity = oldCapacity + ((capacityIncrement > 0) ?

capacityIncrement : oldCapacity);

//如果新容量小于当前需要的容量，则把需要的容量赋值给需要扩容的新容量

if (newCapacity - minCapacity < 0)

newCapacity = minCapacity;

//如果新扩容容量大于最大数组容量，则执行巨大扩容

if (newCapacity - MAX_ARRAY_SIZE > 0)

newCapacity = hugeCapacity(minCapacity);

elementData = Arrays.copyOf(elementData, newCapacity);

}

//巨大扩容函数，如果所需容量大于最大数组容量，则返回int形最大值(2^31 -1),否则返回最大数组容量

private static int hugeCapacity(int minCapacity) {

if (minCapacity < 0) // overflow

throw new OutOfMemoryError();

return (minCapacity > MAX_ARRAY_SIZE) ?

Integer.MAX_VALUE :

MAX_ARRAY_SIZE;

}

//设置容量值为newSize，如果newSize大于当前容量，则扩容，否则newSize以后的所有元素置null

public synchronized void setSize(int newSize) {

modCount++;

if (newSize > elementCount) {

ensureCapacityHelper(newSize);

} else {

for (int i = newSize ; i < elementCount ; i++) {

elementData[i] = null;

}

}

elementCount = newSize;

}

//返回当前Vector的容量

public synchronized int capacity() {

return elementData.length;

}

//返回Vector元素的个数

public synchronized int size() {

return elementCount;

}

//Vector元素个数是否为0

public synchronized boolean isEmpty() {

return elementCount == 0;

}

//返回Vector元素的Enumeration，Enumeration 接口是Iterator迭代器的“古老版本”

//Enumeration接口中的方法名称难以记忆，而且没有Iterator的remove()方法。如果现在编写Java程序，应该尽量采用

//Iterator迭代器，而不是用Enumeration迭代器。

//之所以保留Enumeration接口的原因，主要为了照顾以前那些“古老”的程序，那些程序里大量使用Enumeration接口，如果新版

//本的Java里直接删除Enumeration接口，将会导致那些程序全部出错。

public Enumeration elements() {

return new Enumeration() {

int count = 0;

public boolean hasMoreElements() {

return count < elementCount;

}

public E nextElement() {

synchronized (Vector.this) {

if (count < elementCount) {

return elementData(count++);

}

}

throw new NoSuchElementException("Vector Enumeration");

}

};

}

//返回Vector中是否包含对象o

public boolean contains(Object o) {

return indexOf(o, 0) >= 0;

}

// 查找并返回元素(o)在Vector中的索引值

public int indexOf(Object o) {

return indexOf(o, 0);

}

// 从index位置开始向后查找元素(o)。

// 若找到，则返回元素的索引值；否则，返回-1

public synchronized int indexOf(Object o, int index) {

if (o == null) {

for (int i = index ; i < elementCount ; i++)

if (elementData[i]==null)

return i;

} else {

for (int i = index ; i < elementCount ; i++)

if (o.equals(elementData[i]))

return i;

}

return -1;

}

// 从后向前查找元素(o)。并返回元素的索引

public synchronized int lastIndexOf(Object o) {

return lastIndexOf(o, elementCount-1);

}

// 从index位置开始向前查找元素(o)。

// 若找到，则返回元素的索引值；否则，返回-1

public synchronized int lastIndexOf(Object o, int index) {

if (index >= elementCount)

throw new IndexOutOfBoundsException(index + " >= "+ elementCount);

if (o == null) {

for (int i = index; i >= 0; i--)

if (elementData[i]==null)

return i;

} else {

for (int i = index; i >= 0; i--)

if (o.equals(elementData[i]))

return i;

}

return -1;

}

// 返回Vector中index位置的元素。

// 若index越界，则抛出异常

public synchronized E elementAt(int index) {

if (index >= elementCount) {

throw new ArrayIndexOutOfBoundsException(index + " >= " + elementCount);

}

return elementData(index);

}

// 返回Vector中第0位置的元素。

public synchronized E firstElement() {

if (elementCount == 0) {

throw new NoSuchElementException();

}

return elementData(0);

}

// 返回Vector中最后一个元素。

public synchronized E lastElement() {

if (elementCount == 0) {

throw new NoSuchElementException();

}

return elementData(elementCount - 1);

}

// 设置index位置的元素值为obj

public synchronized void setElementAt(E obj, int index) {

if (index >= elementCount) {

throw new ArrayIndexOutOfBoundsException(index + " >= " +

elementCount);

}

elementData[index] = obj;

}

//删除index位置处的元素

public synchronized void removeElementAt(int index) {

modCount++;

if (index >= elementCount) {

throw new ArrayIndexOutOfBoundsException(index + " >= " +

elementCount);

}

else if (index < 0) {

throw new ArrayIndexOutOfBoundsException(index);

}

int j = elementCount - index - 1;

if (j > 0) {

System.arraycopy(elementData, index + 1, elementData, index, j);

}

elementCount--;

elementData[elementCount] = null; /* to let gc do its work */

}

//在index位置插入元素obj

public synchronized void insertElementAt(E obj, int index) {

modCount++;

if (index > elementCount) {

throw new ArrayIndexOutOfBoundsException(index

+ " > " + elementCount);

}

ensureCapacityHelper(elementCount + 1);

System.arraycopy(elementData, index, elementData, index + 1, elementCount - index);

elementData[index] = obj;

elementCount++;

}

//在vector后面添加对象obj

public synchronized void addElement(E obj) {

modCount++;

ensureCapacityHelper(elementCount + 1);

elementData[elementCount++] = obj;

}

// 在Vector中查找并删除元素obj。

// 成功的话，返回true；否则，返回false。

public synchronized boolean removeElement(Object obj) {

modCount++;

int i = indexOf(obj);

if (i >= 0) {

removeElementAt(i);

return true;

}

return false;

}

//删除Vector中所有元素

public synchronized void removeAllElements() {

modCount++;

// Let gc do its work

for (int i = 0; i < elementCount; i++)

elementData[i] = null;

elementCount = 0;

}

//返回Vector的克隆。 该副本将包含对内部数据数组的克隆的引用，而不是对此对象的原始内部数据数组的引用。

public synchronized Object clone() {

try {

@SuppressWarnings("unchecked")

Vector v = (Vector) super.clone();

v.elementData = Arrays.copyOf(elementData, elementCount);

v.modCount = 0;

return v;

} catch (CloneNotSupportedException e) {

// this shouldn't happen, since we are Cloneable

throw new InternalError(e);

}

}

//返回包含Vector所有元素的数组

public synchronized Object[] toArray() {

return Arrays.copyOf(elementData, elementCount);

}

// 返回Vector的模板数组。所谓模板数组，即可以将T设为任意的数据类型

@SuppressWarnings("unchecked")

public synchronized T[] toArray(T[] a) {

// 若数组a的大小 < Vector的元素个数；

// 则新建一个T[]数组，数组大小是“Vector的元素个数”，并将“Vector”全部拷贝到新数组中

if (a.length < elementCount)

return (T[]) Arrays.copyOf(elementData, elementCount, a.getClass());

// 若数组a的大小 >= Vector的元素个数；

// 则将Vector的全部元素都拷贝到数组a中。

System.arraycopy(elementData, 0, a, 0, elementCount);

if (a.length > elementCount)

a[elementCount] = null;

return a;

}

// Positional Access Operations

@SuppressWarnings("unchecked")

E elementData(int index) {

return (E) elementData[index];

}

//获取index处的元素

public synchronized E get(int index) {

if (index >= elementCount)

throw new ArrayIndexOutOfBoundsException(index);

return elementData(index);

}

//设置index处的元素为element，并返回被替换掉的元素

public synchronized E set(int index, E element) {

if (index >= elementCount)

throw new ArrayIndexOutOfBoundsException(index);

E oldValue = elementData(index);

elementData[index] = element;

return oldValue;

}

//Vector末尾添加元素

public synchronized boolean add(E e) {

modCount++;

ensureCapacityHelper(elementCount + 1);

elementData[elementCount++] = e;

return true;

}

//移除Vector中第一个出现对象o的元素

public boolean remove(Object o) {

return removeElement(o);

}

//在index位置添加对象element

public void add(int index, E element) {

insertElementAt(element, index);

}

//移除index位置的元素

public synchronized E remove(int index) {

modCount++;

if (index >= elementCount)

throw new ArrayIndexOutOfBoundsException(index);

E oldValue = elementData(index);

int numMoved = elementCount - index - 1;

if (numMoved > 0)

System.arraycopy(elementData, index+1, elementData, index,

numMoved);

elementData[--elementCount] = null; // Let gc do its work

return oldValue;

}

// 清空Vector

public void clear() {

removeAllElements();

}

// Bulk Operations

// 返回Vector是否包含集合c

public synchronized boolean containsAll(Collection> c) {

return super.containsAll(c);

}

//在Vector末尾添加集合c

public synchronized boolean addAll(Collection extends E> c) {

modCount++;

Object[] a = c.toArray();

int numNew = a.length;

ensureCapacityHelper(elementCount + numNew);

System.arraycopy(a, 0, elementData, elementCount, numNew);

elementCount += numNew;

return numNew != 0;

}

// 删除集合c的全部元素

public synchronized boolean removeAll(Collection> c) {

return super.removeAll(c);

}

// 删除“非集合c中的元素”

public synchronized boolean retainAll(Collection> c) {

return super.retainAll(c);

}

//在index位置添加集合c中的元素

public synchronized boolean addAll(int index, Collection extends E> c) {

modCount++;

if (index < 0 || index > elementCount)

throw new ArrayIndexOutOfBoundsException(index);

Object[] a = c.toArray();

int numNew = a.length;

ensureCapacityHelper(elementCount + numNew);

int numMoved = elementCount - index;

if (numMoved > 0)

System.arraycopy(elementData, index, elementData, index + numNew,

numMoved);

System.arraycopy(a, 0, elementData, index, numNew);

elementCount += numNew;

return numNew != 0;

}

// 返回两个对象是否相等

public synchronized boolean equals(Object o) {

return super.equals(o);

}

// 计算哈希值

public synchronized int hashCode() {

return super.hashCode();

}

// 调用父类的toString()

public synchronized String toString() {

return super.toString();

}

// 获取Vector中fromIndex(包括)到toIndex(不包括)的子集

public synchronized List subList(int fromIndex, int toIndex) {

return Collections.synchronizedList(super.subList(fromIndex, toIndex),

this);

}

// 删除Vector中fromIndex到toIndex的元素

protected synchronized void removeRange(int fromIndex, int toIndex) {

modCount++;

int numMoved = elementCount - toIndex;

System.arraycopy(elementData, toIndex, elementData, fromIndex,

numMoved);

// Let gc do its work

int newElementCount = elementCount - (toIndex-fromIndex);

while (elementCount != newElementCount)

elementData[--elementCount] = null;

}

// java.io.Serializable的写入函数

private void writeObject(java.io.ObjectOutputStream s)

throws java.io.IOException {

final java.io.ObjectOutputStream.PutField fields = s.putFields();

final Object[] data;

synchronized (this) {

fields.put("capacityIncrement", capacityIncrement);

fields.put("elementCount", elementCount);

data = elementData.clone();

}

fields.put("elementData", data);

s.writeFields();

}

public synchronized ListIterator listIterator(int index) {

if (index < 0 || index > elementCount)

throw new IndexOutOfBoundsException("Index: "+index);

http:// return new ListItr(index);

}

public synchronized ListIterator listIterator() {

return new ListItr(0);

}

public synchronized Iterator iterator() {

return new Itr();

}

private class Itr implements Iterator {

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() {

// Racy but within spec, since modifications are checked

// within or after synchronization in next/previous

return cursor != elementCount;

}

public E next() {

synchronized (Vector.this) {

checkForComodification();

int i = cursor;

if (i >= elementCount)

throw new NoSuchElementException();

cursor = i + 1;

return elementData(lastRet = i);

}

}

public void remove() {

if (lastRet == -1)

throw new IllegalStateException();

synchronized (Vector.this) {

checkForComodification();

Vector.this.remove(lastRet);

expectedModCount = modCount;

}

cursor = lastRet;

lastRet = -1;

}

@Override

public void forEachRemaining(Consumer super E> action) {

Objects.requireNonNull(action);

synchronized (Vector.this) {

final int size = elementCount;

int i = cursor;

if (i >= size) {

return;

}

@SuppressWarnings("unchecked")

final E[] elementData = (E[]) Vector.this.elementData;

if (i >= elementData.length) {

throw new ConcurrentModificationException();

}

while (i != size && modCount == expectedModCount) {

action.accept(elementData[i++]);

}

// update once at end of iteration to reduce heap write traffic

cursor = i;

lastRet = i - 1;

checkForComodification();

}

}

final void checkForComodification() {

if (modCount != expectedModCount)

throw new ConcurrentModificationException();

}

}

final class ListItr extends Itr implements ListIterator {

ListItr(int index) {

super();

cursor = index;

}

public boolean hasPrevious() {

return cursor != 0;

}

public int nextIndex() {

return cursor;

}

public int previousIndex() {

return cursor - 1;

}

public E previous() {

synchronized (Vector.this) {

checkForComodification();

int i = cursor - 1;

if (i < 0)

throw new NoSuchElementException();

cursor = i;

return elementData(lastRet = i);

}

}

public void set(E e) {

if (lastRet == -1)

throw new IllegalStateException();

synchronized (Vector.this) {

checkForComodification();

Vector.this.set(lastRet, e);

}

}

public void add(E e) {

int i = cursor;

synchronized (Vector.this) {

checkForComodification();

Vector.this.add(i, e);

expectedModCount = modCount;

}

cursor = i + 1;

lastRet = -1;

}

}

@Override

public synchronized void forEach(Consumer super E> action) {

Objects.requireNonNull(action);

final int expectedModCount = modCount;

@SuppressWarnings("unchecked")

final E[] elementData = (E[]) this.elementData;

final int elementCount = this.elementCount;

for (int i=0; modCount == expectedModCount && i < elementCount; i++) {

action.accept(elementData[i]);

}

if (modCount != expectedModCount) {

throw new ConcurrentModificationException();

}

}

@Override

@SuppressWarnings("unchecked")

public synchronized boolean removeIf(Predicate super E> filter) {

Objects.requireNonNull(filter);

// figure out which elements are to be removed

// any exception thrown from the filter predicate at this stage

// will leave the collection unmodified

int removeCount = 0;

final int size = elementCount;

final BitSet removeSet = new BitSet(size);

final int expectedModCount = modCount;

for (int i=0; modCount == expectedModCount && i < size; i++) {

@SuppressWarnings("unchecked")

final E element = (E) elementData[i];

if (filter.test(element)) {

removeSet.set(i);

removeCount++;

}

}

if (modCount != expectedModCount) {

throw new ConcurrentModificationException();

}

// shift surviving elements left over the spaces left by removed elements

final boolean anyToRemove = removeCount > 0;

if (anyToRemove) {

final int newSize = size - removeCount;

for (int i=0, j=0; (i < size) && (j < newSize); i++, j++) {

i = removeSet.nextClearBit(i);

elementData[j] = elementData[i];

}

for (int k=newSize; k < size; k++) {

elementData[k] = null; // Let gc do its work

}

elementCount = newSize;

if (modCount != expectedModCount) {

throw new ConcurrentModificationException();

}

modCount++;

}

return anyToRemove;

}

@Override

@SuppressWarnings("unchecked")

public synchronized void replaceAll(UnaryOperator operator) {

Objects.requireNonNull(operator);

final int expectedModCount = modCount;

final int size = elementCount;

for (int i=0; modCount == expectedModCount && i < size; i++) {

elementData[i] = operator.apply((E) elementData[i]);

}

if (modCount != expectedModCount) {

throw new ConcurrentModificationException();

}

modCount++;

}

@SuppressWarnings("unchecked")

@Override

public synchronized void sort(Comparator super E> c) {

final int expectedModCount = modCount;

Arrays.sort((E[]) elementData, 0, elementCount, c);

if (modCount != expectedModCount) {

throw new ConcurrentModificationException();

}

modCount++;

}

@Override

public Spliterator spliterator() {

return new VectorSpliterator<>(this, null, 0, -1, 0);

}

/** Similar to ArrayList Spliterator */

static final class VectorSpliterator implements Spliterator {

private final Vector list;

private Object[] array;

private int index; // current index, modified on advance/split

private int fence; // -1 until used; then one past last index

private int expectedModCount; // initialized when fence set

/** Create new spliterator covering the given range */

VectorSpliterator(Vector list, Object[] array, int origin, int fence,

int expectedModCount) {

this.list = list;

this.array = array;

this.index = origin;

this.fence = fence;

this.expectedModCount = expectedModCount;

}

private int getFence() { // initialize on first use

int hi;

if ((hi = fence) < 0) {

synchronized(list) {

array = list.elementData;

expectedModCount = list.modCount;

hi = fence = list.elementCount;

}

}

return hi;

}

public Spliterator trySplit() {

int hi = getFence(), lo = index, mid = (lo + hi) >>> 1;

return (lo >= mid) ? null :

new VectorSpliterator(list, array, lo, index = mid,

expectedModCount);

}

@SuppressWarnings("unchecked")

public boolean tryAdvance(Consumer super E> action) {

int i;

if (action == null)

throw new NullPointerException();

if (getFence() > (i = index)) {

index = i + 1;

action.accept((E)array[i]);

if (list.modCount != expectedModCount)

throw new ConcurrentModificationException();

return true;

}

return false;

}

@SuppressWarnings("unchecked")

public void forEachRemaining(Consumer super E> action) {

int i, hi; // hoist accesses and checks from loop

Vector lst; Object[] a;

if (action == null)

throw new NullPointerException();

if ((lst = list) != null) {

if ((hi = fence) < 0) {

synchronized(lst) {

expectedModCount = lst.modCount;

a = array = lst.elementData;

hi = fence = lst.elementCount;

}

}

else

a = array;

if (a != null && (i = index) >= 0 && (index = hi) <= a.length) {

while (i < hi)

action.accept((E) a[i++]);

if (lst.modCount == expectedModCount)

return;

}

}

throw new ConcurrentModificationException();

}

public long estimateSize() {

return (long) (getFence() - index);

}

public int characteristics() {

return Spliterator.ORDERED | Spliterator.SIZED | Spliterator.SUBSIZED;

}

}

}

四、总结

Vector实际上是通过一个数组去保存数据的。当我们构造Vecotr时；若使用默认构造函数，则Vector的默认容量大小是10。

当Vector容量不足以容纳全部元素时，Vector的容量会增加。若容量增加系数 >0，则将容量的值增加“容量增加系数”；否则，将容量大小增加一倍。

Vector的克隆函数，即是将全部元素克隆到一个数组中。

五、Vector遍历方式

1. 随机访问遍历，通过索引值去遍历

由于Vector实现了RandomAccess接口，它支持通过索引值去随机访问元素。

Integer value = null;

int size = vec.size();

for (int i=0; i

value = (Integer)vec.get(i);

}

2. 通过迭代器遍历。即通过Iterator去遍历

Integer value = null;

Iterator iterator = vec.iterator();

while (iterator.hasNext()) {

value = iterator.next();

}

3. 通过增强for循环去遍历

Integer value = null;

for (Integer integ:vec) {

value = integ;

}

4. 通过Enumeration遍历

Integer value = null;

Enumeration enu = vec.elements();

while (enu.hasMoreElements()) {

value = (Integer)enu.nextElement();

}

测试这些遍历方式效率的代码如下：

public class Test {

public static void main(String[] args) {

Vector vector = new Vector<>();

for (int i = 0; i < 100000; i++)

vector.add(i);

iteratorThroughRandomAccess(vector);

iteratorThroughIterator(vector);

iteratorThroughFor2(vector);

iteratorThroughEnumeration(vector);

}

public static void iteratorThroughRandomAccess(List list) {

long startTime, endTime;

startTime = System.currentTimeMillis();

for (int i = 0; i < list.size(); i++) {

}

endTime = System.currentTimeMillis();

long time = endTime - startTime;

System.out.println("iteratorThroughRandomAccess：" + time + " ms");

}

public static void iteratorThroughIterator(List list) {

long startTime, endTime;

startTime = System.currentTimeMillis();

Iterator iterator = list.iterator();

while (iterator.hasNext()) {

iterator.next();

}

endTime = System.currentTimeMillis();

long time = endTime - startTime;

System.out.println("iteratorThroughIterator：" + time + " ms");

}

public static void iteratorThroughFor2(List list) {

long startTime, endTime;

startTime = System.currentTimeMillis();

for (Object o : list) {

}

endTime = System.currentTimeMillis();

long time = endTime - startTime;

System.out.println("iteratorThroughFor2：" + time + oQQIve" ms");

}

public static void iteratorThroughEnumeration(Vector vec) {

long startTime, endTime;

startTime = System.currentTimeMillis();

for (Enumeration enu = vec.elements(); enu.hasMoreElements(); ) {

enu.nextElement();

}

endTime = System.currentTimeMillis();

long time = endTime - startTime;

System.out.println("iteratorThroughEnumeration：" + time + " ms");

}

}

输出如下：

iteratorThroughRandomAccess：3 ms

iteratorThroughIterator：6 ms

iteratorThroughFor2：5 ms

iteratorThroughEnumeration：5 ms

所以：遍历Vector，使用索引的随机访问方式最快，使用迭代器最慢。

value = (Integer)vec.get(i);

}

2. 通过迭代器遍历。即通过Iterator去遍历

Integer value = null;

Iterator iterator = vec.iterator();

while (iterator.hasNext()) {

value = iterator.next();

}

3. 通过增强for循环去遍历

Integer value = null;

for (Integer integ:vec) {

value = integ;

}

4. 通过Enumeration遍历

Integer value = null;

Enumeration enu = vec.elements();

while (enu.hasMoreElements()) {

value = (Integer)enu.nextElement();

}

测试这些遍历方式效率的代码如下：

public class Test {

public static void main(String[] args) {

Vector vector = new Vector<>();

for (int i = 0; i < 100000; i++)

vector.add(i);

iteratorThroughRandomAccess(vector);

iteratorThroughIterator(vector);

iteratorThroughFor2(vector);

iteratorThroughEnumeration(vector);

}

public static void iteratorThroughRandomAccess(List list) {

long startTime, endTime;

startTime = System.currentTimeMillis();

for (int i = 0; i < list.size(); i++) {

}

endTime = System.currentTimeMillis();

long time = endTime - startTime;

System.out.println("iteratorThroughRandomAccess：" + time + " ms");

}

public static void iteratorThroughIterator(List list) {

long startTime, endTime;

startTime = System.currentTimeMillis();

Iterator iterator = list.iterator();

while (iterator.hasNext()) {

iterator.next();

}

endTime = System.currentTimeMillis();

long time = endTime - startTime;

System.out.println("iteratorThroughIterator：" + time + " ms");

}

public static void iteratorThroughFor2(List list) {

long startTime, endTime;

startTime = System.currentTimeMillis();

for (Object o : list) {

}

endTime = System.currentTimeMillis();

long time = endTime - startTime;

System.out.println("iteratorThroughFor2：" + time + oQQIve" ms");

}

public static void iteratorThroughEnumeration(Vector vec) {

long startTime, endTime;

startTime = System.currentTimeMillis();

for (Enumeration enu = vec.elements(); enu.hasMoreElements(); ) {

enu.nextElement();

}

endTime = System.currentTimeMillis();

long time = endTime - startTime;

System.out.println("iteratorThroughEnumeration：" + time + " ms");

}

}

输出如下：

iteratorThroughRandomAccess：3 ms

iteratorThroughIterator：6 ms

iteratorThroughFor2：5 ms

iteratorThroughEnumeration：5 ms

所以：遍历Vector，使用索引的随机访问方式最快，使用迭代器最慢。

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