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一、前言
知識補充：Arrays.copyOf函數(shù)：
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;
}可見copyOf()在內(nèi)部新建一個數(shù)組，調(diào)用arrayCopy()將original內(nèi)容復制到copy中去，并且長度為newLength。返回copy;
繼續(xù)看一下System.arraycopy函數(shù)：
public static native void arraycopy(Object src, int srcPos,
Object dest, int destPos,
int length);
src - 源數(shù)組。
srcPos - 源數(shù)組中的起始位置。
dest - 目標數(shù)組。
destPos - 目標數(shù)據(jù)中的起始位置。
length - 要復制的數(shù)組元素的數(shù)量。
該方法是用了native關鍵字，調(diào)用的為C++編寫的底層函數(shù)，可見其為JDK中的底層函數(shù)。
二、Vector簡介
public class Vector<E>
extends AbstractList<E>
implements List<E>, RandomAccess, Cloneable, java.io.Serializable- Vector類實現(xiàn)了一個可增長的對象數(shù)組，內(nèi)部是以動態(tài)數(shù)組的形式來存儲數(shù)據(jù)的。
- Vector具有數(shù)組所具有的特性、通過索引支持隨機訪問、所以通過隨機訪問Vector中的元素效率非常高、但是執(zhí)行插入、刪除時效率比較低下。
- 繼承了AbstractList,此類提供 List 接口的骨干實現(xiàn)，以最大限度地減少實現(xiàn)”隨機訪問”數(shù)據(jù)存儲（如數(shù)組）支持的該接口所需的工作.對于連續(xù)的訪問數(shù)據(jù)（如鏈表），應優(yōu)先使用 AbstractSequentialList，而不是此類.
- 實現(xiàn)了List接口,意味著Vector元素是有序的,可以重復的,可以有null元素的集合.
- 實現(xiàn)了RandomAccess接口標識著其支持隨機快速訪問,實際上,我們查看RandomAccess源碼可以看到,其實里面什么都沒有定義.因為ArrayList底層是數(shù)組,那么隨機快速訪問是理所當然的,訪問速度O(1).
- 實現(xiàn)了Cloneable接口,標識著可以它可以被復制.注意,ArrayList里面的clone()復制其實是淺復制
- 實現(xiàn)了Serializable 標識著集合可被序列化。
三、Vector源碼
public class Vector<E>
extends AbstractList<E>
implements List<E>, RandomAccess, Cloneable, java.io.Serializable
{
//保存Vector數(shù)據(jù)的數(shù)組
protected Object[] elementData;
//實際數(shù)據(jù)的數(shù)量
protected int elementCount;
//容量增長的系數(shù)
protected int capacityIncrement;
// Vector的序列版本號
private static final long serialVersionUID = -2767605614048989439L;
//指定Vector初始大小和增長系數(shù)的構造函數(shù)
public Vector(int initialCapacity, int capacityIncrement) {
super();
if (initialCapacity < 0)
throw new IllegalArgumentException("Illegal Capacity: "+
initialCapacity);
this.elementData = new Object[initialCapacity];
this.capacityIncrement = capacityIncrement;
}
//指定初始容量的構造函數(shù)
public Vector(int initialCapacity) {
this(initialCapacity, 0);
}
//Vector構造函數(shù)，默認容量為10
public Vector() {
this(10);
}
//初始化一個指定集合數(shù)據(jù)的構造函數(shù)
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);
}
//將Vector全部元素拷貝到anArray數(shù)組中
public synchronized void copyInto(Object[] anArray) {
System.arraycopy(elementData, 0, anArray, 0, elementCount);
}
//當前的數(shù)組中元素個數(shù)大于記錄的元素個數(shù)時，重新賦值給當前數(shù)組所記錄的元素
public synchronized void trimToSize() {
modCount++;
int oldCapacity = elementData.length;
if (elementCount < oldCapacity) {
elementData = Arrays.copyOf(elementData, elementCount);
}
}
//確定Vector的容量
public synchronized void ensureCapacity(int minCapacity) {
if (minCapacity > 0) {
// 將Vector的改變統(tǒng)計數(shù)+1
modCount++;
ensureCapacityHelper(minCapacity);
}
}
//確定容量的幫助函數(shù)，如果所需容量大于當前的容量時則執(zhí)行擴容
private void ensureCapacityHelper(int minCapacity) {
// overflow-conscious code
if (minCapacity - elementData.length > 0)
grow(minCapacity);
}
//數(shù)組所允許的最大容量
private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;
//執(zhí)行擴容函數(shù)
private void grow(int minCapacity) {
// overflow-conscious code
//記錄當前容量
int oldCapacity = elementData.length;
//如果擴容系數(shù)大于0則新容量等于當前容量+擴容系數(shù)，如果擴容系數(shù)小于等于0則新容量等于當前容量的2倍
int newCapacity = oldCapacity + ((capacityIncrement > 0) ?
capacityIncrement : oldCapacity);
//如果新容量小于當前需要的容量，則把需要的容量賦值給需要擴容的新容量
if (newCapacity - minCapacity < 0)
newCapacity = minCapacity;
//如果新擴容容量大于最大數(shù)組容量，則執(zhí)行巨大擴容
if (newCapacity - MAX_ARRAY_SIZE > 0)
newCapacity = hugeCapacity(minCapacity);
elementData = Arrays.copyOf(elementData, newCapacity);
}
//巨大擴容函數(shù)，如果所需容量大于最大數(shù)組容量，則返回int形最大值(2^31 -1),否則返回最大數(shù)組容量
private static int hugeCapacity(int minCapacity) {
if (minCapacity < 0) // overflow
throw new OutOfMemoryError();
return (minCapacity > MAX_ARRAY_SIZE) ?
Integer.MAX_VALUE :
MAX_ARRAY_SIZE;
}
//設置容量值為newSize，如果newSize大于當前容量，則擴容，否則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;
}
//返回當前Vector的容量
public synchronized int capacity() {
return elementData.length;
}
//返回Vector元素的個數(shù)
public synchronized int size() {
return elementCount;
}
//Vector元素個數(shù)是否為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接口中的方法名稱難以記憶，而且沒有Iterator的remove()方法。如果現(xiàn)在編寫Java程序，應該盡量采用
//Iterator迭代器，而不是用Enumeration迭代器。
//之所以保留Enumeration接口的原因，主要為了照顧以前那些“古老”的程序，那些程序里大量使用Enumeration接口，如果新版
//本的Java里直接刪除Enumeration接口，將會導致那些程序全部出錯。
public Enumeration<E> elements() {
return new Enumeration<E>() {
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中是否包含對象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);
}
// 從index位置開始向后查找元素(o)。
// 若找到，則返回元素的索引值；否則，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;
}
// 從后向前查找元素(o)。并返回元素的索引
public synchronized int lastIndexOf(Object o) {
return lastIndexOf(o, elementCount-1);
}
// 從index位置開始向前查找元素(o)。
// 若找到，則返回元素的索引值；否則，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，則拋出異常
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中最后一個元素。
public synchronized E lastElement() {
if (elementCount == 0) {
throw new NoSuchElementException();
}
return elementData(elementCount - 1);
}
// 設置index位置的元素值為obj
public synchronized void setElementAt(E obj, int index) {
if (index >= elementCount) {
throw new ArrayIndexOutOfBoundsException(index + " >= " +
elementCount);
}
elementData[index] = obj;
}
//刪除index位置處的元素
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后面添加對象obj
public synchronized void addElement(E obj) {
modCount++;
ensureCapacityHelper(elementCount + 1);
elementData[elementCount++] = obj;
}
// 在Vector中查找并刪除元素obj。
// 成功的話，返回true；否則，返回false。
public synchronized boolean removeElement(Object obj) {
modCount++;
int i = indexOf(obj);
if (i >= 0) {
removeElementAt(i);
return true;
}
return false;
}
//刪除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。 該副本將包含對內(nèi)部數(shù)據(jù)數(shù)組的克隆的引用，而不是對此對象的原始內(nèi)部數(shù)據(jù)數(shù)組的引用。
public synchronized Object clone() {
try {
@SuppressWarnings("unchecked")
Vector<E> v = (Vector<E>) 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所有元素的數(shù)組
public synchronized Object[] toArray() {
return Arrays.copyOf(elementData, elementCount);
}
// 返回Vector的模板數(shù)組。所謂模板數(shù)組，即可以將T設為任意的數(shù)據(jù)類型
@SuppressWarnings("unchecked")
public synchronized <T> T[] toArray(T[] a) {
// 若數(shù)組a的大小 < Vector的元素個數(shù)；
// 則新建一個T[]數(shù)組，數(shù)組大小是“Vector的元素個數(shù)”，并將“Vector”全部拷貝到新數(shù)組中
if (a.length < elementCount)
return (T[]) Arrays.copyOf(elementData, elementCount, a.getClass());
// 若數(shù)組a的大小 >= Vector的元素個數(shù)；
// 則將Vector的全部元素都拷貝到數(shù)組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];
}
//獲取index處的元素
public synchronized E get(int index) {
if (index >= elementCount)
throw new ArrayIndexOutOfBoundsException(index);
return elementData(index);
}
//設置index處的元素為element，并返回被替換掉的元素
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中第一個出現(xiàn)對象o的元素
public boolean remove(Object o) {
return removeElement(o);
}
//在index位置添加對象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;
}
// 刪除集合c的全部元素
public synchronized boolean removeAll(Collection<?> c) {
return super.removeAll(c);
}
// 刪除“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;
}
// 返回兩個對象是否相等
public synchronized boolean equals(Object o) {
return super.equals(o);
}
// 計算哈希值
public synchronized int hashCode() {
return super.hashCode();
}
// 調(diào)用父類的toString()
public synchronized String toString() {
return super.toString();
}
// 獲取Vector中fromIndex(包括)到toIndex(不包括)的子集
public synchronized List<E> subList(int fromIndex, int toIndex) {
return Collections.synchronizedList(super.subList(fromIndex, toIndex),
this);
}
// 刪除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的寫入函數(shù)
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<E> listIterator(int index) {
if (index < 0 || index > elementCount)
throw new IndexOutOfBoundsException("Index: "+index);
return new ListItr(index);
}
public synchronized ListIterator<E> listIterator() {
return new ListItr(0);
}
public synchronized 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() {
// 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<E> {
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<E> 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<E> spliterator() {
return new VectorSpliterator<>(this, null, 0, -1, 0);
}
/** Similar to ArrayList Spliterator */
static final class VectorSpliterator<E> implements Spliterator<E> {
private final Vector<E> 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<E> 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<E> trySplit() {
int hi = getFence(), lo = index, mid = (lo + hi) >>> 1;
return (lo >= mid) ? null :
new VectorSpliterator<E>(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<E> 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、總結
- Vector實際上是通過一個數(shù)組去保存數(shù)據(jù)的。當我們構造Vecotr時；若使用默認構造函數(shù)，則Vector的默認容量大小是10。
- 當Vector容量不足以容納全部元素時，Vector的容量會增加。若容量增加系數(shù) >0，則將容量的值增加“容量增加系數(shù)”；否則，將容量大小增加一倍。
- Vector的克隆函數(shù)，即是將全部元素克隆到一個數(shù)組中。
五、Vector遍歷方式
1. 隨機訪問遍歷，通過索引值去遍歷
由于Vector實現(xiàn)了RandomAccess接口，它支持通過索引值去隨機訪問元素。
Integer value = null;
int size = vec.size();
for (int i=0; i<size; i++) {
value = (Integer)vec.get(i);
}2. 通過迭代器遍歷。即通過Iterator去遍歷
Integer value = null;
Iterator<Integer> iterator = vec.iterator();
while (iterator.hasNext()) {
value = iterator.next();
}3. 通過增強for循環(huán)去遍歷
Integer value = null;
for (Integer integ:vec) {
value = integ;
}4. 通過Enumeration遍歷
Integer value = null;
Enumeration enu = vec.elements();
while (enu.hasMoreElements()) {
value = (Integer)enu.nextElement();
}測試這些遍歷方式效率的代碼如下：
public class Test {
public static void main(String[] args) {
Vector<Integer> 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<Integer> 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 + " 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");
}
}
輸出如下：
iteratorThroughRandomAccess：3 ms
iteratorThroughIterator：6 ms
iteratorThroughFor2：5 ms
iteratorThroughEnumeration：5 ms
所以：遍歷Vector，使用索引的隨機訪問方式最快，使用迭代器最慢。
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