目录一、前言二、Vector简介三、Vector源码四、总结五、Vector遍历方式
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<E> extends AbstractList<E> implements List<E>, 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<E> extends AbstractList<E> implements List<E>, 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<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<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> 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<E> 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<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、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<size; 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<Integer> 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<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"); }}
输出如下：
iteratorThroughRandomAccess：3 msiteratorThroughIterator：6 msiteratorThroughFor2：5 msiteratorThroughEnumeration：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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