JDK源码分析——Java.util.Vector的浅析
首先我们来看JDK源码中Java.util.Vector的代码，剔除所有的方法和静态变量，
Java.lang.Vector的核心代码如下：
public class Vector<E>
extends AbstractList<E>
implements List<E>, RandomAccess, Cloneable, java.io.Serializable
{
protected Object[] elementData;
/**
* The number of valid components in this {@code Vector} object.
* Components {@code elementData[0]} through
* {@code elementData[elementCount-1]} are the actual items.
*
* @serial
*/
protected int elementCount;
protected int capacityIncrement; //可以设定固定增量
}
通过上面的代码我们能看到他是数组存储。
它的构造方法有四个：
/**
* Constructs an empty vector with the specified initial capacity and
* capacity increment.
*
* @param initialCapacity the initial capacity of the vector
* @param capacityIncrement the amount by which the capacity is
* increased when the vector overflows
* @throws IllegalArgumentException if the specified initial capacity
* is negative
*/
public Vector(int initialCapacity, int capacityIncrement) { //设置初始容量，及固定增量
super();
if (initialCapacity < 0)
throw new IllegalArgumentException("Illegal Capacity: "+
initialCapacity);
this.elementData = new Object[initialCapacity];
this.capacityIncrement = capacityIncrement;
}
/**
* Constructs an empty vector with the specified initial capacity and
* with its capacity increment equal to zero.
*
* @param initialCapacity the initial capacity of the vector
* @throws IllegalArgumentException if the specified initial capacity
* is negative
*/
public Vector(int initialCapacity) { //设置初始容量，不设定固定增量（如果不设定固定增量，则成倍增长）
this(initialCapacity, 0);
}
/**
* Constructs an empty vector so that its internal data array
* has size {@code 10} and its standard capacity increment is
* zero.
*/
public Vector() { //不设置初始容量，初始容量为10，不设定固定增量（如果不设定固定增量，则成倍增长）
this(10);
}
/**
* Constructs a vector containing the elements of the specified
* collection, in the order they are returned by the collection's
* iterator.
*
* @param c the collection whose elements are to be placed into this
* vector
* @throws NullPointerException if the specified collection is null
* @since 1.2
*/
public Vector(Collection<? extends E> c) { //以集合初始化
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);
}
下面我们再看一下 Vector类的 add(E e)方法的源代码：
/**
* Appends the specified element to the end of this Vector.
*
* @param e element to be appended to this Vector
* @return {@code true} (as specified by {@link Collection#add})
* @since 1.2
*/
public synchronized boolean add(E e) { //从这里的synchronized我们可以知道他是线程同步的
modCount++;
ensureCapacityHelper(elementCount + 1); //是否需要扩容
elementData[elementCount++] = e; //后放的永远在最后一位
return true; // 好像他永远返回true 哦
}
/**
* This implements the unsynchronized semantics of ensureCapacity.
* Synchronized methods in this class can internally call this
* method for ensuring capacity without incurring the cost of an
* extra synchronization.
*
* @see #ensureCapacity(int)
*/
private void ensureCapacityHelper(int minCapacity) {
int oldCapacity = elementData.length;
if (minCapacity > oldCapacity) { //最新的值如果超出现有容量 扩容
Object[] oldData = elementData;
int newCapacity = (capacityIncrement > 0) ?
(oldCapacity + capacityIncrement) : (oldCapacity * 2); //如果固定增量不为0则增长固定增量，否则成倍增长
if (newCapacity < minCapacity) { //新容量还小，这个可能性应该不大
newCapacity = minCapacity;
}
elementData = Arrays.copyOf(elementData, newCapacity); // 改变数组的大小，这个方法可以记一下
}
}
下面我们再看一下 Vector类的 remove(Object o)方法的源代码：
/**
* Removes the first occurrence of the specified element in this Vector
* If the Vector does not contain the element, it is unchanged. More
* formally, removes the element with the lowest index i such that
* {@code (o==null ? get(i)==null : o.equals(get(i)))} (if such
* an element exists).
*
* @param o element to be removed from this Vector, if present
* @return true if the Vector contained the specified element
* @since 1.2
*/
public boolean remove(Object o) {
return removeElement(o); //他是调用的removeElement
}
/**
* Removes the first (lowest-indexed) occurrence of the argument
* from this vector. If the object is found in this vector, each
* component in the vector with an index greater or equal to the
* object's index is shifted downward to have an index one smaller
* than the value it had previously.
*
* <p>This method is identical in functionality to the
* {@link #remove(Object)} method (which is part of the
* {@link List} interface).
*
* @param obj the component to be removed
* @return {@code true} if the argument was a component of this
* vector; {@code false} otherwise.
*/
public synchronized boolean removeElement(Object obj) {
modCount++;
int i = indexOf(obj); //查找obj第一次出现的位置，（Vector是允许重复值的）
if (i >= 0) {
removeElementAt(i);
return true;
}
return false;
}
/**
* Deletes the component at the specified index. Each component in
* this vector with an index greater or equal to the specified
* {@code index} is shifted downward to have an index one
* smaller than the value it had previously. The size of this vector
* is decreased by {@code 1}.
*
* <p>The index must be a value greater than or equal to {@code 0}
* and less than the current size of the vector.
*
* <p>This method is identical in functionality to the {@link #remove(int)}
* method (which is part of the {@link List} interface). Note that the
* {@code remove} method returns the old value that was stored at the
* specified position.
*
* @param index the index of the object to remove
* @throws ArrayIndexOutOfBoundsException if the index is out of range
* ({@code index < 0 || index >= size()})
*/
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; // 数组中index以后的元素个数
if (j > 0) {
System.arraycopy(elementData, index + 1, elementData, index, j); // 数组中index以后的元素，整体前移，（这个方法挺有用的！！）
}
elementCount--;
elementData[elementCount] = null; /* to let gc do its work */
}
从上面的代码我们可以看出 Vector每次只删除最靠前的那个相符的变量。
下面我们再看一下 Vector类的 equals(Object o)方法的源代码：
/**
* Compares the specified Object with this Vector for equality. Returns
* true if and only if the specified Object is also a List, both Lists
* have the same size, and all corresponding pairs of elements in the two
* Lists are <em>equal</em>. (Two elements {@code e1} and
* {@code e2} are <em>equal</em> if {@code (e1==null ? e2==null :
* e1.equals(e2))}.) In other words, two Lists are defined to be
* equal if they contain the same elements in the same order.
*
* @param o the Object to be compared for equality with this Vector
* @return true if the specified Object is equal to this Vector
*/
public synchronized boolean equals(Object o) {
return super.equals(o); //他是调用的父类AbstractList的equals
}
/**
* Compares the specified object with this list for equality. Returns
* {@code true} if and only if the specified object is also a list, both
* lists have the same size, and all corresponding pairs of elements in
* the two lists are <i>equal</i>. (Two elements {@code e1} and
* {@code e2} are <i>equal</i> if {@code (e1==null ? e2==null :
* e1.equals(e2))}.) In other words, two lists are defined to be
* equal if they contain the same elements in the same order.<p>
*
* This implementation first checks if the specified object is this
* list. If so, it returns {@code true}; if not, it checks if the
* specified object is a list. If not, it returns {@code false}; if so,
* it iterates over both lists, comparing corresponding pairs of elements.
* If any comparison returns {@code false}, this method returns
* {@code false}. If either iterator runs out of elements before the
* other it returns {@code false} (as the lists are of unequal length);
* otherwise it returns {@code true} when the iterations complete.
*
* @param o the object to be compared for equality with this list
* @return {@code true} if the specified object is equal to this list
*/
public boolean equals(Object o) {
if (o == this)
return true;
if (!(o instanceof List))
return false;
ListIterator<E> e1 = listIterator();
ListIterator e2 = ((List) o).listIterator();
while(e1.hasNext() && e2.hasNext()) {
E o1 = e1.next();
Object o2 = e2.next();
if (!(o1==null ? o2==null : o1.equals(o2))) //判断了三次
//这个方法挺不错的，应该比if(!(null==o1&&null==o2)||(null!=o1&&o1.equals(o2)))（大于等于三次判断）
return false;
}
return !(e1.hasNext() || e2.hasNext()); //长度是否一致
}
下面我们再看一下 Vector类的 hashCode()方法的源代码：
/**
* Returns the hash code value for this Vector.
*/
public synchronized int hashCode() {
return super.hashCode(); //他也是调用的父类AbstractList的hashCode
}
/**
* Returns the hash code value for this list.
*
* <p>This implementation uses exactly the code that is used to define the
* list hash function in the documentation for the {@link List#hashCode}
* method.
*
* @return the hash code value for this list
*/
public int hashCode() {
int hashCode = 1;
Iterator<E> i = iterator();
while (i.hasNext()) {
E obj = i.next();
hashCode = 31*hashCode + (obj==null ? 0 : obj.hashCode());
// 在这里用obj，比用i.next()效率高一倍（一直没明白他这为什么用31）
}
return hashCode;
}