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;
    }