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;    }