ConcurrentSkipListSet的add(E e)方法注释误人子弟！

一个项目用到ConcurrentSkipListSet.add(E e)方法，过程中总觉得元素添加数量有问题，故调试之。


查看add()方法的javadoc，其注释为：
如果此 set 中不包含指定元素，则添加指定元素。更确切地讲，如果此 set 不包含满足 e.equals(e2) 的元素
e2，则向 set 中添加指定的元素 e。如果此 set 已经包含该元素，则调用不更改该 set 并返回
false。


根据注释，只要元素的equals()方法判断不相等就能加入到Set中，可调试发现不是这么回事：

 

public boolean add(E e) {
    return m.putIfAbsent(e, Boolean.TRUE) == null;
}

 
m为 ConcurrentSkipListMap，它的putIfAbsent()方法实现是：

    public V putIfAbsent(K key, V value) {
        if (value == null)
            throw new NullPointerException();
        return doPut(key, value, true);
    }

    private V doPut(K kkey, V value, boolean onlyIfAbsent) {
        Comparable super K> key = comparable(kkey);
        for (;;) {
            Node b = findPredecessor(key);
            Node n = b.next;
            for (;;) {
                if (n != null) {
                    Node f = n.next;
                    if (n != b.next)               // inconsistent read
                        break;;
                    Object v = n.value;
                    if (v == null) {               // n is deleted
                        n.helpDelete(b, f);
                        break;
                    }
                    if (v == n || b.value == null) // b is deleted
                        break;
                    int c = key.compareTo(n.key);
                    if (c > 0) {
                        b = n;
                        n = f;
                        continue;
                    }
                    if (c == 0) {
                        if (onlyIfAbsent || n.casValue(v, value))
                            return (V)v;
                        else
                            break; // restart if lost race to replace value
                    }
                    // else c  z = new Node(kkey, value, n);
                if (!b.casNext(n, z))
                    break;         // restart if lost race to append to b
                int level = randomLevel();
                if (level > 0)
                    insertIndex(z, level);
                return null;
            }
        }
    }

 实际上Set中的添加是根据元素的compareTo()方法在比较！如果compareTo()返回0，就认为2个元素相等，跟equals()方法根本没有关系！JAVA SDK 的JavaDoc也不靠谱啊~~

 

 

评论

1 楼 tsyj810883979 2011-10-21  

/**
     * The underlying map. Uses Boolean.TRUE as value for each
     * element.  This field is declared final for the sake of thread
     * safety, which entails some ugliness in clone()
     */
    private final ConcurrentNavigableMap<E,Object> m;

    /**
     * Constructs a new, empty set that orders its elements according to
     * their {@linkplain Comparable natural ordering}.
     */
    public ConcurrentSkipListSet() {
        m = new ConcurrentSkipListMap<E,Object>();
    }

    /**
     * Constructs a new, empty set that orders its elements according to
     * the specified comparator.
     *
     * @param comparator the comparator that will be used to order this set.
     *        If <tt>null</tt>, the {@linkplain Comparable natural
     *        ordering} of the elements will be used.
     */
    public ConcurrentSkipListSet(Comparator<? super E> comparator) {
        m = new ConcurrentSkipListMap<E,Object>(comparator);
    }
JDK源码中的注释