Map和ConcurrentMap

ConcurrentMap为map增加原子方法，用于同步操作。

V putIfAbsent(K key, V value);

如果map中还没有key就添加，有key就直接返回key对应的value

     * <pre>

     *   if (!map.containsKey(key))

     *       return map.put(key, value);

//put方法the previous value associated with key, or null if there was no mapping for key

     *   else

     *       return map.get(key);</pre>

使用示例：确保并发情况下同一个type不好创建多次

List<WebSocketMessageInbound> connects = connections.get(type);

        if (null == connects) {

            connects = new ArrayList<WebSocketMessageInbound>();

            connects=connections.putIfAbsent(type, connects);//connects is null

            connects=connections.get(type);connects is not null

        }

        return connects;

boolean remove(Object key, Object value);

if (map.containsKey(key) &amp;&amp; map.get(key).equals(value)) {

     *       map.remove(key);

     *       return true;

     *   } else return false;

boolean replace(K key, V oldValue, V newValue);

V replace(K key, V value);

Map

put 方法 return：the previous value associated with key, or null if there was no mapping for key

返回的是上一次的值而不是本次set的值

List<WebSocketMessageInbound> connects = connections.get(type);

        if (null == connects) {

            connects = new ArrayList<WebSocketMessageInbound>();

            connects=connections.putIfAbsent(type, connects);//connects is null

            connects=connections.get(type);connects is not null

        }

        return connects;