Map中的Null key, Null Value

ConcurrentHashMap的key和value都不能为null,否则抛NullPointerException
java.util.concurrent.ConcurrentHashMap.put(K, V)

    /**
     * Maps the specified key to the specified value in this table.
     * Neither the key nor the value can be null.
     *
     * <p> The value can be retrieved by calling the <tt>get</tt> method
     * with a key that is equal to the original key.
     *
     * @param key key with which the specified value is to be associated
     * @param value value to be associated with the specified key
     * @return the previous value associated with <tt>key</tt>, or
     *         <tt>null</tt> if there was no mapping for <tt>key</tt>
     * @throws NullPointerException if the specified key or value is null
     */
    public V put(K key, V value) {
        if (value == null)
            throw new NullPointerException();
        int hash = hash(key.hashCode());
        return segmentFor(hash).put(key, hash, value, false);
    }

hashMap和linkedhashMap都允许null key和null value，
treeMap不允许null key，但允许null value

http://www.trinea.cn/android/android-source-code-analysis/concurrenthashmap-nullpointerexception-null-key/