code: public static <K, V> Map<K, V> singletonMap(final K key, final V value) {
  		return Collections.singletonMap(key, value);
  	}
docs: 返回一个只含一个元素但结构特殊的Map，节约空间.
  
  注意返回的Map不可写, 写入会抛出UnsupportedOperationException.
  
  @see java.util.Collections#singletonMap(Object, Object)