code: public static <K, V> Map<K, V> unmodifiableMap(final Map<? extends K, ? extends V> m) {
  		return Collections.unmodifiableMap(m);
  	}
docs: 返回包装后不可修改的Map.
  
  如果尝试修改，会抛出UnsupportedOperationException
  
  @see java.util.Collections#unmodifiableMap(Map)