Hi, Xiuyuan, the method <code> public static List unmodifiableList(List list) Returns an unmodifiable view of the specified list. This method allows modules to provide users with "read-only" access to internal lists. Query operations on the returned list "read through" to the specified list, and attempts to modify the returned list, whether direct or via its iterator, result in an UnsupportedOperationException. </code> Since you only modify the original list, so there is no UnsupportedOperatonException. If you try to modify the returned list, the exception will be thrown. Hope it helps,
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