在Java 9中，下划线关键字的用途是什么？

public class UnderscoreKeywordTest {
   public static void main(String args[]) {
      int _ = 50
      System.out.println(_);
   }
}

UnderscoreKeywordTest.java:3: error: as of release 9, '_' is a keyword, and may not be used as an identifier
int _ = 50;
^
UnderscoreKeywordTest.java:4: error: as of release 9, '_' is a keyword, and may not be used as an identifier
System.out.println(_);