The underscore character (_) can no longer be used as a legal identifier name. Earlier attempts to remove the underscore in an identifier name were incomplete. The use of the underscore would generate a combination of errors and warnings. Since Java 9, the warnings are now errors. Consider the following sample code:

public class UnderscoreTest {
  public static void main(String[] args) {
    int _ = 319;
    if ( _ > 300 ) {
      System.out.println("Your value us greater than 300.");
    }
    else {
      System.out.println("Your value is not greater than 300.");
    }
  }
}

The preceding code, in Java 8, will result in compiler warnings for the int _ = 319; and if ( _ > 300) statements. The warning is: As of Java 9, '_' is a keyword, and may not be used as an identifier. So, in Java 9, 10 or 11, you are not able to use the underscore by itself as a legal identifier.