Warning when _ used as an identifier in Java 8. Compilation error in Java 9 (though, bytecode from Java 8 is executed).

2017-11-22.log

alexandre ~/keyword $ /usr/lib/jvm/java-8-oracle/bin/javac Keyword.java
Keyword.java:3: warning: '_' used as an identifier
		int _ = 1;
		    ^
  (use of '_' as an identifier might not be supported in releases after Java SE 8)
Keyword.java:4: warning: '_' used as an identifier
		System.out.println(_);
		                   ^
  (use of '_' as an identifier might not be supported in releases after Java SE 8)
2 warnings
alexandre ~/keyword $ /usr/lib/jvm/java-8-oracle/bin/java Keyword
1
alexandre ~/keyword $ /usr/lib/jvm/java-9-oracle/bin/java Keyword
1
alexandre ~/keyword $ /usr/lib/jvm/java-9-oracle/bin/javac Keyword.java
Keyword.java:3: error: as of release 9, '_' is a keyword, and may not be used as an identifier
		int _ = 1;
		    ^
Keyword.java:4: error: as of release 9, '_' is a keyword, and may not be used as an identifier
		System.out.println(_);
		                   ^
2 errors

Keyword.java

public class Keyword {
	public static void main(String[] args) {
		int _ = 1;
		System.out.println(_);
	}
}