warning: [options] source value 8 is obsolete and will be removed in a future release
warning: [options] target value 8 is obsolete and will be removed in a future release
warning: [options] To suppress warnings about obsolete options, use -Xlint:-options.
negative\PrivateInterface.java:3: error: private interface methods are not supported in -source 8
    private String helper() { return "Java 9+"; }
                         ^
  (use -source 9 or higher to enable private interface methods)
1 error
3 warnings
