$ javac RoundTrip.java Fx.java   (no flags)     (27.0.1.0.2+1)
src/Fx.java:27: error: PEM is a preview API and is disabled by default.
        if (o instanceof java.security.PEM p) return "PEM(type=" + p.type() + ") - NOT decoded";
                                      ^
  (use --enable-preview to enable preview APIs)
src/RoundTrip.java:10: error: PEMDecoder is a preview API and is disabled by default.
        PEMDecoder decoder = PEMDecoder.of();
        ^
  (use --enable-preview to enable preview APIs)
src/RoundTrip.java:10: error: PEMDecoder is a preview API and is disabled by default.
        PEMDecoder decoder = PEMDecoder.of();
                             ^
  (use --enable-preview to enable preview APIs)
src/RoundTrip.java:11: error: PEMEncoder is a preview API and is disabled by default.
        PEMEncoder encoder = PEMEncoder.of();
        ^
  (use --enable-preview to enable preview APIs)
src/RoundTrip.java:11: error: PEMEncoder is a preview API and is disabled by default.
        PEMEncoder encoder = PEMEncoder.of();
                             ^
  (use --enable-preview to enable preview APIs)
5 errors
exit=1

$ java RoundTrip   (compiled with --enable-preview, run without it)     (27.0.1.0.2+1)
Error: LinkageError occurred while loading main class RoundTrip
	java.lang.UnsupportedClassVersionError: Preview features are not enabled for RoundTrip (class file version 71.65535). Try running with '--enable-preview'

$ javac --enable-preview --release 27 RoundTrip.java Fx.java
src/Fx.java:27: warning: [preview] PEM is a preview API and may be removed in a future release.
        if (o instanceof java.security.PEM p) return "PEM(type=" + p.type() + ") - NOT decoded";
                                      ^
src/RoundTrip.java:10: warning: [preview] PEMDecoder is a preview API and may be removed in a future release.
        PEMDecoder decoder = PEMDecoder.of();
        ^
src/RoundTrip.java:10: warning: [preview] PEMDecoder is a preview API and may be removed in a future release.
        PEMDecoder decoder = PEMDecoder.of();
                             ^
src/RoundTrip.java:11: warning: [preview] PEMEncoder is a preview API and may be removed in a future release.
        PEMEncoder encoder = PEMEncoder.of();
        ^
exit=0