SOLVED: Serial monitor input error under Linux

Just starting out with Arduino on a basic keypad sketch.
Version 1.8.13 under Linux
Getting this error when attempting to print a value to the serial monitor:

Exception in thread "EventThread /dev/ttyACM0" java.lang.NoSuchMethodError: java.nio.ByteBuffer.flip()Ljava/nio/ByteBuffer;
at jssc.SerialPort$

I have given myself permission to uucp which is the group for /dev/ttyACM0 so that does not appear to be the problem. Any assistance would be appreciated.

EDIT: found solution - inconsistent java pathnames in ArchLinux java jdk as per:

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