Class has been compiled by a more recent version of the Java Environment

This is just a version mismatch. You have compiled your code using java version 9 and your current JRE is version 8. Try upgrading your JRE to 9.

49 = Java 5
50 = Java 6
51 = Java 7
52 = Java 8
53 = Java 9
54 = Java 10
55 = Java 11
56 = Java 12
57 = Java 13
58 = Java 14
59 = Java 15
60 = Java 16
61 = Java 17
62 = Java 18
63 = Java 19

IDE: Eclipse Oxygen.3

To temporarily correct the problem do the following:

Project menu > Properties > Java Compiler > Compiler compliance level > 1.8

A permanent fix likely involves installing JDK 9.

FYI 1.8 is what Java 8 is called.

Side bar

I recently returned to Java after a foray into C# (a breath of fresh air) and installed Eclipse Oxygen onto a clean system that had never had Java installed on it before. This default everything with a brand new install of Eclipse Oxygen yet somehow or other Eclipse can't get its own parameters to match the jdk that's installed. This is the second project I created and the second time I ran into this headache. Time to go back to C#?

Related Question

has been compiled by a more recent version of the Java Runtime (class file version 53.0)

You can try this way

javac --release 8 yourClass.java