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I just came across a handy way of checking the version of Java that’s installed on a computer. Try typing:
java -version
This seems to work for me on Windows, MacOS and Linux, regardless of the Java vendor (i.e. my RHEL5 box has a GNU version of Java rather than Sun Java, although I’m not sure what happens for the Microsoft Java VM). Of course, if the command is not found, that probably means that you don’t have Java installed (or at least not on your path).