You can open up terminal and simply type
java -version // this will check your jre version javac -version // this will check your java compiler version if you installed
this should show you the version of java installed on the system (assuming that you have set the path of the java in system environment).
And if you haven’t, add it via
and if you unsure if you have java at all on your system just use
find in terminal
find / -name "java"