At least you need Java (preferable Oracle Java) to get openHAB going (as it’s based on Java in general). And for using myopenhab you need the latest java version, as the older ones couldn’t handle the right certificate.
So: Yes, it makes sense to install java in advance.
Regarding myopenhab: There was an older service called my.openhab (with a dot in between). It was changed to myopenhab some days ago. The above mentioned tutorial for openHAB 1 still refers to the old my.openhab.
But for a start, you don’t need myopenhab.
And if you start with openHAB2 (and openHABian, as sihui suggests), you don’t have to pay attention to java, as openHABian does this for you automatically.
If you’re only just starting, then I would definitely recommend looking at an openhabian install on the raspberry pi. Takes all the pain out of installing, etc. and lets you get on with the job at hand, ie. automating your home.
Openhabian is expanding too, and will also do setups of grafana, etc. if you want to go down that route later.