What is the IntelliJ shortcut key to create a javadoc comment?
You can use the action 'Fix doc comment'. It doesn't have a default shortcut, but you can assign the Alt+Shift+J shortcut to it in the Keymap, because this shortcut isn't used for anything else.
By default, you can also press Ctrl+Shift+A two times and begin typing Fix doc comment
in order to find the action.
Typing /**
+ then pressing Enter above a method signature will create Javadoc stubs for you.