IntelliJ IDEA quick quoted string plus variable
Quintium correctly pointed out that you can "type out of" the quotes by pressing ".
Let me show you another solution, using a Live Template.
Inside a valid code block (e.g. main method...) where your variable userName is in scope, type the following
soutv
Now press Tab and you will see the following
From here it's easy to select userName as the variable.
If you do this very often you can make your own live templates.
In any case it also saves a lot of time to use the sout live template to quickly get System.out.println()