I've installed 3 and 3.5 user app before without issue.

However, I would usually run the .bin file and install everything at
once.

This time, I'm actually reading the 3.5.1 install instructions which
indicate:

1) install java
2) Run the mysql/jboss install
3) Create the user app driver
4) Then run the actual IDM install by typing: java -jar IdmUserApp.jar
(section 5.6)

However, when I try that (it says to make sure I'm not doing this as
root, so I'm not).

I get a:

bash: java: command not found