Preserve whitespace while joining in Vim

I have this text:

abc
   def

There are 3 spaces on the line before "def" and I want to make sure that space is preserved.

I usually join with J in normal mode. There is, however, a :join command that behaves the same way except you can follow it with an optional !.

By using the abbreviation it is shortened to:

:j!

And the result I get is:

abc   def

Space preserved!

See :help :join for more information.