Is it possible to ignore J if only one line is selected?

Question

Today I made a macro that involved selecting a few lines to join. The problem was that sometimes I only selected parts of one line and in this case two lines was still joined with J. Is it possible to avoid this?

So with which key combination can I do these two transformations: (< and > indicates selection)

content <of line 1
content of> line 2

Should be joined like this after pressing gJ:

content of line 1content of line2

And this

all <content on> one line

Should stay the same after pressing gJ. For now it joins the current line with the next one which is not what I want.

Answer 1

As :join[!] command does not really do anything if range has equal start and end values, you can make use of it instead of original J / gJ

xnoremap <silent>J  :join<CR>
xnoremap <silent>gJ :join!<CR>