I can run .jar files through cmd, but I cannot double click them

Question

First, I am studying Java, but I am still very new to it, so complicated code will go over my head.

When I download a .jar file to say install a mod to minecraft and

Answer 1

If you don't care too much about understanding the underlying problem, this might help:

http://johann.loefflmann.net/en/software/jarfix/index.html

--and a double-clickable .jar needs to have Main-Class correctly set in MANIFEST.MF