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

I had the same problem, and it turns out that the .jar file association in the registry was broken. The following steps fixed it:

Open the Registry Editor, and navigate to HKEY_CLASSES_ROOT\jarfile\shell\open\command. Modify the value of the Default key as follows: "[Location of your JRE]\bin\javaw.exe" -jar "%1" %*
Replace [Location of your JRE] with the root directory of your JRE installation.
For example:
"C:\Program Files\Java\jre7\bin\javaw.exe" -jar "%1" %*
Repeat the above steps for HKEY_LOCAL_MACHINE\SOFTWARE\Classes\jarfile\shell\open\command.

I have a Windows 7 machine, but I assume this would work for 32-bit environments as well.

EDIT: Fixed the path in the example - it should use the "Program Files" directory and not "Program Files (x86)".