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That is the entry for .jar
Now what is in the jarfile entry?

It is possible to change the registry by using the regedit program. It is possible to view the registry without making changes to it.
Open regedit, find the ".jar" entry, open that entry and...

There needs to be an entry in the registry that gives the OS a commandline to execute when a file is double clicked.

You don't see the commandline, you should see the name associated with a commandline. If you look in the registry for that type of file you will see the list of names with their commandlines.
...

It's not called a path entry. There are entries in the registry for all the file extensions that the OS knows about. Associated with each extension is a list of command lines that can be used to...

The Environment variable: PATH is used to find a command file like: java.exe. It doesn't associate a command line with a filetype like: jar.
The registry has an entry for .jar file which refers to...

Do you know how to see what commandline the OS uses when you double click on a file?
Win7 has an interface to associate extensions with commands. It doesn't work on my system. I use the regedit...

If you can manually execute it, it could be the OS's registry entry for executing it is messed up.

To copy the contents of the command prompt window:
Click on Icon in upper left corner
Select...

I've often had problems with java installs walking over my existing registry entries. I found a quick workaround that allows me to restore my entries after some product changed some of the registry....

To see if there are any errors, open a command prompt window, change to the directory with the jar file and enter: java -jar THEJARFILENAME.jar
Copy the full contents of the window and paste it...

The OS should be setup to use the java -jar command when a jar file is double clicked.
What OS are you on?
Do you have a JRE installed?

What problems are you having? Were there error messages? copy the full text and paste it here.