Executable JAR Trouble

I've been having problems with creating executable jar's. I'm doing it by putting my class in a folder, creating a file called manifest.mf defining the version as 1.5 and the main class as Example.class and putting it in with it Example.class. I go through the cmd prompt and type

jar cvfm Example.jar manifest.mf Example.class

and it creates the jar file. However, when I click on it, I get an error saying a fatal error has occured.

[456 byte] By [etgohomeoka] at [2007-11-27 10:53:07]