You need a class with a main() method, and a manifest file that says which class in the jar it is.
If your class is named this.that.Bootstrap, your manifest will look something like this:
Main-Class: this/that/Bootstrap
then jar the class file together with the manifest. For the manifest to be read in properly, you need to mark it specially:
where the 'm' flag introduces the manifest.
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Michael Ernest, co-author of:
The Complete Java 2 Certification Study Guide