title: How to convert a folder to a jar file (MAC)?
body: <p>I need to add a jar file to my Eclipse project, but all I've got is the folder with the necessary Java files. 
  I found a way to convert from folder to jar: compress the folder and change the extension to .jar. But when I add it to my project, the jar file seems to be empty (no Java files).</p>
  
  <p>Is there a better way to obtain a jar file or to fix this problem?</p>