Java. Lang. classnotfoundexception: import the existing file system in eclipse

Today, we want to integrate the original GuiCodeIntegrated into a project, but never understood how to import it, and started to try the existing project into workspace under General of import, which never opened, depressed for a long time, copy the trojan directly and then report the error Java. lang. classnotfoundexception: a dialog box is displayed, indicating that there is no main function. I can see that, obviously, I have been entangled. I haven't come up with any solution for a long time. After hitting the ball in the afternoon, I asked the Team Leader about the existing project into workspace under general. After that, right-click the project to be imported, open import, go to the relevant interface, select File System, and then finish step by step. Finally, click Finish to solve the problem.