Selection does not contain a main type

When you import a. java file in a Java file system in MyEclipse, an error balloon appears.

With a netizen, the explanation is: "This problem arises because the. java file is not within the SRC path of the project, which means that the source code is not compiled by Eclipse, the bytecode does not exist and cannot be run."
Right-click on the project name-> Builder path-> Configure build path-> Select the source panel, and then click Add Folder.

Then right click on the project name, Builder Path-> use as Source folder.

(There is the wrong place also hope to a netizen pointed out that the level is limited, thank you.) )