Files and folders may reside in the project directory structure without belonging to your project:
In Project Explorer's directory view, you have the option of displaying all files and folders that reside in the project directory structure, even if they do not belong to the project. The icon for each item denotes whether it currently belongs to the project.
To toggle the display of nonproject items in the project directory structure
For information about adding a nonproject file or folder to the project, see Adding a File and Adding a Folder, respectively.
Visual J++ uses a project filter to determine which types of files typically belong to a Java project. The project filter is an inclusive filter, meaning that the extensions in the filter identify valid project files. The following extensions are included in the project filter:
When a file is added to the project directory structure from outside the Visual J++ environment, the project filter is used to determine whether the file actually belongs to the project.
Important When you add a file within the Visual J++ environment, the file is always added to the project, regardless of its file type.