Follow the hat !

The Eclipse UI doesn't provide an option (contextual menu ...) to convert a "classical" project to a Java project, yet we often find that with this issue (forget when creating the project, Import ...). This option may have been present when writing a right-click on the project > properties > natures of the project (may be a bug Eclipse).

There is a simple way to solve the problem, without closing your project or even Eclipse. Simply open your .project file and to add the following line:

<natures>
...
<nature>org.eclipse.jdt.core.javanature</nature>
...
</natures>

So much for this little trick.