If you created a generic project in Eclipse and would like to turn it into a java project you have to modify the .project file. This is the content of the generic project:

<?xml version="1.0" encoding="UTF-8"?>
<projectDescription>
	<name>de.slopjong.emf.forum</name>
	
	<projects>
	</projects>
	<buildSpec>
	</buildSpec>
	<natures>
	</natures>
</projectDescription>

You have to modify the sections buildSpec and natures.

<buildSpec>
	<buildCommand>
		<name>org.eclipse.jdt.core.javabuilder</name>
	</buildCommand>
</buildSpec>
	<natures>
		<nature>org.eclipse.jdt.core.javanature</nature>
	</natures>

You should then refresh the package explorer to let the changes take effect.