Changing Eclipse project into a Java Project

In the .project file add java nature and builders:

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

and in the .classpath file add Java libs reference:

<classpath>
    <classpathentry kind="con" path="org.eclipse.jdt.launching.JRE_CONTAINER"/>
</classpath>