Change Eclipse Project Nature

In my latest Project I was working with an existing Project, which wasn't develoved in Eclipse, so I checked it out and wondered, how to convert this "simple Project" to a Java Project. Googling around spit out the following recipe:

1. Close your project.
2. Open your project file (which is located in your root directory of the appropriate project) called .project with your default editor. Search for

<natures>
</natures>
and change it to
<natures>
<nature>org.eclipse.jdt.core.javanature</nature>
</natures>

Store the changed file.
3. Open the project again - now it's a java project.

... but there are additional changes which have to be performed:

You have to add

<buildCommand>
<name>org.eclipse.jdt.core.javabuilder</name>
<arguments>
</arguments>
</buildCommand>

withing the <buildSpec> Tag