The ComponentName constructor taking two Strings can be used to refer to a component in another application. But, the first argument is not the package name of the class; it is the package name of the application—the package attribute of the manifest element in that application’s AndroidManifest.xml. So your first example should be
ComponentName cn = new ComponentName("de.zordid.sampleapp",
"de.zordid.sampleapp.widget.WidgetProvider");
That constructor could certainly be used to refer to components in your own application, but since you already have hold of a Context from your own application you might as well use it and use one of the other constructors. In my opinion, the one taking a Class should be preferred whenever usable. You could use the one taking a String if you only know the class dynamically for some reason; in that case, it should take the fully-qualified class name as above.