public Set<String> stringPropertyNames() {
        Hashtable<String, String> h = new Hashtable<>();
        enumerateStringProperties(h);
        return h.keySet();
    }
public Set < String > stringPropertyNames ( ) { Hashtable < String , String > h = new Hashtable < > ( ) ; enumerateStringProperties ( h ) ; return h . keySet ( ) ; }
Returns a set of keys in this property list where
the key and its corresponding value are strings,
including distinct keys in the default property list if a key
of the same name has not already been found from the main
properties list.  Properties whose key or value is not
of type <tt>String</tt> are omitted.
<p>
The returned set is not backed by the <tt>Properties</tt> object.
Changes to this <tt>Properties</tt> are not reflected in the set,
or vice versa.

@return  a set of keys in this property list where
the key and its corresponding value are strings,
including the keys in the default property list.
@see     java.util.Properties#defaults
@since   1.6