In the generated classes, some of the extends clauses are generated as "extends someNS:someClass", which is erroneous. However, some other classes are generated without such error (which is generated as "extends someNS\someClass").
To reproduce this, just import the GML feature schema:
<xsd:import namespace="http://www.opengis.net/gml" schemaLocation="http://schemas.opengis.net/gml/2.1.2/feature.xsd" />