While creating an extension of SwitchableField, i needed to override 2 classes that might be improved:

1. SwitchableFieldFactory might use the D class parameter to extend AbstractFieldFactory and D might extend SwitchableFieldDefinition (which leads to SwitchableFieldFactory<D extends SwitchableFieldDefinition> extends AbstractFieldFactory<D, PropertysetItem>)
2. SwitchableField might declare its member fieldMap as protected or provide a getFieldByName() method