class MyObject {
    private Long myField;
    ....
}

class MyMapper {
    @Mapping(target="myField", source="myField", defaultValue="0L")
    abstract MyObject map(MyOtherObject obj);
}

class MyMapperImpl {
    abstract MyObject map(MyOtherObject obj) {
        ...
        if(obj.getMyField() != null) {
            myObject.setMyField( obj.getMyField() );
        } else {
            obj.setMyField( (long) 0L );         //   <--- This cast is unnecessary and produces a warning when compiling with -Xlint:all
        }
    }
}