使用泛型时不兼容的类型

Question

I have a class that looks like this:

public class TrafficReportMapper implements RowMapper<TrafficReport> {

   @Override
   public TrafficReport mapRow(ResultSet rs, int rowNum) {
     // Do something here
     return new TrafficReport(....);
   }
}

TrafficReport is a POJO and I want to extend it with something like DayTrafficReport and to create a new mapper that extends TrafficReportMapper . So I changed TrafficReportMapper as follows:

public class TrafficReportMapper<T extends TrafficReport> implements RowMapper<T> {

   @Override
   public T mapRow(ResultSet rs, int rowNum) {
     // Exactly the same code as before
     return new TrafficReport(....);
   }
}

Only this time I get

Incompatible Types: Required T; found TrafficReport

But the thing is that TrafficReport must be of type T . I'm not sure what I'm missing here.

Answer 1

You can declare your class as follows:

public class TrafficReportMapper<T extends TrafficReport> implements RowMapper<TrafficReport> 

... assuming:

interface RowMapper<T>  {
    public T mapRow(ResultSet rs, int rowNum);
    ...
}

Then if you have:

class TrafficReport{}

...and...

class DayTrafficReport extends TrafficReport{}

You can initialize as such:

TrafficReportMapper<DayTrafficReport> trm = new TrafficReportMapper<DayTrafficReport>();

Answer 2

If you create TrafficReportMapper<DayTrafficReport> your method will become:

@Override
public DayTrafficReport mapRow(ResultSet rs, int rowNum) {
    return new TrafficReport(....);
}

(not really since generics get erased, but it's a good way to think about it.)

But TrafficReport is not a subtype of DayTrafficReport , so you can't return it like this.