generics problem

[phil mitchel]

Hi all,

Hope I got the right forum - let me know if this is the wrong place.

Here's my problem: in the code below the Client has a compile time error:
Type mismatch: cannot convert from Joiner<capture#1-of ?> to Joiner<File>

My understanding of <?> is 'we don't know what type yet' so what I can't understand is why <file> can't be a type of <?>
By a similar token I've tried making hte return type of the abstract method in the enum factory <Object> - still won't work, you get compile time error in the enum factory that <File> is an incompatable return type.

Perhaps I'm missing some fundamental concept here but I've been googling it all day and I'm totally at a loss - can anyone explain this to me ??

Many thanks in advance.

Phil

  

public interface Joiner<T>{ 
        void addTable(T table,String name) throws IllegalArgumentException; 
        void addJoin(String tName1,int index1, String tName2,int index2,Comparator c)throws IllegalArgumentException; 
        T getResults ();         
} 
 
public enum JoinerFactory {         
        CSVJoiner        {public Joiner<File> getInstance(){return new CSVJoiner();}}; 
        public abstract Joiner<?> getInstance(); 
} 

class CSVJoiner implements Joiner<File>{ 
        public void addTable(File table, String name) {} 
        public void addJoin(String name1, int index1, String name2, int index2,Comparator c) {} 
        public File getResults() {return null;} 
} 

public class Client { 
        public static void main(String [] args){ 
                 Joiner<File> csvJoiner = JoinerFactory.CSVJoiner.getInstance(); // this is where the problem is
        } 
} 
 

[Garrett Rowe]

The abstract method in the JoinerFactory enum declares that it returns a Joiner<?>, even though the CSVJoiner instance of JoinerEnum returns a more specific type, the public interface only exposes the method that returns a Joiner<?>, that's why you're getting the compiler error. If you declare the abstract method to return a Joiner<File>, the error goes away. But that takes away the option for you to return a different type from another enum instance.

One workaround is to use a version of the Typesafe Enum Pattern:

abstract class JoinerFactory<T> {

   private JoinerFactory(){}

   public static JoinerFactory<File> CSVJoiner = new JoinerFactory<File>() {
      public Joiner<File> getInstance(){
         return new CSVJoiner();
      }
   };

   public abstract Joiner<T> getInstance();
}

[phil mitchel]

thanks Garrett - still a bit confused though:

The abstract method in the JoinerFactory enum declares that it returns a Joiner<?>,

 public abstract Joiner<?> getInstance();    

- yep so isn't that saying, I'm going to return a Joiner of some type ?

in which case...

 CSVJoiner        {public Joiner<File> getInstance(){return new CSVJoiner();}};   

is retunring some type - in this case it happens to be of type <File>
- what am I missing here ?

I think from what you're syaing I can infer that <File> does not have an 'isa' relationship with <?> - is this correct ?

[Rob Spoor]

Your enum specifies the return type is Joiner<?>; while in practice this will be Joiner<File>, the compiler does not know that. As far as the compiler is concerned, a Joiner<Integer> could be returned; the ? means that it can be anything.

Since "anything" does not match "File", you will have to cast the Joiner<?> to Joiner<File> (with all dangers if you decide to return a Joiner<String> later), or go for Garrett's solution.

[phil mitchel]

ah, ok think I got it: the compiler only sees the signature of the abstract method, it doesn't see that the CSVJoiner enum overides the getInstance method and returns the <File> so complains because <?> could be anything. Will look into the Typesafe enum suggested.

thanks for the help !