Generic problem

[Don Kiddick]

Trying to genercise my Entity interface :

 
public interface Entity {
 
    Object getId();
    Comparable getVersion();
 
}

So the getVersion method returns a typed Comparable.

I'm trying this :

 
public interface Entity {
 
    Object getId();
    <T extends Comparable> Comparable<T> getVersion();
 
}

The idea being that the getVersion method returns a Comparable object that is comparable with itself.

with concrete implementation :

  
class Employee implements Entity {
 
    //...
 
    private Long version;
 
    public Long getVersion() {
        return version;
    }
     
    //...
  

I get an error saying I'm not implementing the getVersion method. Ack!
Anyone know how to solve this?

thanks, D.

[Jesper de Jong]

"I get an error saying I'm not implementing the getVersion method. Ack!"

Are you sure? I tried compiling your code and did not get an error, just a warning:

C:\Jesper\workspace\Sandbox\src>javac -Xlint:unchecked org\jesperdj\sandbox\*.java
org\jesperdj\sandbox\Employee.java:5: warning: getVersion() in org.jesperdj.sandbox.Employee implements <T>getVersion() in org.jesperdj.sandbox.Entity; return type requires unchecked conversion
found : java.lang.Long
required: java.lang.Comparable<T>
public Long getVersion() {
^
1 warning

[Ilja Preuss]

If the return type if fixed for each implementation of the interface, you should add the type parameter to the interface:

public interface Entity<T extends Comparable<T>> {
     Object getId();
     T getVersion();
}
 
class Employee implements Entity<Long> {
...

Also note that the above type parameter is the correct declaration of a "type that is comparable to itself". Your declaration was just a "type that is comparable to a (potentially different) comparable type", as far as I can tell.