Problem when trying to use generics

Question

I have a class

public abstract class AbstractE<T, E extends Enum<E> & Flags>
{
     public interface Flags{} /*marker interface*/
     //... 
     //other code
}

and an interface

public interface IXYZAdapter
{
   public <E extends Enum<E> & Flags> Set<E> getFlags();
}

Where Flags is an interface defined in AbstractE itself.

M extends AbstractE thus:

public class M extends AbstractE<Long, M.EF> implements IXYZAdapter
{
    public enum EF implements AbstractE.Flags{flag1, flag2}
    @Override /*from IXYZAdapter*/
    public Set<M.EF> getFlags()
    {return EnumSet.allOf(EF.class);}
    
}

Now, from the main code, I try to get a handle on the interface IXYZAdapter and invoke the getFlags method

IXYZAdapter adapter = (IXYZAdapter)m; //where m is an instance of AbstractE
Set s = adapter.getFlags();

I get the following compile time error in the main program last line (Set s = adapter.getFlags();)

invalid inferred types for E; inferred type does not conform to declared bound(s)
inferred: AbstractE.Flags

bound(s): java.lang.Enum<AbstractE.Flags>,AbstractE.Flags

What am I doing wrong? I am using Java 6

Edited to specify the error location

When I try to compile, the line Set s = adapter.getFlags() - generates the error — Dhan
– Dhan, Commented Apr 5, 2011 at 6:50
what is return new EnumSet.allOf(EF.class); supposed to mean/do? — davogotland
– davogotland, Commented Apr 5, 2011 at 7:00
I removed the "new" keyword before EnumSet.allOf It was typed in there by mistake — Dhan
– Dhan, Commented Apr 5, 2011 at 7:11
also your first line public abstract class AbstractE<T, E extends Enum<E> & Flags> gives me compiler error, it needs to be public abstract class AbstractE<T, E extends Enum<E> & AbstractE.Flags>. please use copy/paste when asking this type of intricate question :) — davogotland
– davogotland, Commented Apr 5, 2011 at 7:12

Thomas · Accepted Answer · 2011-04-05 07:48:40Z

2

Try this:

public interface IXYZAdapter <E extends Enum<E> & AbstractE.Flags>
{
   public Set<E> getFlags();
}

And

public class M extends AbstractE<Long, M.EF> implements IXYZAdapter<M.EF> 
{
}

Or

Set<M.EF> s = adapter.getFlags();

The problem is that with Set s = adapter.getFlags(); The system doesn't know which type to infer for E in IXYZAdapter and thus the E in AbstractE doesn't match.

Edit:

Another option might be:

interface IXYZAdapter <E extends Enum<E> & AbstractE.Flags>
{
  public Set<? extends E> getFlags();
}

class M extends AbstractE<Long, M.EF> implements IXYZAdapter<M.EF>
{
  public enum EF implements AbstractE.Flags{flag1, flag2}
  public Set<? extends M.EF> getFlags()
  {return EnumSet.allOf(EF.class);}

}

And the call: Set<? extends AbstractE.Flags> s = adapter.getFlags();

This would allow you to get a set of flags without casting and force the flags to be declared as enum.

edited Apr 5, 2011 at 7:48

answered Apr 5, 2011 at 6:52

Thomas

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2 Comments

davogotland Over a year ago

both of these solutions require that the programmer knows the enum type when calling getFlags, or else there are at least warnings.

davogotland Over a year ago

well, that's not really true actually. depends on how you write the main method :) i'll give an example!

davogotland · Accepted Answer · 2011-04-05 07:27:37Z

0

using the first solution thomas provided, the main method can be written like this to become warning free without actually having to know about the enum type:

public static void main(String[] args) {
    M m = new M();
    IXYZAdapter<?> adapter = (IXYZAdapter<?>)m;
    Set<?> flags = adapter.getFlags();
    Iterator<?> it = flags.iterator();

    while(it.hasNext()) {
        System.out.println(it.next());
    }
}

answered Apr 5, 2011 at 7:27

davogotland

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3 Comments

Thomas Over a year ago

You don't get warnings but you'd still have to cast the elements of the set to AbstractE.Flags. In that case I'd change the signature to Set<? extends AbstractE.Flags> getFlags(); and then call Set<? extends AbstractE.Flags> s = adapter.getFlags();.

davogotland Over a year ago

could you elaborate? this code worked fine, and i got the names of the enums to print this way. it sounds as though you imply that the above code shouldn't work. am i missing something?

Thomas Over a year ago

No, the code above works. But try this: AbstractE.Flags f = it.next();. You'd have to do this in your case: AbstractE.Flags f = (AbstractE.Flags)it.next();

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