Re: return a Collection (Set or List)

Roedy Green wrote:
> On Tue, 13 May 2008 11:29:57 GMT, thufir <hawat.thufir@[EMAIL PROTECTED]
>
> wrote, quoted or indirectly quoted someone who said :
> 
>> found   : java.util.Collection<a00720398.data.Guest>
>> required: java.util.List<a00720398.data.Guest>
>>        guests = console.scanFile(new File("Guests.txt"), new
GuestFactory
>> ());
>>                                 ^
> 
> All Lists are Collections but not all Collections are Lists.  Java
> insists on an explicit cast when the conversion might conceivably
> fail, e.g. Collection -> List.
> 
> You need to have your method return a List<Guest> or you need to cast
> the assignment with:  
> 
> guests  = (List <Guest>)xxxx;

This works, but is a little hackish.  As you say, sometimes you will 
return a Set.  The above will fail if you do return a set.

Consider substituting two methods for this one.  "getAsList" and 
"getAsSet" Then the caller can indicate which type is desired and always 
receive the correct type.

You can use built-in methods to transform Lists to Sets and vice versa. 
  It's usually just a couple of method calls.  Actually you could do 
this externally too and not have to touch your class.  Always get a 
list.  If you need a set, just transform the List into a Set.

You could always just:

    List myList
    myList = getSomeList();
    Set mySet = new SomeSet();
    mySet.addAll( myList );

This feels a bit safer to me than casting something that might fail.