Re: return a Collection (Set or List)

Mark Space <>
Tue, 13 May 2008 11:37:53 -0700
Roedy Green wrote:

On Tue, 13 May 2008 11:29:57 GMT, thufir <>
wrote, quoted or indirectly quoted someone who said :

found : java.util.Collection<>
required: java.util.List<>
       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.

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