Re: Polymorfic Collection construction

From:

"Oliver Wong" <owong@castortech.com>

Newsgroups:

comp.lang.java.programmer

Date:

Fri, 28 Apr 2006 20:30:23 GMT

Message-ID:

<zXu4g.318$cZ3.43@clgrps13>

<acamposr@gmail.com> wrote in message
news:1146231735.021101.230440@g10g2000cwb.googlegroups.com...

Hello. I want to implement a method for filtering a class:

public Collection filter(Collection c) {

   Collection filteredCollection = "An object of the same class that
c"

   for(Object obj : c) {
      if(satisfiesSomeCondition(obj))
          filteredCollection.add(obj)
   }

   return filteredCollection;
}

As you see, I want to get any kind of Collection and create a new
collection (of the same type that the first one). How can I do that?

    You could cop out and ask the user to provide you with the instance of
collection to populate.

<code tested="false">
public <T extends Collection> T filter(T collectionToReadFrom, T
collectionToPopulate) {
  /*Weird things happen if collectionToReadFrom == collectionToPopulate*/
  for(Object obj : collectionToReadFrom) {
    if(satisfiesSomeCondition(obj)) {
      collectionToPopulate.add(obj);
    }
  }
  return collectionToPopulate;
}

....

ArrayList input = /*whatever*/;
ArrayList output = filter(input, new ArrayList());
</code>

    - Oliver