Java sorting collections with generics

Question

I get a random collection and would like to sort it using generics. As an example, see the code below:

abstract class Foo<T extends Bacon>{
  public Foo(Collection<T> bacons){
    List sorted = Util.asSortedList(bacons, Util.BACON_COMPARATOR);
  }
}

class Util{
  public static final Comparator<Bacon> BACON_COMPARATOR = new Comparator<Bacon>() {
      @Override
      public int compare(final Bacon one, final Bacon two) {
          return one.getId().compareTo(two.getId());
      }
  };

  public static <T> List<T> asSortedList(Collection<T> c, Comparator<T> comparator) {
      List<T> list = new ArrayList<T>(c);
      Collections.sort(list,comparator);
      return list;
  }
}

Please note that other types (non-Bacon) may be passed into asSortedList(). How do I correctly modify this scenario so that the Bacon example works, and I can still pass in other types to asSortedList?

The code shown above gives me the following error:

The method asSortedList(Collection<T>, Comparator<T>) in the type Util is not applicable for the arguments (Collection<T>, Comparator<Bacon>)

That looks good. What does not work? Do you get any compile errors for that? — Thilo
– Thilo, Commented Dec 18, 2014 at 1:30

Louis Wasserman · Accepted Answer · 2014-12-18 01:45:28Z

5

Just change asSortedList to

public static <T> List<T> asSortedList(
    Collection<T> c, Comparator<? super T> comparator)

...since T is a subtype of Bacon, and your Comparator is a Comparator<Bacon>.

answered Dec 18, 2014 at 1:45

Louis Wasserman

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