Re: CIAO, How can I sort an ArrayList<K> of Generic Types ?

Lew <>
Wed, 21 May 2008 22:54:55 -0400
Giordano wrote:

On 21 Mag, 17:02, Vivien Barousse <> wrote:

Collections.sort is expecting a List containing objects implementing the
Comparable interface.

Did your K class implements Comparable ?

Thanks Vivien.. the K isn't a class, but the way to indicate a generic
type (so K could be an Integer, a String or any class you are using at
the moment).

That only works inside a generic definition that uses K as a type parameter.

Why don't you work up an SSCCE
to show us the context in which you are attempting this.

However, at the end of the facts, in my case the K is always
equivalent to a String, so i decide to implement(?).. re-write
everything using the type String (I think that i could write a
comparator class that accept the generic K and then switch into the
right compare() method in the case this K were an Integer, a
Character, a String, etc.. etc...)

switch inside a class to handle type resolution is an antipattern.
Inheritance and polymorphism are the way to do that, not explicit type casts.

You need something like

public class Foo <T extends Comparable <? super T>>
   private List <T> stuff = new ArrayList <T> ();

   public List <T> getStuff()
     return stuff;

   public void sort()
     Collections.sort( stuff );

(Add the right imports to turn the above into an SSCCE.)

Then client code can do things like:

  Foo <Integer> foo = new Foo <Integer> ();
  List <Integer> stuff = foo.getStuff();
  addItemsTo( stuff );

You might have noticed that this adds nothing that List doesn't already have:

  List <Integer> stuff = new ArrayList <Integer> ();
  addItemsTo( stuff );
  Collections.sort( stuff );

In other words, List /already is/ how you can sort an array of generic type.


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