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

From:
Lew <conrad@lewscanon.com.invalid>
Newsgroups:
comp.lang.java.help
Date:
Fri, 23 May 2008 00:44:16 -0400
Message-ID:
<g_Wdne1Gj7Y91qvVnZ2dnUVZ_uOdnZ2d@comcast.com>
Giordano wrote, quoted or indirectly quoted someone who said :

ArrayList<K> list = new ArrayList<K>();
/* ERROR --> */ java.util.Collections.sort ( list );


Roedy Green wrote:

hint. Look at the Javadoc for Collections.sort.

It says:

public static <T extends Comparable<? super T>> void sort(List<T>
list)

In other words T had bloody well better implement Comparable in a
generic way.


What do you mean by "implement Comparable in a generic way"?

Here's a prototypical use of Collections.sort(), with T = String,
output:

tangled: { first, last, always, never, maybe, }
  sorted: { always, first, last, maybe, never, }

<sscce name="SortOf.java">
package testit;

import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collections;
import java.util.List;

public class SortOf
{
   private static final String [] TANGLED = new String[]
        { "first", "last", "always", "never", "maybe", };

   static <T> void show( List <T> list )
   {
     System.out.print( "{ " );
     for ( T t : list )
     {
         System.out.print( t );
         System.out.print( ", " );
     }
     System.out.println( "}" );
   }

   public void demonstrate()
   {
     List<String> strangs = new ArrayList <String> ();
     strangs.addAll( Arrays.asList( TANGLED ));

     System.out.print( "tangled: " );
     show( strangs );

     Collections.sort( strangs );

     System.out.print( " sorted: " );
     show( strangs );
   }

   public static void main( String[] args )
   {
     (new SortOf()).demonstrate();
   }
}
</sscce>

--
Lew

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