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From: Daniele Futtorovic <da.futt.news@laposte-dot-net.invalid>
Newsgroups: comp.lang.java.programmer
Subject: Re: Novice to Generics Trying to Implement a Generic Priority Queue
Date: Mon, 11 Apr 2011 21:42:28 +0200
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On 11/04/2011 21:10, KevinSimonson allegedly wrote:
> On Apr 11, 11:52 am, r...@zedat.fu-berlin.de (Stefan Ram) wrote:
>> KevinSimonson<kvnsm...@hotmail.com>  writes:
>>> Exception in thread "main" java.lang.ClassCastException:
>>> [Ljava.lang.Object; cannot be cast to [Ljava.lang.Comparable;
>>
>> ( Da[] )new java.lang.Comparable[ size ]
>
> Stefan, thanks!  That solved the problem and my program works just
> fine now.

This might be somewhat OK in this case, but it's hardly advisable.

A Comparable[] /is not a/ Da[].

You'd normally pass the Class object around in such cases:

   public PriorityQueue( Class<Da> component, int size )
     throws BadSizeException
   {
     if (0<= size)
     { queue = (Da[]) Array.newInstance( component, size );
        nmbrEntries = 0;
     }
     else  {
       throw new BadSizeException();
     }
   }

Since the construction becomes a bit awkward, you can add a factory method:

public static <T extends Comparable> PriorityQueue<T> newQueue( Class<T> 
type, int size )
   throws BadSizeException
{
   return new PriorityQueue<T>( type, size );
}

(Not compiled.)


>
> Kevin Simonson
>
>
> ##############################################################################
>
> Script started on Mon Apr 11 13:03:16 2011
> sh-4.1$ java IntPq 10 314 159 265 358 979 323 846 264 338 327 l
> Added entry 314.
> Added entry 159.
> Added entry 265.
> Added entry 358.
> Added entry 979.
> Added entry 323.
> Added entry 846.
> Added entry 264.
> Added entry 338.
> Added entry 327.
> 0: [979]
>    1: [358]
>      3: [338]
>        7: [159]
>        8: [264]
>      4: [327]
>        9: [314]
>    2: [846]
>      5: [265]
>      6: [323]
> sh-4.1$ cat PriorityQueue.java
> public class PriorityQueue<  Da extends Comparable>
> {
>    public static class BadSizeException   extends Exception {}
>    public static class UnderflowException extends Exception {}
>    public static class OverflowException  extends Exception {}
>
>    Da[] queue;
>     int nmbrEntries;
>
>    public PriorityQueue ( int size)
>                         throws BadSizeException
>    {
>      if (0<= size)
>      { queue       = (Da[]) new java.lang.Comparable[ size];
>        nmbrEntries = 0;
>      }
>      else
>      { throw new BadSizeException();
>      }
>    }
>
>    public boolean hasEntries ()
>    {
>      return 0<  nmbrEntries;
>    }
>
>    public boolean hasRoom ()
>    {
>      return nmbrEntries<  queue.length;
>    }
>
>    public void addEntry ( Da entry)
>                         throws OverflowException
>    {
>      if (queue.length == nmbrEntries)
>      { throw new OverflowException();
>      }
>      Da parent;
>      int index;
>      int searcher;
>      for ( searcher = nmbrEntries++
>          ;    0<  searcher
>            &&  (parent = queue[ index = searcher - 1>>
> 1]).compareTo( entry)<= 0
>          ; searcher = index)
>      { queue[ searcher] = parent;
>      }
>      queue[ searcher] = entry;
>    }
>
>    public Da extract ()
>                      throws UnderflowException
>    {
>      if (nmbrEntries == 0)
>      { throw new UnderflowException();
>      }
>      Da extractee = queue[ 0];
>      Da rplcmnt   = queue[--nmbrEntries];
>      int searcher = 0;
>      int lastborn;
>      int lrgrChld;
>      for (;;)
>      { lastborn = searcher + 1<<  1;
>        if (nmbrEntries<  lastborn)
>        { break;
>        }
>        lrgrChld
>          =      lastborn<  nmbrEntries
>              &&  queue[ lastborn - 1].compareTo( queue[ lastborn])<= 0
>            ? lastborn
>            : lastborn - 1;
>        if (queue[ lrgrChld].compareTo( rplcmnt)<= 0)
>        { break;
>        }
>        queue[ searcher] = queue[ lrgrChld];
>        searcher         = lrgrChld;
>      }
>      queue[ searcher] = rplcmnt;
>      return extractee;
>    }
>
>    private void listTree ( int subroot
>                          , int indnttn)
>    {
>      if (subroot<  nmbrEntries)
>      { int spc;
>        for (spc = indnttn; 0<  spc; spc--)
>        { System.out.print( ' ');
>        }
>        System.out.println( subroot + ": [" + queue[ subroot] + ']');
>        subroot  = (subroot<<  1) + 1;
>        indnttn += 2;
>        listTree( subroot    , indnttn);
>        listTree( subroot + 1, indnttn);
>      }
>    }
>
>    public void list ()
>    {
>      listTree( 0, 0);
>    }
> }
> sh-4.1$ exit
> exit
> Script done on Mon Apr 11 13:04:29 2011