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From: Lew <noone@lewscanon.com>
Newsgroups: comp.lang.java.programmer
Subject: Re: Novice to Generics Trying to Implement a Generic Priority Queue
Date: Fri, 08 Apr 2011 20:39:25 -0400
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markspace wrote:
> KevinSimonson wrote:
>>    public PriorityQueue ( int size)
>>    {
>>      if (0<= size)
>>      { @SupressWarnings( "unchecked")
>>        queue       = (Da[]) new Object[ size];
>
>> PriorityQueue.java:14:<identifier>  expected
>>        queue       = (Da[]) new Object[ size];

Kevin, you really do a fine job of posting a question with a good example, 
well presented.

> Interesting. I thought I could put that annotaion on an assignment. I guess
> not. Oh well.

Annotations are allowed only at declarations.

>     public PriorityQueue(int size) throws BadSizeException {
>         if (0 <= size) {
>             @SuppressWarnings("unchecked")
>             Da[] temp = (Da[]) new Object[ size ];
>             queue = temp;
>             nmbrEntries = 0;
>         } else {...

>>    private static boolean inOrder ( Da left
>>                                   , Da right)
>>    {
>>      return left.compareTo< ? super Da>( right)<= 0;
>>    }
>
>> PriorityQueue.java:25: illegal start of expression
>>      return left.compareTo<  ? super Da>( right)<= 0;

> This bit goes on the declaration, not the invocation ;-)
>
> public class PriorityQueue<Da extends Comparable<? super Da>> {...

In the actual use, you don't need any angle-brackety stuff:

  private static boolean inOrder( Da left, Da right)
  {
    return left.compareTo( right ) <= 0;
  }

You do, however, have to guard against a possible 'NullPointerException'.  The 
method is 'private', so it's up to its callers not to screw that up.  You 
enforce that with an assertion:

  private static boolean inOrder( Da left, Da right)
  {
    assert left != null;
    return left.compareTo( right ) <= 0;
  }

If the method were 'public' you'd need to add an explicit guard against the 
'NullPointerException':

  public static boolean inOrder( Da left, Da right)
  {
    if ( left == null )
    {
      return true;
    }
    assert left != null;
    return left.compareTo( right ) <= 0;
  }

The assertion is rather trivial here, so you could simply:

  public static boolean inOrder( Da left, Da right)
  {
    return left == null || left.compareTo( right ) <= 0;
  }

Don't forget your Javadocs!

-- 
Lew
Honi soit qui mal y pense.
http://upload.wikimedia.org/wikipedia/commons/c/cf/Friz.jpg