Re: Generic generics help

From:
Mark Space <markspace@sbcglobal.net>
Newsgroups:
comp.lang.java.programmer,comp.lang.java.misc,comp.lang.java,comp.lang.java.help
Date:
Thu, 28 Aug 2008 23:21:50 -0700
Message-ID:
<g984ic$j1n$1@registered.motzarella.org>
wizard of oz wrote:

public class SparseMatrix<R, C, E> {

   private TreeSet<R> rowHeaders = new TreeSet<R> ();
   private TreeSet<C> colHeaders = new TreeSet<C> ();

   public void add (R rowKey, C colKey, E element) {

       ensureExists (rowHeaders, rowKey);
       ensureExists (colHeaders, colKey);
   }

   private void ensureExists (TreeSet<? extends Object> treeSet, Object

The problem is that the compiler is complaining about the treeSet.add


Well, the problem is that "rowHeaders" and "colHeaders" contain types of
R and C respectively, and you're trying to add a type of Object.

If you want to ensure that some type R "rowKey" exists in rowHeaders, then

   boolean exists = rowHeaders.contains( rowKey );

will do the whole thing for you. I think however you are confused as to
what a Set like TreeSet will actually do for you. You seem to be trying
to ensure that the /pair/ R, E exists, in which case you need a Map, not
a Set.

   E tempElement;
   if( (tempElement = rowMap.get( rowKey )) != null ) {
     // the pair (rowKey, element) exist
   }

You might even put rowKey and colKey together in one single object
though and just do look-ups on that. Faster and easier.

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