Re: Generic generics help

"wizard of oz" <>
Fri, 29 Aug 2008 14:02:00 GMT
Thanks to Mark and Lucas for your replies.

You are both of course correct, in fact I simply add the new keys to the row
and column headers. Because they are sets, I don't get any dups.

However the question is still relevant. What if I want to have a single
method that can manipulate either one of the headers and accept a parameter
appropriate to the "type" of header (i.e. an R or C)?

The signature
    private void ensureExists (TreeSet<? extends Object> treeSet, Object
key) {

Allows me to pass either type of header set (row or column), but I can't
pass the parameter, so my question is about how do I get something like
these to work:
    private void someMethod (TreeSet<? extends Object> treeSet, <? extends
Object> key) {
    private void someMethod (TreeSet<? extends Object> treeSet, ? key) {
    private void someMethod (TreeSet<X> treeSet, X key) {

None of the above worked for me.
Another variant is
    private TreeSet<X> subset (TreeSet<X> treeSet, X startKey, X endKey) {
Where X is either an R or C. and the subset returns a subset of values
between the start and end key values.

Surely this is possible!?!?!

"Mark Space" <> wrote in message

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

  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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