Re: generic sorting?

From:
Dan Andrews <danharrisandrews@gmail.com>
Newsgroups:
comp.lang.java.programmer
Date:
Sat, 23 Sep 2006 10:31:37 -0600
Message-ID:
<ef3ndc$2js$1@utornnr1pp.grouptelecom.net>
Patricia Shanahan wrote:

danharrisandrews@gmail.com wrote:

willitheowl@gmail.com wrote:

public class IndirectComparator<...> implements Comparator<...> // 1
{
public int compare(int i, int j)
{
    return weight[i].compareTo(weight[j]); // 2
}

}


Hi Bob,

Not sure if this is what you had in mind, but see if this helps
(below). Some days I have a heck of a time wrapping my head around
generics and I find Eclipse to have very straight forward tips when I
go astray.

public final class IndirectComparator<T, R extends Comparable<? super
R>>
    implements Comparator<T> // 1
{
    List<R> weights;
    List<T> objs;
    public IndirectComparator(List<R> weights, List<T> objs) {
        this.weights = weights;
        this.objs = objs;
    }

    public int compare(T i, T j) {
        return weights.get(objs.indexOf(i)).compareTo(
        weights.get(objs.indexOf(j))); // 2
    }

}


If the lists are at all long, using indexOf could cause a performance
problem. It changes the sort complexity from O(n*log(n)) to O(n*n*log(n)).

However, if that is a a problem, it does suggest the alternative of
constructing a HashMap with the objects as keys, and the weights as values:

public final class IndirectComparator<T, R extends Comparable<? super
R>>
    implements Comparator<T> // 1
{
    Map<T,R> weightMap = new HashMap<T,R>();

    public IndirectComparator(List<R> weights, List<T> objs) {
        Iterator<R> wIt = weights.iterator();
        Iterator<T> oIt = objs.iterator();
        while(oIt.hasNext()){
          weightMap.put(oIt.next(),wIt.next());
        }
    }

    public int compare(T i, T j) {
        return weightMap.get(i).compareTo(weightMap.get(j));
    }
}

Patricia


Patricia,

Yes that's better and what I wrote wouldn't work anyhow as "return
weights.get( objs.indexOf( i )).compareTo( weights.get(objs.indexOf(j) )
); // 2" would end up messing up the index order as the list got sorted.

I also thought if there is a one to one relationship between the Integer
weights and the objects then a TreeMap<Integer,T> would do the sort in
O(nlogn) in one step without bothering with implementing a Comparator.

Cheers,

Dan Andrews
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