Hello,
Question about NoSuchElementException usage in Taste:
I'm using Taste in an environment where lots of requests for recommended items
are for users who are new to the system - users who have no previously consumed
items. When I try to get recommendations for such users, the following is
thrown:
java.util.NoSuchElementException
at
org.apache.mahout.cf.taste.impl.model.BooleanUserGenericDataModel.getUser(BooleanUserGenericDataModel.java:81)
at
org.apache.mahout.cf.taste.impl.model.file.BooleanPrefUserFileDataModel.getUser(BooleanPrefUserFileDataModel.java:163)
at
org.apache.mahout.cf.taste.impl.recommender.BooleanUserGenericUserBasedRecommender.recommend(BooleanUserGenericUserBasedRecommender.java:86)
This looks correct, and comes from this piece of code in
BooleanUserGenericDataModel (the same happens in non-Boolean versions of the
code):
public User getUser(Object id) {
User user = userMap.get(id);
if (user == null) {
throw new NoSuchElementException();
}
return user;
}
I'm wondering if this is good/best thing to do here. Here is some reasoning:
- it's not really an error to ask a RE to recommend items for a user the RE
knows nothing about - it's the cold start issue
- I believe throwing exceptions is expensive, and I'm wondering if it would be
cheaper to do something else
- one could say this falls in the "using exceptions for flow control" category
(because that's what I'll have to do on the higher level)
- I briefly looked at returning null, but it looks like this would require
checks for null to avoid NPE in a number of places
I'm not sure this needs to change, but it's been on my mind for a while now.
On my end I can simply catch this ugly exception and eat it, although this
"NoSuchElementException" is so non-specific that if I just catch
NoSuchElementException the code may one day end up catching a
NoSuchElementException from some other part of Taste.
Thoughts?
Otis
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