Collection Filtering

[Mark Herschberg]

I'm looking for an efficent way to filter a collection. We just moved some database tables into memory, holding the data in Collections. Now we need filtering to replace EJB/SQL finders (e.g. find all elements of the collection where the name field equals "Bob").
--Mark
[ April 10, 2003: Message edited by: Mark Herschberg ]

[Michael Morris]

What kind of Collections?
[ April 10, 2003: Message edited by: Michael Morris ]

[Mark Herschberg]

Um, you know... collections collections; like from java.util. Things like Set, List, Map, etc.
--Mark

[Garandi Garandi]

HashMap might be useful.
Garandi

[Michael Morris]

Um, you know... collections collections; like from java.util. Things like Set, List, Map, etc.

I was hoping you would say something definite like Maps using the primary key, even though a Map is not a collection. So you just want use a generic Collection. I guess it really all depends on how many findBys you have, the number of fields in each collection, whether and how the collection is sorted and the collection type.

[Jim Yingst]

Don't forget this previous discussion for fairly arbitrary types of "filtering". For more specific advice, yeah it depends on a lot of different things. If you want to do a lot of searches quickly where you're searcing for some sort of exact match for a key, a HashMap is the way to go. It just takes a bit of time to set it up in the first place, putting in all the key-value pairs. If you want to find some sort of range, eg all names alphabetically between "Jim" and "Mark", then you'd need a TreeMap using the name as a key. Slower than HashMap, but you can extract an ordered subset of the mappings. For other stuff, like say matching a field against an arbitrary regular expression, you may have to just iterate through a collection an apply a check to whatever field you're searching on. Or design some clever custom data structure that makes your particular search easier. Lots of different possibilities...

[Mark Herschberg]

You know what, I got it totally backwards. I was thinking of things working like the comparator method, and was wondering how to put 6 different comparators into a single class. But the way to do it is to have 6 different predicate objects. Duh!
Thanks.
--Mark

[Peter den Haan]

Do not forget to check out the Jakarta Commons Collections. Closures, predicates, all kinds of stuff that you might miss in the Java Collections framework.
- Peter