ConcurrentHashMap/ConcurrentMap/Map.compute

[Paul Sandoz]

On Jan 9, 2013, at 7:47 AM, Joe Bowbeer <joe.bowbeer at gmail.com> wrote:

> In the javadoc for Map.compute in b72, there is an error in the sample
> snippet:
> 
> http://download.java.net/lambda/b72/docs/api/java/util/Map.html
> 
> Attempts to compute a mapping for the specified key and its current mapped
>> value (or null if there is no current mapping). For example, to either
>> create or append a String msg to a value mapping:
>> map.compute(key, (k, v, msg) -> (v == null) ? msg : v.concat(msg))
> 
> 
> Error: BiFunction does not accept three arguments.  In particular, msg is
> extraneous.  It should be defined in the lexical scope?
> 
> 
> Btw, I pondered how to efficiently use compute() or merge() to simulate a
> multi-map and I eventually gave up.
> 
> I eventually wrote the following, which accumulates lists of anagrams,
> given a list of words:
> 
> r.lines().forEach(s -> map.computeIfAbsent(key(s), k -> new
> ArrayList<>()).add(s));
> 

Have you tried using the latest groupBy functionality? e.g.:

  Map<String, Collection<String>> anagrams = r.lines().parallel().reduce(groupBy(s -> key(s)));

It would be interesting to compare performance of reduce vs. reduceUnordered and the concurrent groupBy leveraging CHM.


> 
> Where key() returns the canonical key for a given word, and r is a
> BufferedReader for the dictionary file.
> 
> The following line prints the lists of anagrams:
> 
> map.values().stream().filter(v -> v.size() > 1).forEach(v ->
> System.out.println(v));
> 

Little style tip:

  forEach(System.out::println)

Paul.