[2019 oracle code one] exploring collectors

Exploring Collectors

Speakers: Venkat Subramaniam

For more blog posts, see The Oracle Code One table of contents


  • Common operations; filter, map, reduce
  • Filter – like a coin sorter. Will let some values through and discard others
  • Reduce – go from a stream to a non stream
  • Collect is a reduce operation
  • Functions should be pure – doesn’t change anything *and* doesn’t depend on anything that could change

Collectors – concepts

  • Don’t call add() in a forEach. Should be using a Collector
  • Can’t parallelize code when have shared mutability (ex: add() in forEach)
  • Can’t say “the code worked”. Can say “the code behaved”
  • If use ConcurrentList, it’s a ticking time bomb for when someone changes the list type.
  • Should write collect(Collectors.toList()) or collect(toList()). Already handles concurrency (when running with a parallel stream)
  • Venkat prefers using Collectors as a static import and just calling toList()
  • Collectors are recursive data structures. The second parameter is another Collector
  • Often need to chain collectors to do what want.
  • Ok to write code “the long way” and then refactor once have passing tests

Collectors – Code

  • Java 10+: toUnmodifiableList() – immutable list
  • partitioningBy – when need both the matching and non matching results. Avoids needing two passes of data to get result.
  • joining(“, “) – comma separated
  • groupingBy(Person::getName) – create map with key as name and value as list of Person objects. Conceptualize as buckets. Put items in bucket by key
  • groupingBy(Person::getName, mapping(Person::getAge, toList())) – map after group. Perform mapping right before throw data into bucket.
  • groupingBy(Person::getName, counting()) – value is # matching values
  • groupingBy(Person::getName, collectingAndThen(counting(), Long::intValue)) – transform the result of a collector to a different type

My take

I like that Venkat talked about how to write code “the long way” to explain the power of collectors. This was a good review. And good motivation as we update our OCP book (I have the streams chapter). I like the bucket analogy for groupingBy(). I didn’t know about collectingAndThen() The 45 minutes flew!

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