Tag Archives: collections

[2022 javaone] sequenced collections

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Speaker: Stuart Marks

For more see the table of contents

General

  • New JEP; candidate state
  • JEP-431
  • Targeting Java 20

Encounter order

  • HashSet does not have. Order depends on hashcode
  • Encounter order is insertion order ArrayList, ArrayDeque, LinkedHashSet
  • Encounter order is sort order – TreeSet

First and last elements

  • get(0), getFirst(), first()
  • list.get(list.size()-1), getLast(), last()

Iterating and streaming, forward and reverse

  • Iterating – forEach loop – code same for all types to iterate forward
  • Iterable tied to forward iteration
  • Reverse varies – listIterator(), descendingIterator(), Collections.reverse(), descendingSet()
  • Streams use forward iteration as well
  • Streaming in reverse order needs spliterator as adapter to order

Sequenced Collection

  • Proposed
  • SequencedCollection – new subinterface of Collection.
  • Includes reversed(), addFirst(), addLast(), getFirst(), getLast(), removeFirst(), removeLast()
  • Also SequencedSet, SequencedCollections and SequencedSet
  • Includes default methods so doesn’t break existing code
  • New methods for sequenced map views

Covariant overrides of reversed()

  • Reverses
  • View of reversed list, but same list
  • Updating reversed view/original updates the other
  • Returns same interface you pass in

LinkedHashMap

  • Will be able to get reversed views, remove entry from end, etc
  • sequencedKeySet(), sequencedValues()
  • putFirst(), putLast() – unconditionally put mapping at end regardless of whether addition or replacement

My take

The talk started 15 minutes late due to AV issues. And people waited. That says a lot about people looking forward to this talk! The content was quick but understandable. I like that there was a lot of code shown and run in an IDE without spending time typing. Also, NetBeans which doesn’t show up in as many demos. Despite the late start, it didn’t feel rushed. Ended a few minutes late, but not as late as it started so seems like a win!

[2019 oracle code one] collections corner cases

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Collections Corner Cases

Speaker: Stuart Marks @StuartMarks #CollectionsCornerCases

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


General

  • Covers things in collections framework a long time. Not well known
  • Good mix of PowerPoint and JShell

JShelll

  • If don’t assign an expression to a variable goes in $1, $2, etc
  • /var $1 – prints type of variable (yes the slash is intentional, it is a JShell command.)
  • /open abc.jshell – “imports” file

View Collections

  • Don’t contain their own elements. Instead the elements are stored elsewhere
  • ex: Arrays.asList(array) – creates list backed by array. If change list, the array gets changed to and vice versa.
  • map.keySet(), map.values() and map.entrySet(). If change these three collections, underlying map changes too.
  • Map.Entry – has setValue() method. Can call setValue() on on entry set to change underlying map

Performance/Clarity

  • values() is O(n) on a Map
  • Creating a temporary HashSet for the values makes in O(1)
  • retailAll() – set intersection
  • Don’t always need streams. [My logic is if you wouldn’t have needed a loop, you don’t need streams]

Sorted Collections & Comparators

  • Interfaces: SortedSet, SortedMap, NavigableSet, NavigableMap
  • Implementations: TreeSet, TreeMap, ConcurrentSkipListSet, ConcurrentSkipListMap
  • Ordering provided by Comparator. Could be natural order or Comparable. May or may not be consistent with equals.
  • HashSet and Set.of() – iteration order undefined. For SortedSet, iteration order is well defined – sorted order
  • Comparator rules
    • <0, 0, >0 for less than, equal and greater than
    • Must provide a total ordering. Any two values must return result
    • reflexive, transitive, etc
  • Spent a while on set vs list (unique elements) – don’t forget what type you are working with
  • HashSet – doesn’t allow duplicates using equals()
  • TreeSet – doesn’t allow duplicates where compare(a,b) ==0
  • String class has built in comparator: String.CASE_INSENSITIVE_ORDER. Inconsistent with equals(). So using case insensitive order with a TreeSet and adding ‘a’ and “A’, doesn’t add ‘A’ to list. Also tree.contains(“A”) returns true if “a” in set. If have HashSet with ‘A’ and TreeSet with ‘a’, tree.equals(hash) returns true but hash.equals(tree) returns false.

My take

I learned something new under 5 minutes in. (keySet()/values() and entrySet() are backed.). I was also pleasantly surprised to learn new shell commands. The rest was good too :).

Also, I was reminded that I hate Map.of(). I fell for misreading them because I didn’t parse properly as alternating keys and values.