![Unmodifiable
List<String> jedis = new ArrayList<>();
jedis.add("Luke Skywalker");
List<String> cantChangeMe = Collections.unmodifiableList(jedis);
// java.lang.UnsupportedOperationException
//cantChangeMe.add("Darth Vader");
System.out.println(cantChangeMe); // [Luke Skywalker]
jedis.add("Darth Vader");
System.out.println(cantChangeMe); // [Luke Skywalker, Darth Vader]](https://image.slidesharecdn.com/collectionsforceawakens-160608135642/85/Collections-forceawakens-15-320.jpg)
![Memory usage survey
10,000,000 elements, heap < 32GB
int[] : 40MB
Integer[]: 160MB
ArrayList<Integer>: 215MB
PersistentVector<Integer>: 214MB (Clojure-DS)
Vector<Integer>: 206MB (Dexx, port of Scala-DS)
Data collected using Java Object Layout: http://openjdk.java.
net/projects/code-tools/jol/](https://image.slidesharecdn.com/collectionsforceawakens-160608135642/85/Collections-forceawakens-33-320.jpg)
Collections forceawakens
- 1. Java Collections The Force Awakens Darth @RaoulUK Darth @RichardWarburto
- 3. Collection Problems Java Episode 8 & 9 Persistent & Immutable Collections HashMaps
- 4. Collection bugs 1. Element access (Off-by-one error, ArrayOutOfBound) 2. Concurrent modification 3. Check-then-Act
- 5. Scenario 1 List<String> jedis = new ArrayList<>(asList("Luke", "yoda")); for (String jedi: jedis) { if (Character.isLowerCase(jedi.charAt(0))) { jedis.remove(jedi); } }
- 6. Scenario 2 Map<String, BigDecimal> movieViews = new HashMap<>(); BigDecimal views = movieViews.get(MOVIE); if(views != null) { movieViews.put(MOVIE, views.add(BigDecimal.ONE)); } views != nullmoviesViews.get movieViews.put Then Check Act
- 7. Reducing scope for bugs ● ~280 bugs in 28 projects including Cassandra, Lucene ● ~80% check-then-act bugs discovered are put-if-absent ● Library designers can help by updating APIs as new idioms emerge ● Different data structures can provide alternatives by restricting reads & updates to reduce scope for bugs CHECK-THEN-ACT Misuse of Java Concurrent Collections http://dig.cs.illinois.edu/papers/checkThenAct.pdf
- 8. Collection Problems Java Episode 8 & 9 Persistent & Immutable Collections HashMaps
- 9. Java 8 Lazy Collection Initialization Many allocated HashMaps and ArrayLists never written to, eg Null object pattern Java 8 adds Lazy Initialization for the default initialization case Typically 1-2% reduction in memory consumption http://www.javamagazine.mozaicreader. com/MarApr2016/Twitter#&pageSet=28&page=0
- 11. Java 9 API updates Collection factory methods ● Non-goal to provide persistent immutable collections ● http://openjdk.java.net/jeps/269 java.util.Optional ● ifPresentOrElse(), or(), stream(), getWhenPresent() ● Optional.get() becomes deprecated java.util.Stream ● takeWhile, dropWhile
- 12. Collection Problems Java Episode 8 & 9 Persistent & Immutable Collections HashMaps
- 13. Categorising Collections Mutable Immutable Non-Persistent Persistent Unsynchronized Concurrent Unmodifiable View Available in Core Library
- 14. Mutable ● Popular friends include ArrayList, HashMap, TreeSet ● Memory-efficient modification operations ● State can be accidentally modified ● Can be thread-safe, but requires careful design
- 15. Unmodifiable List<String> jedis = new ArrayList<>(); jedis.add("Luke Skywalker"); List<String> cantChangeMe = Collections.unmodifiableList(jedis); // java.lang.UnsupportedOperationException //cantChangeMe.add("Darth Vader"); System.out.println(cantChangeMe); // [Luke Skywalker] jedis.add("Darth Vader"); System.out.println(cantChangeMe); // [Luke Skywalker, Darth Vader]
- 17. Immutable & Non-persistent ● No updates ● Flexibility to convert source in a more efficient representation ● No locking in context of concurrency ● Satisfies co-variant subtyping requirements ● Can be copied with modifications to create a new version (can be expensive)
- 18. Immutable vs. Mutable hierarchy ImmutableList MutableList + ImmutableList<T> toImmutable() java.util.List + MutableList<T> toList() Eclipse Collections (formaly GSCollections) https://projects.eclipse.org/projects/technology.collections/ ListIterable
- 19. Immutable and Persistent ● Changing source produces a new (version) of the collection ● Resulting collections shares structure with source to avoid full copying on updates
- 20. Persistent List (aka Cons) public final class Cons<T> implements ConsList<T> { private final T head; private final ConsList<T> tail; public Cons(T head, ConsList<T> tail) { this.head = head; this.tail = tail; } @Override public ConsList<T> add(T e) { return new Cons(e, this); } }
- 21. Updating Persistent List A B C X Y Z Before
- 22. Updating Persistent List A B C X Y Z Before A B D After Blue nodes indicate new copies Purple nodes indicates nodes we wish to update
- 23. Concatenating Two Persistent Lists A B C X Y Z Before
- 24. Concatenating Two Persistent Lists - Poor locality due to pointer chasing - Copying of nodes A B C X Y Z Before A B C After
- 25. Persistent List ● Structural sharing: no need to copy full structure ● Poor locality due to pointer chasing ● Copying becomes more expensive with larger lists ● Poor Random Access and thus Data Decomposition
- 26. Updating Persistent Binary Tree Before
- 27. Updating Persistent Binary Tree After
- 28. Persistent Array How do we get the immutability benefits with performance of mutable variants?
- 29. Trie root 10 4520 3. Picking the right branch is done by using parts of the key as a lookup 1. Branch factor not limited to binary 2. Leaf nodes contain actual values a a e b c b c f
- 30. Persistent Array (Bitmapped Vector Trie) ... ... ... ... ... ... ... ... . . . . . . 1 31 0 1 31 Level 1 (root) Level 2 Leaf nodes
- 31. Trade-offs ● Large branching factor facilitates iteration but hinders updates ● Small branching factor facilitates updates but hinders traversal
- 32. Java Persistent Collections - Not available as part of Java Core Library - Existing projects includes - PCollections: https://github.com/hrldcpr/pcollections - Port of Clojure DS: https://github.com/krukow/clj-ds - Port of Scala DS: https://github.com/andrewoma/dexx
- 33. Memory usage survey 10,000,000 elements, heap < 32GB int[] : 40MB Integer[]: 160MB ArrayList<Integer>: 215MB PersistentVector<Integer>: 214MB (Clojure-DS) Vector<Integer>: 206MB (Dexx, port of Scala-DS) Data collected using Java Object Layout: http://openjdk.java. net/projects/code-tools/jol/
- 34. Primitive specialised collections ● Collections often hold boxed representations of primitive values ● Java 8 introduced IntStream, LongStream, DoubleStream and primitive specialised functional interfaces ● Other libraries, eg: Agrona, Koloboke and Eclipse-Collections provide primitive specialised collections today. ● Valhalla investigates primitive specialised generics
- 35. Takeaways ● Immutable collections reduce the scope for bugs ● Always a compromise between programming safety and performance ● Performance of persistent data structure is improving
- 36. Collection Problems Java Episode 8 & 9 Persistent & Immutable Collections HashMaps
- 38. HashMaps Basics ... Han Solo hash = 72309 Chewbacca hash = 72309
- 39. Chaining Probing HashMaps a separate data structure for collision lookups Store inline and have a probing sequence
- 40. Aliases: Palpatine vs Darth Sidious
- 41. Chaining Probing HashMaps aka Closed Addressing aka Open Hashing aka Open Addressing aka Closed Hashing
- 42. Chaining Probing HashMaps Linked List Based Tree Based
- 43. java.util.HashMap Chaining Based HashMap Historically maintained a LinkedList in the case of a collision Problem: with high collision rates that the HashMap approaches O(N) lookup
- 44. java.util.HashMap in Java 8 Starts by using a List to store colliding values. Trees used when there are over 8 elements Tree based nodes use about twice the memory Make heavy collision lookup case O(log(N)) rather than O(N) Relies on keys being Comparable https://github.com/RichardWarburton/map-visualiser
- 45. So which HashMap is best?
- 46. Benchmarking is about building a mental model of the performance tradeoffs
- 47. Example Jar-Jar Benchmark call get() on a single value for a map of size 1 No model of the different factors that affect things!
- 48. Benchmarking HashMaps Load Factor Nonlinear key access Successful vs Failed get() Hash Collisions Comparable vs Incomparable keys Different Keys and Values Cost of hashCode/Equals
- 49. Tree Optimization - 60% Collisions
- 50. Tree Optimization - 10% Collisions
- 51. Probing vs Chaining Probing Maps usually have lower memory consumption Small Maps: Probing never has long clusters, can be up to 91% faster. In large maps with high collision rates, probing scales poorly and can be significantly slower.
- 52. Takeaways There’s no clearcut “winner”. JDK Implementations try to minimise worst case. Linear Probing requires a good hashCode() distribution, Often hashmaps “precondition” their hashes. IdentityHashMap has low memory consumption and is fast, use it! 3rd Party libraries offer probing HashMaps, eg Koloboke & Eclipse-Collections.
- 53. Conclusions
- 54. Interface Popularity List 1576210 Set 980763 Map 803171 Queue 62024 Deque 3464 SortedSet 9121 NavigableSet 1735 SortedMap 8677 NavigableMap 1484
- 55. Implementation Popularity ArrayList 225029 LinkedList 26850 ArrayDeque 1086 HashSet 68940 TreeSet 10108 EnumSet 10512 HashMap 137610 TreeMap 7734 WeakHashMap 3473 IdentityHashMap 2443 EnumMap 1904
- 56. Evolution can be interesting ... Java 1.2 Java 10?
- 59. Further reading Fast Functional Lists, Hash-Lists, Deques and Variable Length Arrays https://infoscience.epfl.ch/record/64410/files/techlists.pdf Smaller Footprint for Java Collections http://www.lirmm.fr/~ducour/Doc-objets/ECOOP2012/ECOOP/ecoop/356.pdf Optimizing Hash-Array Mapped Tries for Fast and Lean Immutable JVM Collections http://michael.steindorfer.name/publications/oopsla15.pdf RRB-Trees: Efficient Immutable Vectors https://infoscience.epfl.ch/record/169879/files/RMTrees.pdf
- 60. Further reading Doug Lea’s Analysis of the HashMap implementation tradeoffs http://www.mail-archive.com/core-libs-dev@openjdk.java.net/msg02147.html Java Specialists HashMap article http://www.javaspecialists.eu/archive/Issue235.html Sample and Benchmark Code https://github.com/RichardWarburton/Java-Collections-The-Force-Awakens
- 61. Further reading Debian code search used for popularity https://codesearch.debian.net/