JavaOne 2017 Immutable Collections CON6079

Immutable CollectionsPaul.Sandoz@oracle.com

@PaulSandoz

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JavaOne 2017 Immutable Collections CON6079 2

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JavaOne 2017 Immutable Collections CON6079

Agenda

• A recap of unmodifiable collections in the JDK

• A brief overview of immutable collections in external Java libraries and JVM-based platforms

• Immutable collections leveraging persistent data structures

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JavaOne 2017 Immutable Collections CON6079

When referring toimmutable collections

there are no claims made as to the immutability of the collection’s elements

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JavaOne 2017 Immutable Collections CON6079

Advantages of immutability

• Don’t need to think about concurrency and data races

• Resistant to misbehaving libraries

• Are constants that may be optimized at runtime

• Implementations can optimize over time, space for representation and transformation

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JavaOne 2017 Immutable Collections CON6079

Immutable collections wish list

• Manifests immutability(of the collections, not their elements)

• Sealed(not publicly extensible)

• Provide a bridge to mutable collections (not extension of)

• Efficient construction, updates, and copying

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JavaOne 2017 Immutable Collections CON6079

Unmodifiable in the JDK

• The JDK has the notion of unmodifiable collections

• Unmodifiable is a runtime property of a collection

• Modifying (add, put, remove, …) methods throw UnsupportedOperationException

• No way to directly query

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JavaOne 2017 Immutable Collections CON6079

Two forms of unmodifiable

• Unmodifiable view or wrapper to a source or backing collection List<T> uvl = Collections .unmodifiableList(sourceList);

• Directly unmodifiableList<T> dul = List.of(1, 2, 3, …);

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JavaOne 2017 Immutable Collections CON6079

Immutability with unmodifiable collections

• When wrapping ensure the source collection is never accessible*List<T> uvl = Collections .unmodifiableList(new ArrayList<>(source));source = null; List<T> dul = stream.collect( collectingAndThen(toList(), Collections::unmodifiableList))

• List.of and friends are is if the source is never accessible

9* Except, of course, to the unmodifiable wrapper

JavaOne 2017 Immutable Collections CON6079

JDK collections as immutable collections

✗ Manifests immutability

✗ Sealed

• Provide a bridge to mutable collections

✗ Efficient construction, updates and copying

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JavaOne 2017 Immutable Collections CON6079

Unmodifiableis a reasonable abstraction for

mutable collectionsbut not for

immutable collections

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JavaOne 2017 Immutable Collections CON6079

Guava’s immutable collections

• Defines sealed types such as ImmutableList, ImmutableMap, …

• These implement the corresponding JDK mutable collection type (ImmutableList implements List)

• Copying is smartImmutableList.copyOf(otherCollection)

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JavaOne 2017 Immutable Collections CON6079

Guava’s collections are a good compromise

✔ Manifests immutability

✔ Sealed

✘ Provide a bridge to mutable collections

✘ Efficient construction (✔✘), updates (✘), and copying (✔✘)

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JavaOne 2017 Immutable Collections CON6079

Eclipse collections: something for everyone

✔ Manifests immutability

✘ Sealed

✔ Provide a bridge to mutable collections

✘ Efficient construction, updates, and copying

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JavaOne 2017 Immutable Collections CON6079

Vavr (Java), Clojure, Scala

✔ Manifests immutability

✔ Sealed*

✔ Provide a bridge to mutable collections*

✔ Efficient construction, updates, and copying

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* Not completely verified but believed to be mostly true

JavaOne 2017 Immutable Collections CON6079

Vavr (Java), Clojure, Scala

✔ Efficient updates (addition, removal, replace, merge)

• The immutable collection implementations leverage persistent data structures for maps, sets and vectors (non-linked lists)

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JavaOne 2017 Immutable Collections CON6079

Persistent data structures• A persistent data structure preserves the previous

version of itself when modified

• Hash Array Mapped Tries (HAMTs) are the basis of efficient persistent (immutable) maps, sets, and vectors

• Provide structural sharing between a new and previous version of a collection

• Effectively constant time for many operations

• Cache friendly

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JavaOne 2017 Immutable Collections CON6079

Trie

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In computer science, a trie, also called digital tree and sometimes radix tree or prefix tree (as they can be searched by prefixes), is a kind of search tree — an ordered tree data structure that is used to store a dynamic set or associative array where the keys are usually strings

A trie for keys "A","to", "tea", "ted", "ten", "i", "in", and "inn".

JavaOne 2017 Immutable Collections CON6079

Hash Array Mapped Trie

• Symbol is a 5 bit sequence

• String is fixed in size, 32 bits, consisting of 7 symbols (last symbol is truncated to 2 bits)

• String is the hashCode of an Object (the key)

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JavaOne 2017 Immutable Collections CON6079

Hash Array Mapped Trie

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32 31 30 29 28 27 26 25 24 23 22 21 20 19 18 17 16 15 14 13 12 11 10 09 08 07 06 05 04 03 02 01

1 1 0 0 1 0 1 0 1 1 1 1 1 1 1 0 1 0 1 1 1 0 1 0 1 0 1 1 1 1 1 0

0xCAFEBABE

s1s2s3s4s5s6s7

JavaOne 2017 Immutable Collections CON6079

HAMT properties

• Wide branching factor, 32

• Limited tree depth, 6

• Effectively constant time lookup O(log32N) = O(log2N/log232) = O(log2N/5)

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JavaOne 2017 Immutable Collections CON6079

HAMT properties• Good structural sharing (for updates, merging and

splitting) but also good memory usage and cache coherency

• The basis for vectors, where index is the hash code (see also Relaxed Radix Balanced trees), and multi-maps

• Can be applied to mutable collections, for efficient construction of an immutable collection

• Efficiently implemented in Java

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JavaOne 2017 Immutable Collections CON6079

A naive implementation

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public class PMap<K, V> { Object[] nodes = new Object[32 * 2]; public Optional<V> get(K k) { return get(k, hash(k), 0); } private Optional<V> get(K k, int h, int d) { int symbol = symbolAtDepth(h, d); Object _k = nodes[symbol * 2]; if (_k == SUB_LAYER_NODE) { PMap<K, V> n = (PMap<K, V>) nodes[symbol * 2 + 1]; return n.get(k, h, d + 1); } else if (k.equals(_k)) return Optional.of((V) nodes[symbol * 2 + 1]); else return Optional.empty(); } static int symbolAtDepth(int h, int d) { return (h >>> (d * 5) & (32 - 1)); }}

JavaOne 2017 Immutable Collections CON6079

A naive implementation

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public class PMap<K, V> { Object[] nodes = new Object[32 * 2]; public Optional<V> get(K k) { return get(k, hash(k), 0); } private Optional<V> get(K k, int h, int d) { int symbol = symbolAtDepth(h, d); Object _k = nodes[symbol * 2]; if (_k == SUB_LAYER_NODE) { PMap<K, V> n = (PMap<K, V>) nodes[symbol * 2 + 1]; return n.get(k, h, d + 1); } else if (k.equals(_k)) return Optional.of((V) nodes[symbol * 2 + 1]); else return Optional.empty(); } static int symbolAtDepth(int h, int d) { return (h >>> (d * 5) & (32 - 1)); }}

JavaOne 2017 Immutable Collections CON6079

A naive implementation

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public class PMap<K, V> { Object[] nodes = new Object[32 * 2]; public Optional<V> get(K k) { return get(k, hash(k), 0); } private Optional<V> get(K k, int h, int d) { int symbol = symbolAtDepth(h, d); Object _k = nodes[symbol * 2]; if (_k == SUB_LAYER_NODE) { PMap<K, V> n = (PMap<K, V>) nodes[symbol * 2 + 1]; return n.get(k, h, d + 1); } else if (k.equals(_k)) return Optional.of((V) nodes[symbol * 2 + 1]); else return Optional.empty(); } static int symbolAtDepth(int h, int d) { return (h >>> (d * 5) & (32 - 1)); } }

JavaOne 2017 Immutable Collections CON6079

A compact representation

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public class PMap<K, V> { // bit map of symbols @Stable private final int bitmap; // [..., k, v, ....] or // [..., SUB_LAYER_NODE, PMap, ...] or // [..., COLLISION_NODE, CollisionNode, ...] or // invariant: a sub-layer will not consist of a single mapping node @Stable private final Object[] nodes;

JavaOne 2017 Immutable Collections CON6079

A better representation

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private Optional<V> get(K k, int h, int dShift) { int symbol = symbolAtDepth(h, dShift); if (bitmapGet(bitmap, symbol) == 0) return Optional.empty(); int nodeCount = bitmapCountFrom(bitmap, symbol); Object _k = nodes[nodeCount * 2]; if (_k == SUB_LAYER_NODE) { PMap<K, V> s = (PMap<K, V>) nodes[nodeCount * 2 + 1]; return s.get(k, h, dShift + 5); } else if (_k.equals(k)) return Optional.of((V) nodes[nodeCount * 2 + 1]); else return Optional.empty();}

private static int bitmapCountFrom(int bitmap, int symbol) { return Integer.bitCount(bitmap & ((1 << symbol) - 1));}

JavaOne 2017 Immutable Collections CON6079

A better representation

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private Optional<V> get(K k, int h, int dShift) { int symbol = symbolAtDepth(h, dShift); if (bitmapGet(bitmap, symbol) == 0) return Optional.empty(); int nodeCount = bitmapCountFrom(bitmap, symbol); Object _k = nodes[nodeCount * 2]; if (_k == SUB_LAYER_NODE) { PMap<K, V> s = (PMap<K, V>) nodes[nodeCount * 2 + 1]; return s.get(k, h, dShift + 5); } else if (_k.equals(k)) return Optional.of((V) nodes[nodeCount * 2 + 1]); else return Optional.empty();}

private static int bitmapCountFrom(int bitmap, int symbol) { return Integer.bitCount(bitmap & ((1 << symbol) - 1));}

JavaOne 2017 Immutable Collections CON6079

A better representation

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private Optional<V> get(K k, int h, int dShift) { int symbol = symbolAtDepth(h, dShift); if (bitmapGet(bitmap, symbol) == 0) return Optional.empty(); int nodeCount = bitmapCountFrom(bitmap, symbol); Object _k = nodes[nodeCount * 2]; if (_k == SUB_LAYER_NODE) { PMap<K, V> s = (PMap<K, V>) nodes[nodeCount * 2 + 1]; return s.get(k, h, dShift + 5); } else if (_k.equals(k)) return Optional.of((V) nodes[nodeCount * 2 + 1]); else return Optional.empty();}

private static int bitmapCountFrom(int bitmap, int symbol) { return Integer.bitCount(bitmap & ((1 << symbol) - 1));}

JavaOne 2017 Immutable Collections CON6079

A better representation

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private Optional<V> get(K k, int h, int dShift) { int symbol = symbolAtDepth(h, dShift); if (bitmapGet(bitmap, symbol) == 0) return Optional.empty(); int nodeCount = bitmapCountFrom(bitmap, symbol); Object _k = nodes[nodeCount * 2]; if (_k == SUB_LAYER_NODE) { PMap<K, V> s = (PMap<K, V>) nodes[nodeCount * 2 + 1]; return s.get(k, h, dShift + 5); } else if (_k.equals(k)) return Optional.of((V) nodes[nodeCount * 2 + 1]); else return Optional.empty();}

private static int bitmapCountFrom(int bitmap, int symbol) { return Integer.bitCount(bitmap & ((1 << symbol) - 1));}

Compiles to POPCNT on x64

JavaOne 2017 Immutable Collections CON6079

A better representation • Space is required only for present nodes, using the

bitmap (made possible with HotSpot optimzations)

• Further refinements (and tradeoffs) possible

• Sub nodes and entries could be separated for more cache friendly traversal (see Steindorfer’s work compressed HAMTs aka CHAMP)

• Hash codes could be cached

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JavaOne 2017 Immutable Collections CON6079

Persistent Map API

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public void forEach(BiConsumer<K, V> action);

public Optional<V> get(K k);

public PMap<K, V> put(K k, V v);

public PMap<K, V> remove(K k);

JavaOne 2017 Immutable Collections CON6079

Persistent collections API• Modifying methods return a new collection

• An implementation shares unmodified structure with the previous collection

• Require mutable builders to efficiently construct in a confined manner

• For example, closure/thread confined construction then freezing

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JavaOne 2017 Immutable Collections CON6079

Demo:Visualizing

HAMT-basedpersistent maps

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https://github.com/PaulSandoz/per/

JavaOne 2017 Immutable Collections CON6079

Summary

• Unmodifiable is a reasonable abstraction for mutable but not immutable

• For efficient immutable collections we need persistent collections

• Sets, maps and vectors using HAMTs have proven to be effective in many libraries and platforms

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JavaOne 2017 Immutable Collections CON6079

What about Java?• We shall continue to improve on unmodifiable

in the JDK

• Selective sedimentation of persistent collections into the Java platform?

• Claim: possibly to optimize such collections very aggressively with internal APIs, HotSpot, and safely contained unsafe mechanisms

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JavaOne 2017 Immutable Collections CON6079

References• Fast And Space Efficient Trie Searches, Bagwell

https://pdfs.semanticscholar.org/93a1/fe7f226cfbc7cb2bceac39308a66c8aef0b0.pdf

• Ideal Hash Trees, Bagwell http://lampwww.epfl.ch/papers/idealhashtrees.pdf

• RRB-Trees: Efficient Immutable Vectors, Bagwell and Rompf https://infoscience.epfl.ch/record/169879/files/RMTrees.pdf

• Optimizing Hash-Array Mapped Tries for Fast Lean Immutable JVM Collections, Steindorfer and Vinju https://michael.steindorfer.name/publications/oopsla15.pdf

• Efficient Immutable Collections - PhD Thesis - Steindorfer https://michael.steindorfer.name/publications/phd-thesis-efficient-immutable-collections.pdf

• Cache-Aware Lock-Free Concurrent Hash Tries, Prokopec, Bagwell, Odersky https://infoscience.epfl.ch/record/166908/files/ctries-techreport.pdf

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