Immutable Collections [email protected] @PaulSandoz JavaOne 2017 Immutable Collections CON6079 1 Safe Harbor Statement The following is intended to outline our general product direction. It is intended for information purposes only, and may not be incorporated into any contract. It is not a commitment to deliver any material, code, or functionality, and should not be relied upon in making purchasing decisions. The development, release, and timing of any features or functionality described for Oracle’s products remains at the sole discretion of Oracle. JavaOne 2017 Immutable Collections CON6079 2 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 JavaOne 2017 Immutable Collections CON6079 3 When referring to immutable collections there are no claims made as to the immutability of the collection’s elements JavaOne 2017 Immutable Collections CON6079 4 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 JavaOne 2017 Immutable Collections CON6079 5 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 JavaOne 2017 Immutable Collections CON6079 6 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 JavaOne 2017 Immutable Collections CON6079 7 Two forms of unmodifiable • Unmodifiable view or wrapper to a source or backing collection List<T> uvl = Collections .unmodifiableList(sourceList); • Directly unmodifiable List<T> dul = List.of(1, 2, 3, …); JavaOne 2017 Immutable Collections CON6079 8 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 * Except, of course, to the unmodifiable wrapper JavaOne 2017 Immutable Collections CON6079 9 JDK collections as immutable collections ✗ Manifests immutability ✗ Sealed • Provide a bridge to mutable collections ✗ Efficient construction, updates and copying JavaOne 2017 Immutable Collections CON6079 10 Unmodifiable is a reasonable abstraction for mutable collections but not for immutable collections JavaOne 2017 Immutable Collections CON6079 11 Guava’s immutable collections • Defines sealed types such as ImmutableList, ImmutableMap, … • These implement the corresponding JDK mutable collection type (ImmutableList implements List) • Copying is smart ImmutableList.copyOf(otherCollection) JavaOne 2017 Immutable Collections CON6079 12 Guava’s collections are a good compromise ✔ Manifests immutability ✔ Sealed ✘ Provide a bridge to mutable collections ✘ Efficient construction (✔✘), updates (✘), and copying (✔✘) JavaOne 2017 Immutable Collections CON6079 13 Eclipse collections: something for everyone ✔ Manifests immutability ✘ Sealed ✔ Provide a bridge to mutable collections ✘ Efficient construction, updates, and copying JavaOne 2017 Immutable Collections CON6079 14 Vavr (Java), Clojure, Scala ✔ Manifests immutability ✔ Sealed* ✔ Provide a bridge to mutable collections* ✔ Efficient construction, updates, and copying * Not completely verified but believed to be mostly true JavaOne 2017 Immutable Collections CON6079 15 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) JavaOne 2017 Immutable Collections CON6079 16 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 JavaOne 2017 Immutable Collections CON6079 17 Trie 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 18 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) JavaOne 2017 Immutable Collections CON6079 19 Hash Array Mapped Trie 0xCAFEBABE 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 s7 s6 s5 s4 s3 s2 s1 JavaOne 2017 Immutable Collections CON6079 20 HAMT properties • Wide branching factor, 32 • Limited tree depth, 6 • Effectively constant time lookup O(log32N) = O(log2N/log232) = O(log2N/5) JavaOne 2017 Immutable Collections CON6079 21 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 JavaOne 2017 Immutable Collections CON6079 22 A naive implementation 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 23 A naive implementation 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 24 A naive implementation 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 25 A compact representation 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 26 A better representation 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 27 A better representation 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 28 A better representation private Optional<V> get(K k, int h, int dShift) { int symbol = symbolAtDepth(h,