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Built Collections are immutable collections using the builder pattern.

Each of the core SDK collections is split in two: a mutable builder class and an immutable "built" class. Builders are for computation, "built" classes are for safely sharing with no need to copy defensively.

Immutable collections work particularly well with immutable values. See built_value.

You can read more about built_collection on medium.

Built Collections:

• are immutable, if the elements/keys/values used are immutable;
• are comparable;
• are hashable;
• use copy-on-write to avoid copying unnecessarily.

See below for details on each of these points.

A project can benefit greatly from using Built Collections throughout. Methods that will not mutate a collection can accept the "built" version, making it clear that no mutation will happen and completely avoiding the need for defensive copying.

For code that is public to other projects or teams not using Built Collections, prefer to accept Iterable where possible. That way your code is compatible with SDK collections, Built Collections and any other collection implementation that builds on Iterable.

It's okay to accept List, Set or Map if needed. Built Collections provide efficient conversion to their SDK counterparts via BuiltList.toList, BuiltListMultimap.toMap, BuiltSet.toSet, BuiltMap.toMap and BuiltSetMultimap.toMap.

Built Collections do not offer any methods that modify the collection. In order to make changes, first call toBuilder to get a mutable builder.

In particular, Built Collections do not implement or extend their mutable counterparts. BuiltList implements Iterable, but not List. BuiltSet implements Iterable, but not Set. BuiltMap, BuiltListMultimap and BuiltSetMultimap share no interface with the SDK collections.

Built Collections can contain mutable elements. However, this use is not recommended, as mutations to the elements will break comparison and hashing.

Core SDK collections do not offer equality checks by default.

Built Collections do a deep comparison against other Built Collections of the same type using Dart's equality operator (==). A Built Collection cannot be compared to another Built Collection when they are of different types, such as a BuiltList and a BuiltSet. Hashing is used to make repeated comparisons fast.

// true: same contents according to `int.operator==`, `MyClass.operator==`
BuiltList([1, 2, 3]) == BuiltList([1, 2, 3]);
BuiltList([MyClass(1), MyClass(2)]) == BuiltList([MyClass(1), MyClass(2)]);

// false: BuiltList and BuiltSet are never equal
BuiltList([1, 2, 3]) == BuiltSet([1, 2, 3]);
// false: different contents according to `int.operator==`
BuiltList([1, 2, 3]) == BuiltList([2, 3, 4]);

Core SDK collections do not compute a deep hashCode.

Built Collections do compute, and cache, a deep hashCode. That means they can be stored inside collections that need hashing, such as hash sets and hash maps. They also use the cached hash code to speed up repeated comparisons.

Built Collections and their builder and helper types collaborate to avoid copying unless it's necessary.

In particular, BuiltList.toList, BuiltListMultimap.toMap, BuiltSet.toSet, BuiltMap.toMap and BuiltSetMultimap.toMap do not make a copy, but return a copy-on-write wrapper. So, Built Collections can be efficiently and easily used with code that needs core SDK collections but does not mutate them.

When you want to provide a collection that explicitly throws when a mutation is attempted, use BuiltList.asList, BuiltListMultimap.asMap, BuiltSet.asSet, BuiltSetMultimap.asMap and BuiltMap.asMap.

Please file feature requests and bugs at the issue tracker.