Built Collections are immutable collections using the builder pattern and are part of Libraries for Object Oriented Dart.

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.

Built Collections:

• are immutable, if the elements/keys/values used are immutable;
• are comparable;
• are hashable;
• reject nulls;
• require generic type parameters;
• reject wrong-type elements;
• 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, only. Hashing is used to make repeated comparisons fast.

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.

A null in a collection is usually a bug, so Built Collections and their builders throw if given a null element, key or value.

A List<dynamic> is error-prone because it can be assigned to a List of any type without warning. So, all Built Collections must be created with explicit element, key or value types.

Collections that happen to contain elements, keys or values that are not of the right type can lead to difficult-to-find bugs. So, all Built Collections and their builders are aggressive about validating types, even with checked mode disabled.

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.

Please file feature requests and bugs at the issue tracker.