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docs: Added the 6th rule of immutable classes #79

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docs: Added the 6th rule of immutable classes #79

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docs: Added the 6th rule of immutable classes
larssn Jun 5, 2024
dd9b8e5
docs: fix spelling
larssn Jun 18, 2024
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9 changes: 2 additions & 7 deletions README.md
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Expand Up @@ -1996,13 +1996,8 @@ To create an immutable object **in Dart** you must follow these 5 rules:
changes have no external effects (this may be useful for lazy initialization, caching and
improving performance).

_Note:_ There should also be a 6th rule stating that the class should be `final` (in the Java sense)
, but in Dart it's impossible to prevent a class from being subclassed. The problem is that one can
always subclass an otherwise immutable class and then add mutable fields to it, as well as override
public methods to return changed values according to those mutable fields. This means that in Dart
it's impossible to create strictly immutable classes. However, you can make it as close as possible
to the real thing by at least not invoking overridable methods from the constructor (which in Dart
means not invoking public non-static methods).
6. Make classes `final` or `sealed` **(Dart 3+ only)** to prevent classes outside the current library to subclass it.
This prevents subclasses adding mutable fields. It also prevents the <a href="https://en.wikipedia.org/wiki/Fragile_base_class#:~:text=The%20fragile%20base%20class%20problem,the%20derived%20classes%20to%20malfunction.">Fragile base class</a> problem.

<br>

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