collectionsdartclone

Clone a List, Map or Set in Dart

Coming from a Java background: what is the recommended way to "clone" a Dart List, Map and Set?

Solution

  • Use of clone() in Java is tricky and questionable1,2. Effectively, clone() is a copy constructor and for that, the Dart List, Map and Set types each have a named constructor named .from() that perform a shallow copy; e.g. given these declarations

      Map<String, int> numMoons, moreMoons;
  numMoons = const <String,int>{ 'Mars' : 2, 'Jupiter' : 27 };
  List<String> planets, morePlanets;

    you can use .from() like this:

      moreMoons = new Map<String,int>.from(numMoons)
    ..addAll({'Saturn' : 53 });
  planets = new List<String>.from(numMoons.keys);
  morePlanets = new List<String>.from(planets)
    ..add('Pluto');

    Note that List.from() more generally accepts an iterator rather than just a List.

    For sake of completeness, I should mention that the dart:html Node class defines a clone() method.

    1 J. Bloch, "Effective Java" 2nd Ed., Item 11.
    2 B. Venners, "Josh Bloch on Design: Copy Constructor versus Cloning", 2002. Referenced from here3. Quote from the article:

    If you've read the item about cloning in my book, especially if you read between the lines, you will know that I think clone is deeply broken. ---J.Bloch

    3 Dart Issue #6459, clone instance(object).