final variables in Dart

(Shameless stolen from my own Stack Overflow question and answer.)

Dart has a concept of final. Most dynamic languages don't have this concept. What is final and what do I use it for?
final variables can contain any value, but once assigned, a final variable can't be reassigned to any other value.
For example:
main() {
  final msg = 'hello';
  msg = 'not allowed'; // **ERROR**, program won't compile
}
final can also be used for instance variables in an object. A final field of a class must be set before the constructor body is run. A final field will not have an implicit setter created for it, because you can't set a new value on a final variable.
class Point {
  final num x, y;
  Point(this.x, this.y);
}

main() {
  var p = new Point(1, 1);
  print(p.x); // 1
  p.x = 2; // WARNING, no such method
}
It's important to realize that final affects the variable, but not the object pointed to by the variable. That is, final doesn't make the variable's object immutable.
For example:
class Address {
  String city;
  String state;
  Address(this.city, this.state);
}

main() {
  final address = new Address("anytown", "hi");
  address.city = 'waikiki';
  print(address.city); // waikiki
}
In the above example, the address variable is marked as final, so it will always point to the object instantiated by the new Address("anytown", "hi") constructor. However, the object itself has state that is mutable, so it's perfectly valid to change the city. The only thing prevented by final is reassigning the address variable.

Popular posts from this blog

Lists and arrays in Dart

(This is Part 3 in a series about Dart. Check out Part 2, Function in Dart .) Warning : We expect the Dart libraries to undergo potentially sweeping changes before Dart goes to alpha. This document is relevant as of 2011-12-22. Intro Dart is a "batteries included" effort to help app developers build modern web apps. An important "battery" is the bundled core Dart libraries, providing common and rich functionality. Dart is building a solution for large, complex web apps, and providing well tested, integrated, and common libraries is key to helping a web app developer be more productive out of the box. The Collection libraries are a crucial set of APIs that Dart developers get for free. Much more than simple arrays and maps, the Collection library includes standard ways to filter, iterate, inspect, compose, and sort your data. This post specifically looks at List<E> , Dart's ordered, indexable collection of objects. Aside: The Dart project is lucky
Read more

The 29 Healthiest Foods on the Planet

The 29 Healthiest Foods on the Planet > The following is a "healthy food hot list" consisting of the 29 food that will give you the biggest nutritional bang for you caloric buck, as well as decrease your risk for deadly illnesses like cancer, diabetes and heart disease.
Read more

Converting Array to List in Scala

Now, this has to have a built-in somewhere in Scala , because it just seems too common. So, how to convert an Array to a List in Scala? Why do I need this? I needed to drop to Java for some functionality, which in this case returns an Array. I wanted to get that Array into a List to practice my functional programming skillz. **Update**: I figured out how to convert Arrays to Lists the Scala way. Turns out it's a piece of cake. val myList = List.fromArray(Array("one", "two", "three")) or val myList = Array("one","two","three").elements.toList The call to elements returns an Iterator , and from there you can convert to a List via toList . Nice. Because my first version wasn't actually tail recursive, what follows is a true tail recursive solution, if I were to implement this by hand. The above, built in mechanism is much better, though. object ArrayUtil { def toList[a](array: Array[a]): List[a] = { d
Read more