Dart Modifiers - final vs. const

Describes the difference between the final and const keywords in the Dart programming language.

Code example @ https://dartpad.dartlang.org/c09c790d954c5d53572cb9661b36ff43

Both `final` and `const` prevent a variable from being reassigned (similar to how `final` works in Java or how `const` works in JavaScript).

The difference has to do with how memory is allocated. Memory is allocated for a `final` variable at runtime, and for a `const` variable at compile-time. The `final` modifier should be the more commonly used, because many program variables won't need any memory since the program logic won't call for them to be initialized. With a `const` variable you are basically telling the computer, "Hey, I need memory for this variable up front because I know I'm going to need it."

Thinking of them in this way makes it easier to understand differences in their syntactical usage. Mainly that a `final` variable may be an instance variable, but a `const` must be a `static` variable on a class. This is because instance variables are created at runtime, and `const` variables--by definition--are not. Thus, `const` variables on a class must be `static`, which means simply that a single copy of that variable exists on a class, regardless of whether that class is instantiated.

This article goes into more depth and explains a very important semantic difference between the two, i.e. `final` modifies variables and `const` modifies values, which essentially boils down to only being able to initialize `const` values which are derivable at compile-time.

https://news.dartlang.org/2012/06/const-static-final-oh-my.html