Extension vs subclass

Learn the difference between extension and subclass.

Extension vs subclass

Extension and subclass are two ways to add functionality to a type. An extension adds methods from outside (no inheritance); a subclass extends the type (with inheritance).

Extension advantages: can extend final classes, add functionality to types you don't own. Subclass advantages: can add state, override methods, provide polymorphism. Which to use depends on the need.

When to use which?

Use an extension: when you only want to add helper methods, when you don't own the type, when inheritance isn't needed. Use a subclass: when you want to add new state (fields), when you'll override, when polymorphism is required.

Extensions are stateless; you cannot add new fields. Subclasses can add state. Also, extensions are resolved statically; subclasses are dynamic. So when polymorphism is needed, a subclass is required.

Exercise A StringShout extension

Define a StringShout extension on String with a String shout() => toUpperCase() + '!'; method. In main add print('hello'.shout());.

Show example solution

extension StringShout on String {
  String shout() => toUpperCase() + '!';
}

void main() {
  print('hello'.shout());
}

Test yourself

Extension on String

extension StringShout on ____ {
  String shout() => toUpperCase() + '!';
}

Show answer

extension StringShout on String {
  String shout() => toUpperCase() + '!';
}

Match extension vs subclass differences

Extension

No inheritance
Subclass

With inheritance
Final class

Extendable via extension
Polymorphism

With subclass
When is an extension used instead of a subclass?
- To override methods
- To add new fields to a type
- For polymorphism
- To only add helper methods to a type
Show answer

To only add helper methods to a type — an extension is ideal for adding helper methods.

Next lesson: Summary - Native extension

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