Lesson 32 of 59

5 mins read

Implicit Interfaces & the implements Keyword

In Plain English

Every class in Dart implicitly defines an **Interface** containing all of its instance members and getters. This means any class can act as an interface for another class via the `implements` keyword. While a class can only `extend` ONE superclass, it can `implement` MULTIPLE interfaces (`class C implements A, B`), providing powerful multiple-contract polymorphism.

The Mental Model

"An implicit interface is like an actor playing a famous role: the actor doesn't share DNA with the historical figure (`extends`), but they memorize and deliver every single line and gesture of the character (`implements`)."

Deep Dive: How It Works

Implicit Interface: You don't need a separate `interface` keyword to define contracts; every standard class automatically acts as an interface.

extends vs implements: `extends` inherits implementation code; `implements` forces the subclass to re-implement every single field and method.

Multiple Interfaces: A single class can implement multiple independent interfaces (`implements Serializable, Comparable, Disposable`).

Syntax Blueprint

class AuthenticLogger implements Printer, Formatter {
  @override
  void printMessage(String msg) { ... }
  @override
  String format(String raw) { ... }
}

Implementing multiple implicit class interfaces.

Core Rules to Remember

Every Class is an Interface: No separate interface declaration required; any class can be implemented.

Multiple Implementation: Implement as many interfaces as required simultaneously.

Full Method Re-implementation: `implements` copies no code; all members must be defined explicitly.

Common Beginner Traps & How to Fix Them

Mistake:Using `implements` and expecting to inherit the parent class's method bodies.

Why it happens: Confusing `extends` (code inheritance) with `implements` (interface contract).

How to fix: Use `extends` if you want to inherit implementation code; use `implements` if you only want the type contract.

Pro Tip: In Dart unit testing, you can mock any complex dependency simply by implementing its class interface (`class MockClient implements HttpClient`).

Live Interactive Example

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Implementing Multiple Implicit Interfaces

Python 3.12

Output Console

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How it works: CacheService satisfied the type contracts of both Describable and Resettable implicit interfaces.

Your Turn: Micro Challenge

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Micro Exercise

Implement Runner Interface

Create class `Runner { void run() {} }`.
Create class `Athlete implements Runner { @override void run() => print("Athlete running!"); }`.
In `main()`, instantiate `Athlete().run();`.

Sandbox Output

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