Lesson 31 of 59

5 mins read

Abstract Classes & Method Contracts

In Plain English

An **Abstract Class** (`abstract class`) cannot be instantiated directly with `new` or generative constructors. It serves as an architectural blueprint defining method signatures (contracts) without implementations, alongside shared concrete helper methods that all derived subclasses inherit and implement.

The Mental Model

"An abstract class is like an appliance electrical specification standard: it dictates that every microwave or toaster must have a 3-prong grounded plug, but leaves the internal heating mechanism up to each manufacturer."

Deep Dive: How It Works

Instantiation Prevention: Calling `AbstractClass()` directly is rejected by the compiler.

Abstract Methods: Method signatures declared without a body (e.g. `void save();`) must be overridden by concrete subclasses.

Partial Implementation: Abstract classes can contain both implemented methods and abstract method stubs.

Syntax Blueprint

abstract class PaymentGateway {
  void processPayment(double amount); // Abstract method
  void logTransaction() => print("Tx logged"); // Shared concrete method
}

Abstract class defining abstract contract with optional concrete defaults.

Core Rules to Remember

Architectural Blueprint: Standardizes interfaces across multiple interchangeable implementations.

Cannot be Instantiated: Prevents incomplete base classes from existing as runtime instances.

Enforced Implementation: Concrete subclasses must implement all abstract methods.

Common Beginner Traps & How to Fix Them

Mistake:Forgetting to implement one of the abstract methods in a concrete subclass.

Why it happens: Missing method triggers a compilation error: "Missing concrete implementation".

How to fix: Implement all abstract methods or declare the subclass itself as abstract.

Pro Tip: In Dart 3, combine `abstract` with `sealed` or `interface` class modifiers to control external library inheritance.

Live Interactive Example

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Polymorphic Storage Repositories with Abstract Classes

Python 3.12

Output Console

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How it works: Instantiated concrete MemoryStore through abstract DataStore interface reference.

Your Turn: Micro Challenge

No pressure! Edit the starter code below and test your solution with instant feedback.

Micro Exercise

Implement Abstract Shape Class

Create `abstract class Shape { double getArea(); }`.
Create class `SquareShape extends Shape { double s; SquareShape(this.s); @override double getArea() => s * s; }`.
In `main()`, print `Area: ${SquareShape(5.0).getArea()}`.

Sandbox Output

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