Lesson 25 of 59

5 mins read

Classes & Objects: Members, Instance Methods & this

In Plain English

Dart is a pure object-oriented language where every value is an instance of a class derived from `Object`. Classes encapsulate state (fields) and behavior (methods). The `this` keyword references the current instance receiver and is used when field names collide with parameter names.

The Mental Model

"A class is like an architectural blueprint for a smart house; an object is the actual physical house built on a lot, with its own independent power meter and temperature settings."

Deep Dive: How It Works

Pure Object Hierarchy: All types, including primitives and functions, inherit from `Object` (or `Object?`).

Instance Methods: Functions declared inside a class that have access to `this` and instance fields.

Cascades on Objects: Multiple methods and field mutations can be chained using `..`.

Syntax Blueprint

class User {
  String name = "";
  int score = 0;

  void incrementScore(int points) {
    score += points;
  }
}

Class definition with instance fields and modifying method.

Core Rules to Remember

Unified Object Model: Everything in Dart is an object, with no separate primitive wrapper types.

Implicit this: `this` is optional in Dart unless disambiguating naming collisions.

Cascade Friendly: Classes interact seamlessly with cascade operator `..`.

Common Beginner Traps & How to Fix Them

Mistake:Using `this.field` redundantly everywhere inside methods.

Why it happens: Java habit. Dart style guidelines advise omitting `this.` unless required for disambiguation.

How to fix: Omit `this.` unless a local variable shadows the field name.

Pro Tip: Use `toString()` overrides on domain classes to produce clean debug representations in logs and console.

Live Interactive Example

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Creating and Interacting with Class Instances

Python 3.12

Output Console

Click "Run Code" to view the rendered output.

How it works: Created an instance of BankAccount, mutated state with deposit(), and printed custom toString().

Your Turn: Micro Challenge

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

Micro Exercise

Define Product Class

Create a class `Product` with fields `String title; double price;` and constructor `Product(this.title, this.price);`.
In `main()`, instantiate `final p = Product("Laptop", 999.0);`.
Print `Product: ${p.title} costs $${p.price}`.

Sandbox Output

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