Lesson 29 of 59

5 mins read

Getters, Setters & Private Fields (_)

In Plain English

Dart achieves privacy at the **library** level (not class level) by prefixing identifiers with an underscore (`_privateField`). Custom **Getters** (`get`) and **Setters** (`set`) allow defining computed virtual properties and validation wrappers that callers access with clean dot-syntax (`rect.area`) without clumsy getter methods like `getArea()`.

The Mental Model

"Getters and setters are like a thermostat on a wall: turning the dial feels like setting a simple property directly, but behind the wall, complex electrical relays and heaters are validated and activated."

Deep Dive: How It Works

Library Privacy (`_`): Prepending an underscore makes fields, methods, and classes private to the containing `.dart` file/library.

Uniform Access Principle: Callers use `obj.property` syntax regardless of whether it is a stored field or a computed getter.

Validation Setters: Use `set value(val)` to validate inputs and enforce business invariants.

Syntax Blueprint

class Temperature {
  double _celsius = 0.0; // Private field

  double get fahrenheit => (_celsius * 9 / 5) + 32;
  set fahrenheit(double f) => _celsius = (f - 32) * 5 / 9;
}

Private backing field with computed getter and mutating setter.

Core Rules to Remember

Library-Level Privacy: Identifiers starting with `_` are visible only within the same file/library.

Uniform Access: Computed getters use standard property access syntax without parentheses.

Encapsulated Validation: Setters guard mutations and update internal state consistently.

Common Beginner Traps & How to Fix Them

Mistake:Writing redundant trivial getters/setters (e.g. `String get name => _name; set name(val) => _name = val;`).

Why it happens: Habit from Java/C#.

How to fix: Start with a public field `String name;`; you can refactor to getters/setters later without breaking client code.

Pro Tip: In Dart, you can safely change a public field to a getter/setter without changing a single line of caller code.

Live Interactive Example

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Encapsulating Temperature with Getters and Setters

Python 3.12

Output Console

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How it works: Accessed and updated private temperature state through computed getters and validated setters.

Your Turn: Micro Challenge

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

Micro Exercise

Create Rectangle with Computed Area Getter

Create class `Square` with `double side; Square(this.side);`.
Add getter `double get area => side * side;`.
In `main()`, instantiate `Square(6.0)` and print `Square Area: ${s.area}`.

Sandbox Output

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