Getters, Setters, and Privacy

Dart lets you control how fields are read and changed, and hide implementation details.

Getters — computed properties

A getter looks like a field from the outside but calculates its value when read:

class Rectangle {
  double width;
  double height;

  Rectangle(this.width, this.height);

  double get area => width * height;
  bool get isSquare => width == height;
}

void main() {
  final rect = Rectangle(4, 5);
  print(rect.area);
  rect.width = 5;
  print(rect.area);
  print(rect.isSquare);
}

20.0
25.0
true

area is always correct, because it's recalculated from the current width and height.

Privacy with underscores

Names starting with _ are private to the library (the file). Pair a private field with a public getter to make something read-only from outside:

class Counter {
  int _count = 0;
  int _step = 1;

  int get count => _count;

  set step(int value) {
    if (value <= 0) throw ArgumentError('step must be positive');
    _step = value;
  }

  void increment() => _count += _step;
}

void main() {
  final c = Counter();
  c.step = 5;
  c.increment();
  c.increment();
  print(c.count);
}

10

• count can be read but not assigned — there's no setter.
• set step(int value) is a setter with validation. Callers write c.step = 5, like assigning a field.

Dart privacy is per file, not per class: code elsewhere in the same file can still see _count. In practice, one main class per file makes this behave like class privacy.

Common mistakes

• Writing getters and setters that just read and write a field. Dart doesn't need them — use a public field, and switch to a getter/setter later if you need logic; callers won't have to change.