Classes and Objects

A class is a blueprint that describes a kind of thing: the data it holds (fields, also called properties) and what it can do (methods). An object is one actual thing built from that blueprint, also called an instance.

Real-world analogy: a class is a cookie cutter; objects are the cookies. One cutter, many cookies — each can have different icing, but they all share the same shape.

Example

class Dog {
  final String name;
  final String breed;
  int tricks = 0;

  Dog(this.name, this.breed);

  String bark() => '$name says woof!';

  void learnTrick() {
    tricks++;
  }

  String describe() => '$name the $breed knows $tricks tricks';
}

void main() {
  final rex = Dog('Rex', 'beagle');
  final luna = Dog('Luna', 'collie');

  print(rex.bark());
  rex.learnTrick();
  rex.learnTrick();
  print(rex.describe());
  print(luna.describe());
}

Expected output

Rex says woof!
Rex the beagle knows 2 tricks
Luna the collie knows 0 tricks

Line by line:

• final String name; declares a field that's set once, when the object is created.
• int tricks = 0; is a field with a starting value that can change.
• Dog(this.name, this.breed); is the constructor. this.name means "store this argument in the name field" — no body needed.
• bark, learnTrick, and describe are methods. Inside them, fields are used directly by name.
• Dog('Rex', 'beagle') creates an instance. (Older Dart code writes new Dog(...); new is optional and no longer used.)

Each object has its own data: teaching Rex tricks doesn't affect Luna.

Common mistakes

• Forgetting to initialize a non-nullable field: Field 'name' should be initialized because its type 'String' doesn't allow null. Set it in the constructor, give it a default, or make it nullable.
• Printing an object: print(rex) shows Instance of 'Dog' until you override toString (a later lesson).

Practice

Create a BankAccount class with an owner and a balance starting at 0.0, with deposit(amount) and summary() methods. summary() should return 'Ada has 150.0' after depositing 100 and 50.

class BankAccount {
  final String owner;
  double balance = 0.0;

  BankAccount(this.owner);

  void deposit(double amount) {
    balance += amount;
  }

  String summary() => '$owner has $balance';
}

void main() {
  final account = BankAccount('Ada');
  account.deposit(100);
  account.deposit(50);
  print(account.summary());
}

Ada has 150.0