Dart Classes

A class is a plan for making objects. Each object made from a class holds its own stored values.

Fields on an object

A field is a stored value that belongs to an object. The constructor Dog(this.name, this.age) fills both fields when an object is made.

class Dog {
  String name;
  int age;
  Dog(this.name, this.age);
}

void main() {
  var rex = Dog('Rex', 4);
  print(rex.name);
  print(rex.age);
}

A method on a class

A method is a function that belongs to a class. describe reads the fields of its own object and prints them as one line.

class Dog {
  String name;
  int age;
  Dog(this.name, this.age);
  void describe() {
    print('$name is $age years old');
  }
}

void main() {
  var rex = Dog('Rex', 4);
  rex.describe();
}

Two objects from one class

One class can make many objects in a program. Each object keeps its own field values, so the two lines differ.

class Dog {
  String name;
  int age;
  Dog(this.name, this.age);
  void describe() {
    print('$name is $age years old');
  }
}

void main() {
  var rex = Dog('Rex', 4);
  var bella = Dog('Bella', 2);
  rex.describe();
  bella.describe();
}

A getter on a class

A getter is a value that a class computes from its fields. label joins the name and the age, and main reads it with no parentheses.

class Dog {
  String name;
  int age;
  Dog(this.name, this.age);
  String get label => '$name ($age)';
}

void main() {
  var rex = Dog('Rex', 4);
  print(rex.label);
}

Example

class Dog {
  String name;
  int age;
  Dog(this.name, this.age);
  void describe() {
    print('$name is $age years old');
  }

  String get label => '$name ($age)';
}

void main() {
  var rex = Dog('Rex', 4);
  var bella = Dog('Bella', 2);
  rex.describe();
  bella.describe();
  print(rex.label);
}

Key points

Use the class name with parentheses to make an object.
Use a dot to reach a field or a method of an object.
Each object keeps its own field values.
A getter has no parentheses when an object reads it.

In this example, the class Dog holds two fields, a constructor, a method, and a getter. The constructor fills the name and the age of each object. describe prints one line for Rex and one line for Bella. The getter label then builds a short text for Rex, and the last print shows it.

Try it Yourself

Edit the code, then press Run.

Running is simulated · DartPad

class Dog {
  String name;
  int age;
  Dog(this.name, this.age);
  void describe() {
    print('$name is $age years old');
  }

  String get label => '$name ($age)';
}

void main() {
  var rex = Dog('Rex', 4);
  var bella = Dog('Bella', 2);
  rex.describe();
  bella.describe();
  print(rex.label);
}

Exercise

Which line makes an object from the class Dog?

var rex = Dog('Rex', 4); class rex = Dog(); print(Dog); field rex = Dog;