Dart Lesson 40 of 102 3 min read

Classes and Objects in Dart

Learn object-oriented basics in Dart: define a class, create objects, and use fields, methods, this and toString with clear examples.

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A class is a blueprint. An object is a thing built from that blueprint. The class Car describes what every car has and can do; your car and your neighbour’s car are two objects of that class.

Defining a class #

class Dog {
  // Fields: the data each dog has.
  String name;
  int age;

  // Constructor: runs when a dog is created.
  Dog(this.name, this.age);

  // Method: something a dog can do.
  void bark() {
    print('$name says Woof!');
  }
}

void main() {
  var dog = Dog('Bruno', 3);
  dog.bark();
  print(dog.age);
}

Bruno says Woof!
3

Class names use UpperCamelCase. Fields and methods use lowerCamelCase.

Each object has its own data #

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

  void birthday() {
    age++;
    print('$name is now $age');
  }
}

void main() {
  var a = Dog('Bruno', 3);
  var b = Dog('Kali', 7);

  a.birthday();
  print(b.age); // unchanged
}

Bruno is now 4
7

Both dogs share the same methods, but each keeps its own name and age.

The this keyword #

Inside a class, this refers to the current object. You usually do not need to write it. It is required only when a local name hides a field.

class Counter {
  int value = 0;

  void add(int value) {
    this.value += value; // field += parameter
  }
}

void main() {
  var c = Counter();
  c.add(5);
  c.add(2);
  print(c.value);
}

Methods that return values #

class Rectangle {
  double width;
  double height;
  Rectangle(this.width, this.height);

  double area() => width * height;
  double perimeter() => 2 * (width + height);
  bool isSquare() => width == height;
}

void main() {
  var r = Rectangle(4, 6);
  print(r.area());
  print(r.perimeter());
  print(r.isSquare());
}

24.0
20.0
false

A readable print with toString #

Printing an object shows Instance of 'Dog' by default. Override toString to say something useful.

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

  @override
  String toString() => 'Dog($name, $age)';
}

void main() {
  print(Dog('Bruno', 3));
}

Dog(Bruno, 3)

Objects are passed by reference #

A variable holds a reference to an object, not a copy. Two variables can point to the same object.

class Dog {
  String name;
  Dog(this.name);
}

void rename(Dog d) => d.name = 'Max';

void main() {
  var a = Dog('Bruno');
  var b = a;      // same dog
  rename(b);
  print(a.name);
}

Max

Vocabulary #

Term	Meaning
Class	The blueprint
Object / instance	One thing made from the class
Field / property	A variable that belongs to an object
Method	A function that belongs to an object
Constructor	The special function that creates an object
Member	Any field, method or constructor of a class

Try it yourself #

Create a BankAccount class with an owner and a balance. Add deposit and withdraw methods, where withdraw refuses to go below zero, and a toString that prints Owner: balance. Make two accounts and move money between them.