Dart Class

Summary: in this tutorial, you’ll learn how to define a Dart class and create objects from the class.

Introduction to the Dart classes

Dart is an object-oriented programming language. In Dart, everything is an object. An object consists of states and behaviors:

• The state describes the values that an object has at a specified time.
• The behaviors are actions that an object can do to manipulate its state.

To create an object, you need to define a class first. A class is a blueprint for creating objects.

Variables are used to model the state of objects. When variables are defined inside a class, they are called properties of the class.

Functions are used to model the behaviors of objects. When functions are defined inside a class, they are called methods.

Defining a class

To define a class, you use the class keyword followed by a class name and curly braces:

class MyClass {
}Code language: Dart (dart)

By convention, the class names follow the PascalCase naming convention. For example, the following defines the Point class:

class Point {
}Code language: Dart (dart)

Creating objects from a class

To create an object from a class, you use the class name followed by the parentheses. It’s like calling a function.

For example, the following creates a new object called p1 from the Point class:

Point p1 = Point();Code language: Dart (dart)

Because the Dart compiler can infer the type of the p1 as Point, you can use the var keyword to make the statement more concise:

var p1 = Point();Code language: Dart (dart)

In this example, p1 is an object of the Point class. In other words, p1 is an instance of the Point class:

class Point {
}

void main() {
  var p1 = Point();
}Code language: Dart (dart)

Adding properties to the class

The following adds the x-coordinate and y-coordinate to the Point class and initialized their values to zeros:

class Point {
  int x = 0;
  int y = 0;
}Code language: Dart (dart)

By doing this, all objects of the Point class will have the x and y properties. To access a property from an object, you use the dot notation as follows:

objectName.propertyCode language: Dart (dart)

For example, the following creates a new Point object and assigns the values to the x and y properties:

class Point {
  int x = 0;
  int y = 0;
}

void main() {
  var p1 = Point();
  p1.x = 10;
  p1.y = 20;
}Code language: Dart (dart)

It’s important to note that you can create many objects from the Point class. For example, the following creates two Point objects p1 and p2:

class Point {
  int x = 0;
  int y = 0;
}

void main() {
  var p1 = Point();
  p1.x = 10;
  p1.y = 20;

  var p2 = Point();
  p2.x = 100;
  p2.x = 200;
}Code language: Dart (dart)

In this example, p1 and p2 will have a separate set of properties x and y.

Cascade notation

The following creates the p1 object and assigns its properties to values:

var p1 = Point();
p1.x = 10;
p1.y = 20;Code language: Dart (dart)

In this example, the p1 object is repeated multiple times, each for an assignment. To make it more concise, Dart provides a cascade operator (..) that allows you to chain multiple assignments on the same object without repeating the object name. For example:

class Point {
  int x = 0;
  int y = 0;
}

void main() {
  var p1 = Point()
    ..x = 10
    ..y = 20;
}Code language: Dart (dart)

It’s important to note that the semicolon (;) only appears on the last line.

Adding a method to a class

A method is like a function. When you define a function inside a class, it becomes a method. For example, the following adds a method called move() to the Point class that moves the point to a new coordinate:

class Point {
  int x = 0;
  int y = 0;

  void move(int x1, int y1) {
    x = x1;
    y = y1;
  }
}Code language: Dart (dart)

Unlike a function, the move() method can access the x and y properties.

To call a method, you also use a dot notation syntax:

objectName.methodName(arguments);Code language: JavaScript (javascript)

For example, the following shows how to call the move() method:

p1.move(100, 200);Code language: CSS (css)

The following shows a complete program that defines the Point class and displays its object to the console:

class Point {
  int x = 0;
  int y = 0;

  void move(int x1, int y1) {
    x = x1;
    y = y1;
  }
}

void main() {
  var p1 = Point()
    ..x = 10
    ..y = 20;

  p1.move(100, 200);

  print(p1);
}
Code language: Dart (dart)

Output:

Instance of 'Point'Code language: Dart (dart)

When you pass the p1 object to the print() function, the print() function converts the p1 object into a string before displaying it. By default, an object has the following string representation:

Instance of 'ClassName'Code language: Dart (dart)

To make a custom string representation of an object, you need to override a method called toString() method which will be covered in a later tutorial.

For now, you can define a show() method that displays the Point object to the console like this:

class Point {
  int x = 0;
  int y = 0;

  void move(int x1, int y1) {
    x = x1;
    y = y1;
  }

  void show() {
    print('Point($x,$y)');
  }
}

void main() {
  var p1 = Point()
    ..x = 10
    ..y = 20;
  p1.move(100, 200);
  p1.show();
}
Code language: Dart (dart)

Output:

Point(100,200)Code language: Dart (dart)

The is operator

The is operator returns true if an object is an instance of a class. If an object is not an instance of the class, the is operator returns false:

myObject is MyClass

For example, the following expression returns true because p1 is an instance of the Point class:

var p1 = Point();
print(p1 is Point);Code language: PHP (php)

Summary

• Objects have states and behaviors.
• Properties represent the object’s state and methods define the object’s behaviors.
• A class is a blueprint for creating objects.
• Use the class keyword to define a class.
• Use the is operator to check if an object is an instance of a class.