Abstract Class in Dart

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Introduction

In this section, you will learn about dart abstract class. Before learning about abstract class, you should have a basic understanding of class, object, constructor, and inheritance. Previously you learned how to define a class. These classes are concrete classes. You can create an object of concrete classes, but you cannot create an object of abstract classes.

Abstract Class

Abstract classes are classes that cannot be initialized. It is used to define the behavior of a class that can be inherited by other classes. An abstract class is declared using the keyword abstract.

Syntax

abstract class ClassName {
  //Body of abstract class

  method1();
  method2();
}

Abstract Method

An abstract method is a method that is declared without an implementation. It is declared with a semicolon (;) instead of a method body.

Syntax

abstract class ClassName {
  //Body of abstract class
  method1();
  method2();
}

Why We Need Abstract Class

Subclasses of an abstract class must implement all the abstract methods of the abstract class. It is used to achieve abstraction in the Dart programming language.

Example 1: Abstract Class In Dart

In this example below, there is an abstract class Vehicle with two abstract methods start() and stop(). The subclasses Car and Bike implement the abstract methods and override them to print the message.

abstract class Vehicle {
  // Abstract method
  void start();
  // Abstract method
  void stop();
}

class Car extends Vehicle {
  // Implementation of start()
  @override
  void start() {
    print('Car started');
  }

  // Implementation of stop()
  @override
  void stop() {
    print('Car stopped');
  }
}

class Bike extends Vehicle {
  // Implementation of start()
  @override
  void start() {
    print('Bike started');
  }

  // Implementation of stop()
  @override
  void stop() {
    print('Bike stopped');
  }
}

void main() {
  Car car = Car();
  car.start();
  car.stop();

  Bike bike = Bike();
  bike.start();
  bike.stop();
}

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Info

Note: The abstract class is used to define the behavior of a class that can be inherited by other classes. You can define an abstract method inside an abstract class.

Example 2: Abstract Class In Dart

In this example below, there is an abstract class Shape with one abstract method area() and two subclasses Rectangle and Triangle. The subclasses implement the area() method and override it to calculate the area of the rectangle and triangle, respectively.

abstract class Shape {
  int dim1, dim2;
  // Constructor
  Shape(this.dim1, this.dim2);
  // Abstract method
  void area();
}

class Rectangle extends Shape {
  // Constructor
  Rectangle(int dim1, int dim2) : super(dim1, dim2);

  // Implementation of area()
  @override
  void area() {
    print('The area of the rectangle is ${dim1 * dim2}');
  }
}

class Triangle extends Shape {
  // Constructor
  Triangle(int dim1, int dim2) : super(dim1, dim2);

  // Implementation of area()
  @override
  void area() {
    print('The area of the triangle is ${0.5 * dim1 * dim2}');
  }
}

void main() {
  Rectangle rectangle = Rectangle(10, 20);
  rectangle.area();

  Triangle triangle = Triangle(10, 20);
  triangle.area();
}

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Constructor In Abstract Class

You can’t create an object of an abstract class. However, you can define a constructor in an abstract class. The constructor of an abstract class is called when an object of a subclass is created.

Example 3: Constructor In Abstract Class

In this example below, there is an abstract class Bank with a constructor which takes two parameters name and rate. There is an abstract method interest(). The subclasses SBI and ICICI implement the abstract method and override it to print the interest rate.

abstract class Bank {
  String name;
  double rate;

  // Constructor
  Bank(this.name, this.rate);

  // Abstract method
  void interest();

  //Non-Abstract method: It have an implementation
  void display() {
    print('Bank Name: $name');
  }
}

class SBI extends Bank {
  // Constructor
  SBI(String name, double rate) : super(name, rate);

  // Implementation of interest()
  @override
  void interest() {
    print('The rate of interest of SBI is $rate');
  }
}

class ICICI extends Bank {
  // Constructor
  ICICI(String name, double rate) : super(name, rate);

  // Implementation of interest()
  @override
  void interest() {
    print('The rate of interest of ICICI is $rate');
  }
}

void main() {
  SBI sbi = SBI('SBI', 8.4);
  ICICI icici = ICICI('ICICI', 7.3);

  sbi.interest();
  icici.interest();
  icici.display();
}

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Key Points To Remember

You can’t create an object of an abstract class.
It can have both abstract and non-abstract methods.
It is used to define the behavior of a class that other classes can inherit.
Abstract method only has a signature and no implementation.

Video

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