Oppiskele Abstract Classes and Interfaces | Inheritance and Abstraction

Understanding abstraction and interfaces is essential for designing flexible and reusable code in Dart. Abstract classes provide a way to define a blueprint for other classes, specifying methods that must be implemented by subclasses. You use abstract classes when you want to establish a common contract but do not intend to create instances of the abstract class itself. In Dart, you declare an abstract class using the abstract keyword. Interfaces, on the other hand, define a set of methods that a class must implement, without providing any implementation details. Every class in Dart can act as an interface, and you implement an interface using the implements keyword. This allows you to separate what a class should do from how it does it, making your code more modular and testable.

Declaring abstract classes

You declare an abstract class in Dart using the abstract keyword before the class declaration. Abstract classes can have both abstract methods (methods without a body) and concrete methods (methods with a body). Abstract classes cannot be instantiated directly.

Implementing interfaces

In Dart, any class can act as an interface. To implement an interface, a class uses the implements keyword and must provide concrete implementations for all methods defined in the interface, regardless of whether they already have implementations.

main.dart


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abstract class Shape {
  double area();
}

class Rectangle implements Shape {
  double width;
  double height;

  Rectangle(this.width, this.height);

  @override
  double area() {
    return width * height;
  }
}

void main() {
  Rectangle rect = Rectangle(5, 3);
  print('Area of rectangle: [1m${rect.area()}[0m');
}

The Shape class is declared as abstract and defines an abstract method area(). The Rectangle class implements the Shape interface by providing a concrete implementation of the area() method. By doing so, Rectangle fulfills the contract specified by Shape, ensuring that any code working with Shape references can rely on the presence of the area() method. This approach enables polymorphic code, where you can write functions or collections that operate on any object implementing the Shape interface, regardless of its concrete class.

main.dart


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abstract class Printable {
  void printInfo();
}

class Book implements Printable {
  String title;
  Book(this.title);

  @override
  void printInfo() {
    print('Book: $title');
  }
}

class Magazine implements Printable {
  String name;
  Magazine(this.name);

  @override
  void printInfo() {
    print('Magazine: $name');
  }
}

void main() {
  Printable book = Book('Dart Essentials');
  Printable magazine = Magazine('Tech Monthly');

  book.printInfo();
  magazine.printInfo();
}

Study More

While Dart does not have a separate interface keyword, every class definition can serve as an interface. Abstract classes can contain both implemented and unimplemented methods, while interfaces (using implements) require you to provide concrete implementations for all methods, even those with default implementations in the original class.

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Suggested prompts:

Can you give an example of how to declare an abstract class and implement it in Dart?

What are the main differences between using abstract classes and interfaces in Dart?

How does polymorphism work with abstract classes and interfaces in Dart?

Pyyhkäise näyttääksesi valikon

Declaring abstract classes

Implementing interfaces

main.dart


              1234567891011121314151617181920
            
abstract class Shape {
  double area();
}

class Rectangle implements Shape {
  double width;
  double height;

  Rectangle(this.width, this.height);

  @override
  double area() {
    return width * height;
  }
}

void main() {
  Rectangle rect = Rectangle(5, 3);
  print('Area of rectangle: [1m${rect.area()}[0m');
}

main.dart


              1234567891011121314151617181920212223242526272829303132
            
abstract class Printable {
  void printInfo();
}

class Book implements Printable {
  String title;
  Book(this.title);

  @override
  void printInfo() {
    print('Book: $title');
  }
}

class Magazine implements Printable {
  String name;
  Magazine(this.name);

  @override
  void printInfo() {
    print('Magazine: $name');
  }
}

void main() {
  Printable book = Book('Dart Essentials');
  Printable magazine = Magazine('Tech Monthly');

  book.printInfo();
  magazine.printInfo();
}

Study More

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