How to define interfaces in Dart?

Question

In Java, I might have an interface IsSilly and one or more concrete types that implement it:

public interface IsSilly {
    public void makePeopleLaugh();
}

public class Clown implements IsSilly {
    @Override
    public void makePeopleLaugh() {
        // Here is where the magic happens
    }
}

public class Comedian implements IsSilly {
    @Override
    public void makePeopleLaugh() {
        // Here is where the magic happens
    }
}

What's the equivalent to this code in Dart?

After perusing the official docs on classes, it doesn't seem that Dart has a native interface type. So, how does the average Dartisan accomplish the interface segregation principle?

score 176 · Accepted Answer · edited Oct 25 '21 at 01:46

In Dart there is a concept of implicit interfaces.

Every class implicitly defines an interface containing all the instance members of the class and of any interfaces it implements. If you want to create a class A that supports class B’s API without inheriting B’s implementation, class A should implement the B interface.

A class implements one or more interfaces by declaring them in an implements clause and then providing the APIs required by the interfaces.

So your example can be translate in Dart like this :

abstract class IsSilly {
  void makePeopleLaugh();
}

class Clown implements IsSilly {
  void makePeopleLaugh() {
    // Here is where the magic happens
  }
}

class Comedian implements IsSilly {
  void makePeopleLaugh() {
    // Here is where the magic happens
  }
}

Is it possible to define an abstract static methods or an abstract named constructor? — Anton, Feb 27 '20 at 13:27

score 19 · Answer 2 · answered Jan 12 '19 at 03:12

The confusion usually is because doesn't exist the word "interface" like java and another languages. Class declarations are themselves interfaces in Dart.

In Dart every class defines an implicit interface like others says.So then... the key is: Classes should use the implements keyword to be able to use an interface.

abstract class IsSilly {
  void makePeopleLaugh();
}

//Abstract class
class Clown extends IsSilly {   
  void makePeopleLaugh() {
    // Here is where the magic happens
  }
}

//Interface
class Comedian implements IsSilly {
  void makePeopleLaugh() {
    // Here is where the magic happens
  }
}

score 18 · Answer 3 · answered Dec 26 '13 at 21:14

In Dart, every class defines an implicit interface. You can use an abstract class to define an interface that cannot be instantiated:

abstract class IsSilly {
    void makePeopleLaugh();
}

class Clown implements IsSilly {

    void makePeopleLaugh() {
        // Here is where the magic happens
    }

}

class Comedian implements IsSilly {

    void makePeopleLaugh() {
        // Here is where the magic happens
    }

}

score 5 · Answer 4 · edited Aug 03 '23 at 18:49

The other answers do a good job at informing about interfaces with methods.

If you're looking for an interface with properties, you can use getters:

abstract class AppColors {
  Color get primary;

  Color get secondary;
}

class AppColorsImpl implements AppColors {

  @override
  Color get primary => Colors.red;

  @override
  Color get primary => Colors.blue;
}

And yes, you can combine an interface to have both properties and methods too.

--- Updated --- With version 3.0 Dart added support for Class modifiers:

https://dart.dev/language/class-modifiers

score 3 · Answer 5 · answered Jan 11 '20 at 18:33


abstract class ORMInterface {
  void fromJson(Map<String, dynamic> _map);
}

abstract class ORM implements ORMInterface {

  String collection = 'default';

  first(Map<String, dynamic> _map2) {
    print("Col $collection");
  }
}

class Person extends ORM {

  String collection = 'persons';

  fromJson(Map<String, dynamic> _map) {
    print("Here is mandatory");
  }
}

score 2 · Answer 6 · answered Aug 26 '21 at 20:00

This is a class or an interface depending on the situation.

abstract class A {
    void sayHello() {
       print("Hello");
    }
    void sayBye();
}

B class implements A interface, so it has to implements all methods of A.

class B implements A {
    void sayHello() {
       print("B say Hello");
    }
    void sayBye() {
       print("B say Bye");
    }
}

C class extends A class, so it has to implement all abstract methods of A. (Not all). C is inherited the sayHello() methods from A class.

class C extends A {
   void sayBye() {
       print("C say Bye");
   }
}

How to define interfaces in Dart?

6 Answers6

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