Dart : Abstract Method

Abstract Method :

• Instance, getter, and setter methods can be abstract, defining an interface but
  leaving its implementation up to other classes.

• To make a method abstract, use a semicolon (;) instead of a method body:

•   abstract class Doer {
      // Define instance variables and methods...
  
      void doSomething(); // Define an abstract method.
    }
  
    class EffectiveDoer extends Doer {
      void doSomething() {
        // Provide an implementation, so the method is not abstract here...
      }
    }