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An abstract method in Dart is a method signature declared without a concrete implementation. It defines the method’s name, return type, and parameters, but replaces the method body (the curly braces {}) with a semicolon ;. Abstract methods establish a strict contract, compelling any concrete subclass to provide the actual implementation.

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Syntax and Declaration

To declare an abstract method, the enclosing structure must be either an abstract class or a mixin. Standard instance methods, getters, and setters can all be declared as abstract. 
abstract class DataProcessor {
  // Abstract instance method
  void process(String data);

  // Abstract getter
  bool get isReady;

  // Abstract setter
  set configuration(String config);
}

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Rules and Constraints

  1. Enclosure Restriction: You cannot declare an abstract method inside a standard, concrete class. The compiler will throw an error if a class contains an unimplemented method but is not marked with the abstract modifier.
  2. Static Modifier: Abstract methods cannot be static. Because static methods are resolved at compile-time and cannot be overridden by subclasses, an abstract static method represents a logical contradiction in Dart’s object model.
  3. Signature Compatibility: When a concrete subclass implements an abstract method, the overriding method’s signature must be compatible with the abstract signature, but it does not require an exact match. Dart’s type system allows variance: you can use covariant return types (returning a subtype of the original return type), contravariant parameter types (accepting a supertype of the original parameter type), and you can add new optional parameters to the overriding method.
  4. Implicit Interfaces: Because every class in Dart implicitly defines an interface, abstract methods act as interface members. Whether a subclass uses extends or implements on the abstract class, it is forced to provide a concrete implementation for all abstract members.

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Implementation Mechanics

When a concrete class inherits an abstract method, it must provide the method body. It is standard practice to use the @override annotation to explicitly indicate that the method is fulfilling the abstract contract. 
class JsonProcessor extends DataProcessor {
  // Fulfilling the abstract instance method
  @override
  void process(String data) {
    print('Processing JSON: $data');
  }

  // Fulfilling the abstract getter
  @override
  bool get isReady => true;

  // Fulfilling the abstract setter
  @override
  set configuration(String config) {
    print('Configuration applied: $config');
  }
}
If a subclass extends an abstract class but fails to implement all inherited abstract methods, that subclass must also be declared as abstract. The obligation to implement the methods is deferred down the inheritance chain until a concrete class is defined.
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