abstract

Learn abstract classes with abstract.

abstract class

An abstract class is one that cannot be instantiated directly. Abstract methods carry only a signature; the body is written in a subclass. It is declared with the abstract class keywords.

Example: abstract class Shape { double area(); }. Here Shape cannot be instantiated: Shape() is a compile error. The area method is left without a body; every subclass must write its own area calculation. Abstract classes are used to define a common contract; subclasses make the contract concrete.

Abstract methods and subclasses

Bodyless methods of an abstract class are written in the subclass with @override. Until the subclass gives bodies it is itself abstract; to become concrete it must implement all abstract methods.

Example: class Circle extends Shape { final double radius; Circle(this.radius); @override double area() => 3.14 * radius * radius; }. Here Circle is a concrete class because it gave a body to area. Since Shape is abstract, no Shape instance exists, but a Circle instance does. The abstract class sets the skeleton for inheritance.

Exercise Write the Circle class

Keep the existing abstract class Shape { double area(); } definition. Define a class named Circle that inherits from Shape; add a final double radius field and a Circle(this.radius) constructor. Write the @override double area() => 3.14 * radius * radius; method. In main, create an instance with var c = Circle(2); and call print(c.area()).

Show example solution

abstract class Shape {
  double area();
}

class Circle extends Shape {
  final double radius;
  Circle(this.radius);
  @override
  double area() => 3.14 * radius * radius;
}

void main() {
  var c = Circle(2);
  print(c.area());
}

Test yourself

Which statement is true?
- An abstract class cannot be instantiated directly
- An abstract class cannot contain final fields
- An abstract class may have only one subclass
- All methods of an abstract class must have bodies
Show answer

An abstract class cannot contain final fields — Abstract classes may contain final fields; only direct instantiation is forbidden.
Match the abstract parts

abstract class

Class prefix
double area()

Abstract method signature
Circle

Concrete subclass

Add the abstract keyword

class Shape {
  double area();
}____

Show answer

class Shape {
  double area();
}abstract

Next lesson: interface

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