Abstract Class and Abstract Methods

Abstract Class & Abstract Methods

void main() {
  // Abstract Class & Methods

  var circle1 = Circle(3);

  print(circle1.calarea());
}

abstract class Shape {
  void draw();

  double calarea();
}

class Circle extends Shape {
  int radius;

  final pi = 3.14;

  @override
  void draw() {
    print("Drawing a Circle");
  }

  @override
  double calarea() {
    return (this.pi * this.radius * this.radius);
  }

  Circle(this.radius);
}

• You can't make instances of abstract class . Abstract class are always extended by some other class

• Abstract classes may contain some abstract methods as well as normal methods

• When you extend a abstract class you must override it's abstract methods, though it's not mandatory to override other normal methods

• Abstract Classes are useful for code organisation and consistency .You can define the functionality you need as abstract methods inside abstract class

Abstract Class & Abstract Methods : Why do we use them ?

Separate Circle Class

import 'dart:math';

void main() {
  var circle1 = Circle(5);

  print(circle1.calarea());
}

class Circle {
  // instance variables
  double radius;
  final pi = 3.14;

  //methods
  void draw() {
    print("Drawing a circle ");
  }

  double calarea() {
    return this.pi * pow(this.radius, 2);
  }

  //Constructor
  Circle(this.radius);
}

Separate Square Class

import 'dart:math';

void main() {
  var square1 = Square(6);

  print(square1.calarea());
}

class Square {
  // instance variables
  double length;

  //methods
  void draw() {
    print("Drawing a Square");
  }

  double calarea() {
    return pow(this.length, 2);
  }

  //Constructor
  Square(this.length);
}

Circle & Square Class with abstract class & methods

import 'dart:math';

void main() {
  // Abstract Class & Methods

  // You can't make an instance of an abstract class

  // Abstract class always meant to extended by some other class

  // When any class extend an abstract , it must override it's
  // abstract methods

  var circle1 = Circle(3);

  var square1 = Square(3);

  print(circle1.calarea());

  print(square1.calarea());
}

abstract class Shape {
  void test() {
    print("test");
  }

  void draw();

  double calarea();
}

class Circle extends Shape {
  int radius;

  final pi = 3.14;

  @override
  void draw() {
    print("Drawing a Circle");
  }

  @override
  double calarea() {
    return (this.pi * this.radius * this.radius);
  }

  Circle(this.radius);
}

class Square extends Shape {
  double length;

  @override
  void draw() {
    print("Drawing a Square");
  }

  @override
  double calarea() {
    return pow(this.length, 2);
  }

  Square(this.length);
}

You can't declare an abstract method inside a simple class . You can only declare abstract methods inside abstract class

Trying to declare an abstract method inside a normal class

❌ Complier shows an error

Change the normal class to abstract

✅ Compiler is happy

You can have an abstract class with only normal methods , but that defeats the purpose of using an abstract class

Abstract Class & Methods in Flutter

Official Documentation