why and when should we use an abstract class code example

Example 1: is it necessary for abstract class to have abstract method

No, abstract class can have zero abstract methods.

Example 2: what is the need of abstract class

It is helpful if you need to make a generic function that can take a lot of class types as an argument. Eg:


abstract class Shape {
  void area();
  void perimeter();
}


class Rectangle extends Shape {
 int width;
 int height;


 Rectangle(this.width, this.height);


 void area() => this.width * this.height;
 void perimeter() => 2*(this.width + this.height);
}


class Triangle extends Shape {
 int side1;
 int side2;
 int side3;


 Triangle(this.side1, this.side2, this.side3);


  void area() => 0.5 * this.side1 * this.side2 * this.side3;


  void perimeter() => this.side1 + this.side2 + this.side3;
}


void printGeometry(Shape shape) {
 print("The area of this shape is ${shape.area()}";)
 print("The perimeter of this shape is ${shape.perimeter()}";)
}