Objects are created for abstract classes * code example

Example 1: when we should use abstract class

When to use an abstract class
An abstract class is a good choice if we are using the inheritance concept since it provides a common base class implementation to derived classes.
An abstract class is also good if we want to declare non-public members. In an interface, all methods must be public.
If we want to add new methods in the future, then an abstract class is a better choice. Because if we add new methods to an interface, then all of the classes that already implemented that interface will have to be changed to implement the new methods.
If we want to create multiple versions of our component, create an abstract class. Abstract classes provide a simple and easy way to version our components. By updating the base class, all inheriting classes are automatically updated with the change. Interfaces, on the other hand, cannot be changed once created. If a new version of an interface is required, we must create a whole new interface.
Abstract classes have the advantage of allowing better forward compatibility. Once clients use an interface, we cannot change it; if they use an abstract class, we can still add behavior without breaking the existing code.
If we want to provide common, implemented functionality among all implementations of our component, use an abstract class. Abstract classes allow us to partially implement our class, whereas interfaces contain no implementation for any members.

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()}";)
}