Dart Abstract Classes

Abstract classes are the classes in Dart that has one or more abstract method. Abstraction is a part of the data encapsulation where the actual internal working of the function hides from the users. They interact only with external functionality. We can declare the abstract class by using the abstract keyword. There is a possibility that an abstract class may or may not have abstract methods.

Abstract methods are those methods, which are declared without implementation. The concrete methods or normal methods are declared with implementation. An abstract class can contain both types of methods, but a normal class is not allowed to have abstract methods.

We cannot create the instance of an abstract class that means it can't be instantiated. It can only be extended by the subclass, and the subclass must be provided the implantation to the abstract methods which are present in the present class. Then it is necessary to declare abstract subclass.

Rules for Abstract classes:

The rules of the abstract are given below.

1. An abstract class can have an abstract method (method without implementation), or not.
2. If there is at least one abstract method, then the class must be declared abstract.
3. The object of the abstract class cannot be created, but it can be extended.
4. An abstract keyword is used to declare the abstract class.
5. An abstract class can also include normal or concrete (method with the body) methods.
6. All abstract methods of parent class must be implemented in the subclass.

Declaring Abstract Class

An abstract keyword followed by a class name is used to declare the abstract class. An abstract class mostly used to offer a base for the subclass to extends and implement the abstract method.

Syntax:

1. abstract class ClassName {  
2.  // Body of abstract class  
3. }  

Usage of Abstract class

Let's suppose we have a class Person that has method displayInfo(), and we have to sub classes of it Boy and Girl. Each of the person information varies from the other person, so there is no benefit to implementing the displayInfo() in the parent class. Because every subclass must override the parent class method by provides its own implementation. Thus, we can force the subclass to provide implementation to that method, so that is the benefit to make method abstract. We don't require the give implementation in the parent class.

Let's understand the above scenario through the following code.

Example -

1. abstract class Person {  
2. //declaring abstract method  
3.   
4. void displayInfo();  //abstract method   
5.   
6. }  
7. class Boy extends Person   
8. {  
9. // Overriding method  
10. void displayInfo() {  
11.     print("My name is Johnathon");  
12.   
13.        }  
14.   
15. }  
16.   
17. class Girl extends Person   
18. {  
19. // Overriding method  
20. void displayInfo() {  
21.     print("My name is Grecia");  
22.   
23.        }  
24.   
25. }  
26.   
27. void main() {  
28. Boy b = new Boy();  // Creating Object of Boy class  
29. Girl g = new Girl();  // Creating Object of Girl class  
30.   
31. b.displayInfo();  
32. g.displayInfo();  
33. }  

Output

My name is Johnathon
My name is Grecia

Explanation:

As we can see that in the above code, we implemented the abstract method in two subclasses according to its requirement and then we called the displayInfo() method using the object of the both class's object