Abstract classes and abstract methods (keyword abstract)

1. Abstract classes can not be instantiated , no , One or more abstract methods

2. Abstract methods only declarations of methods but no implementations of methods , classes with abstract methods must be declared as abstract classes , Subclasses must override all the abstract methods of the parent class to be instantiated , or the subclass is an abstract class .

3.abstrat can be used to modify classes and methods , However, you cannot modify properties and constructors, but you can have a construction method

Abstract classes and abstract methods (keyword abstract)