Express abstract classes and methods

The relationship between abstract classes and abstract methods ：

Abstract classes can define 0-n Abstract methods ;

The role of abstract classes ：

The main role of abstract classes , Is to create a generic template for subclasses ;
Subclasses can be developed on the basis of this template , This is also in line with the application in the development of some enterprises , First override the abstract method in the parent class , The latter subclass can extend its own content ;
Abstract class design also avoids the randomness of design , Through abstract classes , The design of subclasses will become relatively strict , Limit to a certain extent

Code

According to every comment I write , Try tapping one by one , You know the limitations of abstract method definitions

//4. If a method in a class is an abstract method , Then this class should also become an abstract class .
//5. An abstract class can have 0-n Abstract methods 
public abstract class Person {
    
    //1. In a class , There will be a kind of method , Subclasses are very satisfied with this method , No need to rewrite , Use it directly 
    public void eat(){
    
        System.out.println(" I'm so hungry for a meal ");
    }
    //2. In a class , There will be a kind of method , Subclasses are never satisfied with this method , This method will be overridden .
    //3. Remove the method body of a method , Then be abstract modification , So this method becomes an abstract method 
    public abstract void say();
    public abstract void sleep();
}
//6. Abstract classes can be inherited by other classes ：
//7. A class inherits an abstract class , Then this class can become an abstract class 
//8. Generally, subclasses will not be added abstract modification , It is common for subclasses to override abstract methods in the parent class 
//9. Subclass inherits abstract class , You have to rewrite all the abstract methods 
//10. If the subclass does not override all the abstract methods of the parent class , Then the subclass can also become an abstract class .
class Student extends Person{
    
    @Override
    public void say() {
    
        System.out.println(" I'm from Northeast , I like speaking Northeast Dialect ..");
    }
    @Override
    public void sleep() {
    
        System.out.println(" Northeast people like sleeping on Kang ..");
    }
}
class Demo{
    
    // This is a main Method , It's the entrance to the program ：
    public static void main(String[] args) {
    
        //11. Create objects of abstract classes ：--> Abstract classes cannot create objects 
        //Person p = new Person();
        //12. Create subclass objects ：
        Student s = new Student();
        s.sleep();
        s.say();
        
   //13. Polymorphic writing ： A parent class reference only wants child class objects ：
      Person p  = new Student();
      p.say();
      p.sleep();
  }
}

Interview questions

（1） Abstract class cannot create object , So whether there is a constructor in the abstract class ？
There must be a constructor in an abstract class . The role of the constructor When initializing objects for subclasses, first super Call the constructor of the parent class .

（2） Whether abstract classes can be final modification ？
Can not be final modification , Because the original intention of abstract class design is for subclass inheritance . If by final Modified this abstract class , There is no inheritance , There are no subclasses .