25 abstract abstract

1. First, we elicit the abstract, we used to write in a parent class and write a subclass of the time, to be subclass overrides the parent class method, then the parent class method body original content entity method in no meaning, because the method is performed after the subclasses override method overrides. All we want to parent class method body needs subclass overrides the method to remove, this time on the need to use the abstract keyword

public  class Error {

    public static void main(String[] args) {
        B b=new B();
        b.m();
    }

}

 class A {
     // attribute 
    String name;
     // Method 
    public  void M1 () {
        System.out.println ( "method of the parent class overrides the method body code is meaningless" );
    }

    
}
class B extends A{ 
    public void m() {
        System.out.println ( "method is performed after the rewriting of the subclass" );
    }
}

After transformation

public  class Error {

    public static void main(String[] args) {
        B b=new B();
        b.m();
    }

}

 abstract  class A {// abstract class can have an abstract method
     // attribute 
    String name;
     // methods 
    public  abstract  void m (); // change the abstract methods
    
}
class B extends A{ 
    public void m() {
        System.out.println ( "method is performed after the rewriting of the subclass" );
    }
}

1. With the abstract method in a class, then the class will become abstract class