为什么用抽象类

abstract class Printer{
    void Open(){
        System.out.println("Open");
    }
    
    void Close(){
        System.out.println("Close");
    }
    
    abstract void Print(String s){
    }
}

class HPPrinter extends Printer{
}

class CannonPrinter extends Printer{
    void Close(){
        this.Clean();
        super.Close();
    }
        
    void Clean(){
        System.out.println("Clean");
    }
}

class Test{
    public static void main(String args []){
        int flag = 1;
        
        if( flag == 0){
            HPPrinter hpprinter = new HPPrinter();
            hpprinter.Open();
            hpprinter.Print("abc");
            hpprinter.Close();
        }
        else if( flag == 1){
            CannonPrinter cannonprinter = new CannonPrinter();
            cannonprinter.Open();
            cannonprinter.Print("123");
            cannonprinter.Close();
        }
    }
}