阅读和理解下面程序段: class Manager extends Employee { public Manager(String n,double s,int year,int month,int day) { super(n,s,year,month,day); bonus=0; } public double getSalary() { double baseSalary=super.getSalary(); return baseSalary+bonus; } public void setBonus(double b) { bonus=b; } private double bonus; } Manager是Employee的子类,其理由是_______。