Manager Test and DAO

1. 阅读ManagerTest代码

（1）代码

import java.util.*
package test;
/**
 * This program demonstrates inheritance.
 * @version 1.21 2004-02-21
 * @author Cay Horstmann
 */
public class ManagerTest
{
   public static void main(String[] args)
   {
      // construct a Manager object
      Manager boss = new Manager("Carl Cracker", 80000, 1987, 12, 15);
      boss.setBonus(5000);

      Employee[] staff = new Employee[3];

      // fill the staff array with Manager and Employee objects

      staff[0] = boss;
      staff[1] = new Employee("Harry Hacker", 50000, 1989, 10, 1);
      staff[2] = new Employee("Tommy Tester", 40000, 1990, 3, 15);

      // print out information about all Employee objects
      for (Employee e : staff)
         System.out.println("name=" + e.getName() + ",salary=" + e.getSalary());
   }
}

class Employee
{
   public Employee(String n, double s, int year, int month, int day)
   {
      name = n;
      salary = s;
      GregorianCalendar calendar = new GregorianCalendar(year, month - 1, day);
      hireDay = calendar.getTime();
   }

   public String getName()
   {
      return name;
   }

   public double getSalary()
   {
      return salary;
   }

   public Date getHireDay()
   {
      return hireDay;
   }

   public void raiseSalary(double byPercent)
   {
      double raise = salary * byPercent / 100;
      salary += raise;
   }

   private String name;
   private double salary;
   private Date hireDay;
}

class Manager extends Employee
{
   /**
    * @param n the employee's name
    * @param s the salary
    * @param year the hire year
    * @param month the hire month
    * @param day the hire day
    */
   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;
}

（2）UML类图

（3）文件第26行e.getSalary()，到底是调用Manager类的还是Employee类的getSalary方法？

Manager boss = new Manager("Carl Cracker", 80000, 1987, 12, 15);
boss.setBonus(5000);
Employee[] staff = new Employee[3];
staff[0] = boss;
staff[1] = new Employee("Harry Hacker", 50000, 1989, 10, 1);
staff[2] = new Employee("Tommy Tester", 40000, 1990, 3, 15);

e.getSalary()会根据具体的类执行对应的代码，子类的同名方法会覆盖父类的方法
staff[0]执行的是Manager类的getSalary方法，staff[1]和staff[2]执行的是Employee类的getSalary方法

（4） Manager类的构造函数使用super调用父类的构造函数实现了代码复用，这样有什么好处？为什么不把父类构造函数中的相关代码复制粘贴到Manager的构造函数中，这样看起来不是更直观吗？

使用super调用父类的构造函数减少代码重复现象，代码更加简洁，也减少了打代码的时间。
调用子类的构造方法时会先调用父类的无参构造函数，若父类无无参构造函数，那么在子类的构造方法中必须显式地调用父类的构造方法。因为子类继承了父类，那么就默认的含有父类的公共成员方法和公共成员变量，这些方法和变量在子类里不再重复声明。如果初始化子类的时候，不初始化父类，那么你通过子类调用父类方法或变量的时候会抛出异常。

（5）该代码中哪里体现了多态的好处？请说明。

 for (Employee e : staff)
         System.out.println("name=" + e.getName() + ",salary=" + e.getSalary());

在调用对象的e.getName()和getSalary()方法时，根据对象实例调用相应方法,无需用if语句判断实例对象类型，代码简洁，方便阅读