public class Test {

    public static void main(String[] args) {

      show(new Cat());  // 以 Cat 对象调用 show 方法

      show(new Dog());  // 以 Dog 对象调用 show 方法

      Animal a = new Cat();  // 向上转型

      a.eat();               // 调用的是 Cat 的 eat

      Cat c = (Cat)a;        // 向下转型

      c.work();        // 调用的是 Cat 的 work

  }  

    public static void show(Animal a)  {

      a.eat();

        // 类型判断

        if (a instanceof Cat)  {  // 猫做的事情

            Cat c = (Cat)a;

            c.work();

        } else if (a instanceof Dog) { // 狗做的事情

            Dog c = (Dog)a;

            c.work();

        }

    }

}

abstract class Animal {

    abstract void eat();

}  

class Cat extends Animal {

    public void eat() {

        System.out.println("吃鱼");

    }

    public void work() {

        System.out.println("抓老鼠");

    }

}  

class Dog extends Animal {

    public void eat() {

        System.out.println("吃骨头");

    }

    public void work() {

        System.out.println("看家");

    }

}

/* 文件名 : Employee.java */

public class Employee {

   private String name;

   private String address;

   private int number;

   public Employee(String name, String address, int number) {

      System.out.println("Employee 构造函数");

      this.name = name;

      this.address = address;

      this.number = number;

   }

   public void mailCheck() {

      System.out.println("邮寄支票给： " + this.name

       + " " + this.address);

   }

   public String toString() {

      return name + " " + address + " " + number;

   }

   public String getName() {

      return name;

   }

   public String getAddress() {

      return address;

   }

   public void setAddress(String newAddress) {

      address = newAddress;

   }

   public int getNumber() {

     return number;

   }

}

/* 文件名 : Salary.java */

public class Salary extends Employee

{

   private double salary; // 全年工资

   public Salary(String name, String address, int number, double salary) {

       super(name, address, number);

       setSalary(salary);

   }

   public void mailCheck() {

       System.out.println("Salary 类的 mailCheck 方法 ");

       System.out.println("邮寄支票给：" + getName()

       + " ，工资为：" + salary);

   }

   public double getSalary() {

       return salary;

   }

   public void setSalary(double newSalary) {

       if(newSalary >= 0.0) {

          salary = newSalary;

       }

   }

   public double computePay() {

      System.out.println("计算工资，付给：" + getName());

      return salary/52;

   }

}

/* 文件名 : VirtualDemo.java */

public class VirtualDemo {

   public static void main(String [] args) {

      Salary s = new Salary("员工 A", "北京", 3, 3600.00);

      Employee e = new Salary("员工 B", "上海", 2, 2400.00);

      System.out.println("使用 Salary 的引用调用 mailCheck -- ");

      s.mailCheck();

      System.out.println("\n使用 Employee 的引用调用 mailCheck--");

      e.mailCheck();

    }

}