Java中的多态(polymorphism)和动态绑定(dynamic binding)

以下面代码为例，存在超类Employee和子类Manager，两个类中都有getSalary方法，但实现不同。

一个对象变量可以引用多种实际类型的现象被称为多态(polymorphism)，运行时能够自动选择调用哪个方法的现象称为动态绑定(dynamic binding)。

Employee类型的变量e既可以引用Employee类型又可以引用Manager类型，称为多态；调用e.getSalary时，根据e所引用的实际类型，决定调用Employee.getSalary还是调用Manager.getSalary，称为动态绑定。

需要注意的是，Employee类型的变量e尽管引用了Manager类型，但是不能够调用只定义在Manager子类中的方法(如setBonus)，因为编译器将e视为Employee类型。如果要调用Manager类中的方法，需另外声明Manager类型的变量(如boss)进行调用。

import java.util.*;

/**
 * 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);
      // GregorianCalendar uses 0 for January
      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;
}

参考:
Cay S. Horstmann, Gary Cornell. JAVA核心技术卷I：基础知识(第八版).机械工业出版社.2011年4月第1版.第5.1节.