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.