上次讲了Java中继承的基本概念和理论知识。这次就来结合具体事例来讲解一下。就来看一个具体例子:
import java.util.*; public class ManagerTest { public static void main(String[] args) { 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); 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 { 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; }
(摘自《Java2核心技术》)
Manager是Employee的一员,因此可以扩展Employee类。但是Manager又和普通的Employee有些不同,包括Salary(薪金)。因此就需要重新覆盖Manager类中的getSalary方法。看起来比较简单,只要返回salary和bonus就可以了,就如这样:
public double getSalary(){ return salary+bonus; }
但是编译运行之后就会发现这根本不起效果。原因在于虽然每个Manager对象都有一个名为salary的域,但是在Manager类中病不能直接访问私有的salary域,只有Employee类的方法才能够访问其私有的部分。但是Manager域可以通过公有的接口来访问私有域,Employee中的getSalary方法正符合。
public double getSalary(){ double baseSalary=getSalary(); return baseSalary+bonus; }
现在,修改过的代码运行起来仍然有问题,在于Manager类中同样有getSalary方法,因此会无限次的调用自己知道程序崩溃为止。应该使用Employee类的getSalary方法而不是Manager类。使用super关键字来调用超类的方法,包括构造器。
public double getSalary(){ double baseSalary=super.getSalary(); return baseSalary+bonus; }
这样,就解决问题了。
今天回家就看了,好几天没好好看了………..