在看《Java核心技术（原书第9版中文版）》的时候，看到113页的一个警告，“注意不要编写返回引用可变对象的访问器方法”。以前没看到过，原来Date对象是可变对象。就可变对象这个概念，查了一下网，我认为可变对象，就是在类中可以不通过域更改器方法就能改变值的对象。

1 package com.csst.sort; 2  3 import java.util.Date; 4 import java.util.GregorianCalendar; 5  6 public class Employee { 7     private String name; 8     private double salary; 9     private Date hireDay;10 11     public Employee(String n, double s, int year, int month, int day) {12         name = n;13         salary = s;14         GregorianCalendar Calendar = new GregorianCalendar(year, month - 1, day);15         hireDay = (Date) Calendar.getTime();16     }17 18     public static void main(String[] args) {19 20         Employee employee = new Employee("lele", 600, 1994, 10, 24);21 22         Date date = employee.getHireDay();23         System.out.println(date);24 25         date.setTime(2000500000);26 　　　　　　27         System.out.println(date);28         System.out.println(employee.getHireDay());29     }30 31     public void raiseSalary(double byPercent) {32         double raise = this.salary * byPercent / 100;33         salary = this.salary + raise;34     }35 36     public String getName() {37         return name;38     }39 40     public void setName(String name) {41         this.name = name;42     }43 44     public double getSalary() {45         return salary;46     }47 48     public void setSalary(double salary) {49         this.salary = salary;50     }51 52     public Date getHireDay() {53         return hireDay;54     }55 56     public void setHireDay(Date hireDay) {57         this.hireDay = hireDay;58     }59 60 }

最终结果：

Mon Oct 24 00:00:00 CST 1994

Sat Jan 24 11:41:40 CST 1970

Sat Jan 24 11:41:40 CST 1970

说明hireDay已经改变，将其get方法改为

public Date getHireDay() {		return (Date) hireDay.clone();	}

　　结果为：

Mon Oct 24 00:00:00 CST 1994

Sat Jan 24 11:41:40 CST 1970

Mon Oct 24 00:00:00 CST 1994