注意不要编写返回引用可变对象的访问器方法

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