java 可变对象的引用 clone

最近看java书时，上面有提到 “可变对象访问器” 不理解，看了下代码，于是就自己敲了一遍，理解为：

可变对象访问器：在类A中定义类B做其属性时，直接用get方法返回此属性，那得到的是其引用，然后接收此返回值的对象（类B的一个对象）将可以改变类A对象的属性B的值。

解决的方法就是：在类A中返回此对象属性的克隆（clone），而不是直接返回此对象属性。

详细代码如下：

public 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 Date getHireDay(){
        return hireDay;
    }

    private String name;
    private double salary;
    private Date hireDay;

public static void main(String args[]){
        Employee harry = new Employee("harry",1000.0,2012,6,12);
        Date d = harry.getHireDay();

        System.out.println("d: "+d);
        System.out.println("harry.getHireDay(): "+harry.getHireDay());

        double tenYearsInMilliSeconds = 10*365.25*23*60*60*1000;
        d.setTime(d.getTime() - (long)tenYearsInMilliSeconds);

        System.out.println("d: "+d);
        System.out.println("harry.getHireDay(): "+harry.getHireDay());
    }

运行后的结果：

d: Tue Jun 12 00:00:00 CST 2012
harry.getHireDay(): Tue Jun 12 00:00:00 CST 2012
d: Mon Nov 11 16:30:00 CST 2002
harry.getHireDay(): Mon Nov 11 16:30:00 CST 2002

修改后的代码：

将返回可变对象的引用的clone

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

修改后的结果：

d: Tue Jun 12 00:00:00 CST 2012
harry.getHireDay(): Tue Jun 12 00:00:00 CST 2012
d: Mon Nov 11 16:30:00 CST 2002
harry.getHireDay(): Tue Jun 12 00:00:00 CST 2012