关于Core Java中一句话的理解

最近在翻翻以前看过的书，看到Core Java里的一句话：As a rule of thumb, always use clone whenever you need to return a copy of a mutable data field.大意是说：一般来讲，对于返回变化的变量，最好用clone()方法。结合上下文来理解这句话，其言外之意，就是如果不经处理的返回实例的对象变量或者属性，是一种破坏封装的行为。

还是先来看下最直接的破坏封装的编程行为：

class Person{ public int age; //public修饰变量，导致封装被破坏 public Person(int age){ this.age = age; } public String toString(){ return "The person's age is "+ age; } } public class NotGood { public static void main(String args[]){ Person p = new Person(24); p.age = 40; //能够直接通过变量访问变量，而不是通过方法 System.out.println(p); } }

上面的代码，明显的反应了封装的一个基本思想：通过类自身的方法去访问或者改变变量，而不应该直接通过类的实例化对象直接去操作变量。

回到我们开始是提起的那句话，那为什么不能直接返回可变的变量呢？看代码和输出：

import java.util.*; import java.text.*; class Person{ private int age; private Date birthday; public Person(int age, int year, int month, int day){ this.age = age; GregorianCalendar calendar = new GregorianCalendar(year, month - 1, day); this.birthday = calendar.getTime(); } public int getAge(){ return age; } public Date getBirth(){ return birthday; } public String toString(){ return "The person's age is "+ age +", and born at "+ new SimpleDateFormat("yyyy-MM-dd").format(birthday) +"."; } } public class NotGood { public static void main(String args[]){ Person p = new Person(24, 1984, 12, 1); int i = p.getAge(); i++; System.out.println(p); Date d = p.getBirth(); double tenYearsInMilliSeconds = 10 * 365.25 * 24 * 60 * 60 * 1000; d.setTime(d.getTime() - (long) tenYearsInMilliSeconds); System.out.println(p); } }

结果：

The person's age is 24, and born at 1984-12-01. The person's age is 24, and born at 1974-12-01.

可以看出来，对于基本类型的变量输出没有影响，但是如果变量是个对象的话，修改之后，就会对原始的对象变量的内容。这就Core Java这句话的意思。按照它的建议，应该处理下，最直接的办法就是利用clone()方法。看代码和输出：

import java.util.*; import java.text.*; class Person{ private int age; private Date birthday; public Person(int age, int year, int month, int day){ this.age = age; GregorianCalendar calendar = new GregorianCalendar(year, month - 1, day); this.birthday = calendar.getTime(); } public int getAge(){ return age; } public Date getBirth(){ return (Date)birthday.clone(); //在这里进行修改 } public String toString(){ return "The person's age is "+ age +", and born at "+ new SimpleDateFormat("yyyy-MM-dd").format(birthday) +"."; } } public class NotGood { public static void main(String args[]){ Person p = new Person(24, 1984, 12, 1); System.out.println(p); Date d = p.getBirth(); double tenYearsInMilliSeconds = 10 * 365.25 * 24 * 60 * 60 * 1000; d.setTime(d.getTime() - (long) tenYearsInMilliSeconds); System.out.println(p); } }

输出：

The person's age is 24, and born at 1984-12-01. The person's age is 24, and born at 1984-12-01.