小白要实现一个记录奶酪生产期限的程序,昨天自测完后,提交给测试 MM 进行测试,测试 MM 测试完后说没有问题后小白才下班。今天早上测试 MM 走自动回归测试时,发现了 bug。
import java.util.Date;
public class Cheese {
public static final Cheese cheese=new Cheese();
private final long produceTimes;
private static final long produceDate =new Date(119,8,1).getTime();
private Cheese() {
produceTimes=new Date().getTime()-produceDate;
}
public long produceTimes() {
return produceTimes;
}
public static void main(String[] args) {
System.out.println("current time in day(from 1900:00:00) : "+new Date().getTime()/(1000*60*60*24L));
System.out.println("cheese had produces : "+ cheese.produceTimes()/(1000*60*60*24L) +" days");
}
}
小白把测试环境的代码拉下来,进行调试,发现运行结果果然不对:
current time in day(from 1900:00:00) : 18153
cheese had produces : 18153 days
难道昨天和测试 MM 都眼花了?不可能呀!肯定是代码的问题。拉出看历史提交记录,发现仅有小胖对程序做了改动,将两行代码的顺序做了调换。调换前的程序如下:
import java.util.Date;
public class Cheese {
private final long produceTimes;
private static final long produceDate =new Date(119,8,1).getTime();//这里
public static final Cheese cheese=new Cheese();//这里
private Cheese() {
produceTimes=new Date().getTime()-produceDate;
}
public long produceTimes() {
return produceTimes;
}
public static void main(String[] args) {
System.out.println("current time in day(from 1900:00:00) : "+new Date().getTime()/(1000*60*60*24L));
System.out.println("cheese had produces : "+ cheese.produceTimes()/(1000*60*60*24L) +" days");
}
}
运行结果:
current time in day(from 1900:00:00) : 18153
cheese had produces : 13 days
这才是小白想要的结果,也是测试 MM 期望看到的结果。
追根究底
原来,实例的初始化也是有讲究的。
-
static 字段先设置默认值,其中 cheese 被设置为 null,produceDate 被设置为 0;
-
然后 static 初始器执行,按照声明出现的顺序执行:
如果先执行 cheese 的话,调用 Cheese () 构造方法,此时用 produceDate=0 为值;
如果先执行 produceDate 的话,producteDate 被设置为 2019-09-01,再调用 cheese () 构造方法。 - 最后从构造器返回 cheese 类的初始化。
另外,还学习了新的一招
Date 设置日期为 2019-09-01 为何设置为:
new Date(119,8,1)
进去源码看一眼:
/**
* Allocates a <code>Date</code> object and initializes it so that
* it represents midnight, local time, at the beginning of the day
* specified by the <code>year</code>, <code>month</code>, and
* <code>date</code> arguments.
*
* @param year the year minus 1900.
* @param month the month between 0-11.
* @param date the day of the month between 1-31.
* @see java.util.Calendar
* @deprecated As of JDK version 1.1,
* replaced by <code>Calendar.set(year + 1900, month, date)</code>
* or <code>GregorianCalendar(year + 1900, month, date)</code>.
*/
@Deprecated
public Date(int year, int month, int date) {
this(year, month, date, 0, 0, 0);
}
其中,year 份是从 1900 年开始的年数,即 2019-1900=119;
month 是 0~11 计数的,需要实际月份减 1,即 9-1=8;
date 是 1~31 计数的,实际天就可以 即 1。