有一天,老鼠小白发现了一个奇怪的问题,它的奶酪的生产日期被谁搞丢了,不知道奶酪是否过期,可怎么吃呀?
让我们来看看吧
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"); } }
按照小白的预期,程序该跑出奶酪上市了多少天,可是打印出的结果确实奶酪不会过期
current time in day(from 1900:00:00) : 18153 cheese had produces : 18153 days
这是怎么回事呢?
查看代码提交记录,发现老鼠小蓝有修改记录,仅仅调整了两行程序的顺序,小白原来的代码如下:
import java.util.Date; public class Cheese { private final long produceTimes; private static final long produceDate =new Date(119,8,1).getTime(); public static final Cheese cheese=new Cheese(); 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"); } }
仅仅修改了两个变量的顺序,输出的结果就大相径庭了
current time in day(from 1900:00:00) : 18153 cheese had produces : 13 days
这才是小白鼠想要的结果!
于是小白鼠去请教java的创造者java之父
原来,实例的初始化也是有讲究的。
1.static字段先设置默认值,其中cheese被设置为null,produceDate被设置为0
2.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。
3 最后从构造器返回cheese类的初始化。
Date设置日期为2019-09-01 为何设置为
new Date(119,8,1)
大家可以进去源码看说明情况
/** * 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
【1】 https://docs.oracle.com/javase/specs/jls/se12/html/jls-12.html#jls-12.4