Java Calendar 的设置方法只有在我打印它时才有效
Set method for Java Calendar only works if I do a print of it
我遇到了一个我真的不明白的问题。我正在尝试在日历上使用 set 方法，但它仅在我打印时修改值，代码如下：
Calendar cal = Calendar.getInstance();
cal.set(2020,2,31);
// System.out.println("Necessary print: " + cal.getTime());
cal.set(Calendar.DAY_OF_WEEK, cal.getFirstDayOfWeek());
// reset time
cal.set(cal.get(Calendar.YEAR), cal.get(Calendar.MONTH), cal.get(Calendar.DAY_OF_MONTH), 0, 0, 0);
System.out.println("Date set: " + cal.getTime());
System.out.println("Day of month: " + cal.get(Calendar.DAY_OF_MONTH));
前面的代码给出了输出：
Date set: Mon Mar 02 00:00:00 CET 2020
Day of month: 2
如果我取消注释 "necessary print"，那么我会得到：
Necessary print: Tue Mar 31 23:07:17 CEST 2020
Date set: Mon Mar 30 00:00:00 CEST 2020
Day of month: 30
我真的很想知道为什么打印会影响此方法，在此先感谢您的帮助！
日历 class 的 documentation 指出：
The calendar fields can be changed using three methods: set(), add(), and roll().
set(f, value) changes calendar field f to value. In addition, it sets an internal member variable to indicate that calendar field f has been changed. Although calendar field f is changed immediately, the calendar's time value in milliseconds is not recomputed until the next call to get(), getTime(), getTimeInMillis(), add(), or roll() is made. Thus, multiple calls to set() do not trigger multiple, unnecessary computations. As a result of changing a calendar field using set(), other calendar fields may also change, depending on the calendar field, the calendar field value, and the calendar system. In addition, get(f) will not necessarily return value set by the call to the set method after the calendar fields have been recomputed. The specifics are determined by the concrete calendar class.
TL;DR：您需要调用 cal.getTime() 来更新值。
我遇到了一个我真的不明白的问题。我正在尝试在日历上使用 set 方法，但它仅在我打印时修改值，代码如下：
Calendar cal = Calendar.getInstance();
cal.set(2020,2,31);
// System.out.println("Necessary print: " + cal.getTime());
cal.set(Calendar.DAY_OF_WEEK, cal.getFirstDayOfWeek());
// reset time
cal.set(cal.get(Calendar.YEAR), cal.get(Calendar.MONTH), cal.get(Calendar.DAY_OF_MONTH), 0, 0, 0);
System.out.println("Date set: " + cal.getTime());
System.out.println("Day of month: " + cal.get(Calendar.DAY_OF_MONTH));
前面的代码给出了输出：
Date set: Mon Mar 02 00:00:00 CET 2020 Day of month: 2
如果我取消注释 "necessary print"，那么我会得到：
Necessary print: Tue Mar 31 23:07:17 CEST 2020 Date set: Mon Mar 30 00:00:00 CEST 2020 Day of month: 30
我真的很想知道为什么打印会影响此方法，在此先感谢您的帮助！
日历 class 的 documentation 指出：
The calendar fields can be changed using three methods:
set(),add(), androll().set(f, value)changes calendar fieldftovalue. In addition, it sets an internal member variable to indicate that calendar fieldfhas been changed. Although calendar fieldfis changed immediately, the calendar'stimevalue in milliseconds is not recomputed until the next call toget(),getTime(),getTimeInMillis(),add(), orroll()is made. Thus, multiple calls toset()do not trigger multiple, unnecessary computations. As a result of changing a calendar field usingset(), other calendar fields may also change, depending on the calendar field, the calendar field value, and the calendar system. In addition,get(f)will not necessarily return value set by the call to the set method after the calendar fields have been recomputed. The specifics are determined by the concrete calendar class.
TL;DR：您需要调用 cal.getTime() 来更新值。