为什么 java.util.Calendar 在 2021 年 2 月返回 31？

Question

The following snippet returns:

28
31

I can't figure out, why. I tried so set the calendar object to Feb 2021 in two ways, what am I doing wrong? Month is zero-based, so Feb is "1".

import java.util.Calendar;
import java.util.GregorianCalendar;

public class HelloWorld{

     public static void main(String []args){
        Calendar g = new GregorianCalendar(2021, 1, 28);
        System.out.println(g.getActualMaximum(Calendar.DAY_OF_MONTH));
        

        int year = 2021;
        int month = 1;
        Calendar c = Calendar.getInstance();
        c.set(Calendar.YEAR, year);
        c.set(Calendar.MONTH, month);
        System.out.println(c.getActualMaximum(Calendar.DAY_OF_MONTH));
     }
}

Or does this only happen, because I call Calendar.getInstance() on March 29, 30 or 31 and I don't call

c.set(Calendar.DAY_OF_MONTH, {any day below 29});

Answer 1

java.util date-time API and their formatting API, SimpleDateFormat are outdated and error-prone. It is recommended to stop using them completely and switch to the modern date-time API ^* .

Using modern date-time API:

import java.time.Month;
import java.time.Year;

public class Main {
    public static void main(String[] args) {
        System.out.println(YearMonth.of(2021, Month.FEBRUARY).lengthOfMonth());
    }
}

Output:

28

Learn more about the modern date-time API from Trail: Date Time .

Using legacy API:

import java.util.Calendar;
import java.util.GregorianCalendar;

public class Main {
    public static void main(String[] args) {
        // 1st Feb 2021
        Calendar cal = new GregorianCalendar(2021, Calendar.FEBRUARY, 1);
        System.out.println(cal.getActualMaximum(Calendar.DAY_OF_MONTH));
    }
}

Output:

28

---

^{* For any reason, if you have to stick to Java 6 or Java 7, you can use ThreeTen-Backport which backports most of the java.time functionality to Java 6 & 7. If you are working for an Android project and your Android API level is still not compliant with Java-8, check Java 8+ APIs available through desugaring and How to use ThreeTenABP in Android Project .}

Answer 2

You call Calendar.getInstance() , which returns a Calendar object initialized to the current time, which today means Mar 31, 2021.

You then set YEAR to 2021 and MONTH to 1 (Feb), leaving DAY_OF_MONTH at 31. If you asked the Calendar object for the date, it would resolve the fields and return a date value that explains the issue:

System.out.println(c.getTime()); // prints: Wed Mar 03 11:51:55 EST 2021

See how it says Mar 3 , 2021? That is because Feb 31 rolls over after Feb 28 and becomes Mar 3.

Since the Calendar object is now in March, the c.getActualMaximum(Calendar.DAY_OF_MONTH) correctly returns 31.

To do it right, call clear() and set DAY_OF_MONTH to 1, which is much easier when you use the nice set(year, month, date) helper method.

int year = 2021;
int month = Calendar.FEBRUARY/*1*/;
Calendar c = Calendar.getInstance();
c.clear();
c.set(year, month, 1);
System.out.println(c.getTime()); // prints: Mon Feb 01 00:00:00 EST 2021
System.out.println(c.getActualMaximum(Calendar.DAY_OF_MONTH)); // prints: 28