Java Calendar add can beyond the XXX_OF boundary and Calendar date comparision test code

MyClass.java

import java.text.SimpleDateFormat;
import java.util.Calendar;
import java.util.Date;
import java.util.GregorianCalendar;

public class MyClass {
    public static void main(String args[]) {
        
        // Don be afraid that add(DAY_OF〜, value) makes no differences
        
		Calendar calendar1 = GregorianCalendar.getInstance();
		calendar1.set(Calendar.MONTH, 11); // = December (0 start value)
		calendar1.set(Calendar.DAY_OF_MONTH, 31);
		calendar1.add(Calendar.DATE, 1);
		printCalendarDate(calendar1); // => 2018-01-01
		
		Calendar calendar2 = GregorianCalendar.getInstance();
		calendar2.set(Calendar.MONTH, 11); // = December (0 start value)
		calendar2.set(Calendar.DAY_OF_MONTH, 31);
		calendar2.add(Calendar.DAY_OF_MONTH, 1);
		printCalendarDate(calendar2); // => 2018-01-01
	
		Calendar calendar3 = GregorianCalendar.getInstance();
		calendar3.set(Calendar.MONTH, 11); // = December (0 start value)
		calendar3.set(Calendar.DAY_OF_MONTH, 31);
		calendar3.add(Calendar.DAY_OF_YEAR, 1);
		printCalendarDate(calendar3); // => 2018-01-01
		
		System.out.println("calendar1.equals(calendar2) = " + (calendar1.equals(calendar2) ? "true" : "false")); // => false
		System.out.println("calendar2.equals(calendar3) = " + (calendar2.equals(calendar3) ? "true" : "false")); // => false
		System.out.println("calendar3.equals(calendar1) = " + (calendar3.equals(calendar1) ? "true" : "false")); // => false
		
		System.out.println("isEqualsDate(calendar1, calendar2) = " + (isEqualDate(calendar1, calendar2) ? "true" : "false")); // => true
		System.out.println("isEqualsDate(calendar2, calendar3) = " + (isEqualDate(calendar2, calendar3) ? "true" : "false")); // => true
		System.out.println("isEqualsDate(calendar3, calendar1) = " + (isEqualDate(calendar3, calendar1) ? "true" : "false")); // => true
    }
    
    public static void printCalendarDate(Calendar calendar) {
        SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd");
		String dateString = sdf.format(calendar.getTime());
		System.out.println(dateString);
    }
    
    public static boolean isEqualDate(Calendar c1, Calendar c2) {
        return c1.get(Calendar.YEAR) == c2.get(Calendar.YEAR)
            && c1.get(Calendar.DAY_OF_YEAR) == c2.get(Calendar.DAY_OF_YEAR);
    }
}