We Keep Coding

I want to get last date of the month and add n months to it but calendar instance is taking only 30 days in Java

SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd");
Calendar calender = Calendar.getInstance();
calender.set(Calendar.DAY_OF_MONTH, calender.getActualMaximum(Calendar.DATE));
int months = 1;
calender.add(Calendar.MONTH, months );
String time = sdf .format(calender .getTime());
System.out.println(time);
Since current month is April and last date is 2020-04-30
Next month last date I should get 2020-05-31
but I am getting last date as 2020-05-30
Any thing am i doing wrong ?

java.time
I recommend that you use java.time, the modern Java date and time API, for your date work. It’s much nicer to work with than the old classes Calendar and SimpleDateFormat.
LocalDate endOfNextMonth =
YearMonth // Represent an entire month in a particular year.
.now(ZoneId.of("Europe/Volgograd")) // Capture the current year-month as seen in a particular time zone. Returns a `YearMonth` object.
.plusMonths(1) // Move to the next month. Returns another `YearMonth` object.
.atEndOfMonth(); // Determine the last day of that year-month. Returns a `LocalDate` object.
String time = endOfNextMonth.toString(); // Represent the content of the `LocalDate` object by generating text in standard ISO 8601 format.
System.out.println("Last day of next month: " + time);
Output when running today:
Last day of next month: 2020-05-31
A YearMonth, as the name maybe says, is a year and month without day of month. It has an atEndOfMonth method that conveniently gives us the last day of the month as a LocalDate. A LocalDate is a date without time of day, so what we need here. And its toString method conveniently gives the format that you wanted (it’s ISO 8601).
Depending on the reason why you want the last day of another month there are a couple of other approaches you may consider. If you need to handle date ranges that always start and end on month boundaries, you may either:
Represent your range as a range of YearMonth objects. Would this free you from knowing the last day of the month altogether?
Represent the end of your range as the first of the following month exclusive. Doing math on the 1st of each month is simpler since it is always day 1 regardless of the length of the month.
What went wrong in your code?
No matter if using Calendar, LocalDate or some other class you need to do things in the opposite order: first add one month, then find the end of the month. As you know, months have different lengths, so the important part is getting the end of that month where you want to get the last day. Putting it the other way: setting either a LocalDate or a Calendar to the last day of the month correctly sets it to the last day of the month in qustion but does not instruct it to stay at the last day of the month after subsequent changes to its value, such as adding a month. If you add a month to April 29, you get May 29. If you add a month to April 30, you get May 30. Here it doesn’t matter that 30 is the last day of April while 30 is not the last day of May.
Links
Oracle tutorial: Date Time explaining how to use java.time.
Wikipedia article: ISO 8601

You'd better use LocalDate like this:
LocalDate now = LocalDate.now();
LocalDate lastDay = now.withDayOfMonth(now.lengthOfMonth());
LocalDate nextMonth = lastDay.plusMonths(1);

Don't use deprecated classes from java.util.*.
Use classes from java.time.*.
Example with LocalDate :
public class Testing {
public static void main(String args[]) {
DateTimeFormatter dateTimeFormatter = DateTimeFormatter.ofPattern("yyyy-MM-dd");
LocalDate date = LocalDate.now();
int months = 1;
date = date.plusMonths(months);
date = date.withDayOfMonth(date.lengthOfMonth());
System.out.println(date.format(dateTimeFormatter));
}
}
Output :
2020-05-31
Example with Calendar :
public class Testing {
public static void main(String args[]) {
SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd");
Calendar calender = Calendar.getInstance();
int months = 1;
calender.add(Calendar.MONTH, months);
calender.set(Calendar.DAY_OF_MONTH, calender.getActualMaximum(Calendar.DAY_OF_MONTH));
String time = sdf.format(calender.getTime());
System.out.println(time);
}
}
Output :
2020-05-31

getMonth getYear getDate giving errorneous result

I have a method to calculate the expiry date given manufacturing date of format("yyyy-MM-dd") and the months before the product could be used in int.First I tried with getYear getMonth getDate of Month class as follows I dint got errors results:
public void calculateExpiryDate(List<Item> items)
{
Iterator<Item> itr=items.iterator();
while(itr.hasNext())
{
Item i=itr.next();
Date md=i.getManufacturingDate();
int ubm=i.getUseBeforeMonths();
Calendar c=new GregorianCalendar(md.getYear(),md.getMonth(),md.getDate());
//System.out.println(c);
c.add(Calendar.MONTH, ubm);
Date exp=c.getTime();
i.setExpiryDate(exp);
SimpleDateFormat sdf=new SimpleDateFormat("yyyy MM dd");
System.out.println(md+" "+ubm+" "+sdf.format(exp)+" "+" "+i.getId());
}
}
But when I Quit using it and used setTime instead it solved my problem.I want to know what mistake I was making before and why things don't work that day and if any mistake(cause I was not getting any compile time errors) what is it actually.Following is the version of same code giving proper results.
public void calculateExpiryDate(List<Item> items)
{
Iterator<Item> itr=items.iterator();
while(itr.hasNext())
{
Item i=itr.next();
Date md=i.getManufacturingDate();
int ubm=i.getUseBeforeMonths();
Calendar c=new GregorianCalendar();
c.setTime(md);
//System.out.println(c);
c.add(Calendar.MONTH, ubm);
Date exp=c.getTime();
i.setExpiryDate(exp);
SimpleDateFormat sdf=new SimpleDateFormat("yyyy MM dd");
System.out.println(md+" "+ubm+" "+sdf.format(exp)+" "+" "+i.getId());
}
}

Your problem is that the constructor for GregorianCalendar expects the year as absolute value, but getYear() returns an offset to 1900.
A look into to the Java documentation reveals (beside that the used Date methods are all deprecated):
Date.getYear():
returns a value that is the result of subtracting 1900 from the year that contains or begins with the instant in time represented by this Date object, as interpreted in the local time zone.
GregorianCalendar:
year constructor parameter the value used to set the YEAR calendar field in the calendar
Whereas setTime(Date) used the value returned by md.getTime() which is number of milliseconds since January 1, 1970, 00:00:00 GMT.

How to remove Hour, Minute, and Second from Epoch timestamp

I need to write a function that accepts a java.util.Date and removes the hours, minutes, and milliseconds from it USING JUST MATH (no Date formatters, no Calendar objects, etc.):
private Date getJustDateFrom(Date d) {
//remove hours, minutes, and seconds, then return the date
}
The purpose of this method is to get the date from a millisecond value, without the time.
Here's what I have so far:
private Date getJustDateFrom(Date d) {
long milliseconds = d.getTime();
return new Date(milliseconds - (milliseconds%(1000*60*60)));
}
The problem is, this only removes minutes and seconds. I don't know how to remove hours.
If I do milliseconds - (milliseconds%(1000*60*60*23)), then it goes back to 23:00 hrs on the previous day.
EDIT:
Here's an alternative solution:
public static Date getJustDateFrom(Date d) {
Calendar c = Calendar.getInstance();
c.setTime(d);
c.set(Calendar.HOUR_OF_DAY, 0);
c.set(Calendar.MINUTE, 0);
return c.getTime();
}
Will this solution be affected by time zone differences between the client/server sides of my app?

There are 24 hours in a day. Use milliseconds%(1000*60*60*24).

Simply not possible by your definition.
A millisecond timestamp represents milliseconds elapsed from a fixed point in time (1970-01-01 00:00:00.000 UTC, if I remember correctly). This timestamp can not be converted into a date + time without specifying the timezone to convert to.
So you can only round the timestamp to full days in respect to a specific timezone, not in general. So any fiddling with Date.getTime() and not taking into account any timezone is guaranteed to work in only one time zone - the one you hardcoded for.
Do yourself a favor and use a Calendar.

You can make use of apache's commons lang DateUtils helper utility class.
For example, if you had the datetime of 28 Mar 2002
13:45:01.231, if you passed with Calendar.HOUR, it would return 28 Mar
2002 13:00:00.000. If this was passed with Calendar.MONTH, it would
return 1 Mar 2002 0:00:00.000.
Date newDate = DateUtils.truncate(new Date(1408338000000L), Calendar.DAY_OF_MONTH);
You can download commons lang jar at http://commons.apache.org/proper/commons-lang/

import java.sql.Date;
long dateInEpoch = 1_592_283_050_000L;
ZoneId defaultZoneId = ZoneId.systemDefault();
long currentDate = Date
.from(new Date(dateInEpoch)
.toLocalDate()
.atStartOfDay(defaultZoneId)
.toInstant())
.getTime();
input : 1592283050000
output: 1592245800000

Compare same date using java

The date is selected by the user using a drop down for year, month and day. I have to compare the user entered date with today's date. Basically see if they are the same date. For example
the user entered 02/16/2012. And if today is 02/16/2012 then I have to display a message. How do I do it?
I tried using milliseconds but that gives out wrong results.

And what kind of object are you getting back? String, Calendar, Date? You can get that string and compare it, at least that you think you'll have problems with order YYYY MM DD /// DD MM YYY in that case I suggest to create a custom string based on your spec YYYYMMDD and then compare them.
Date d1 = new Date();
Date d2 = new Date();
String day1 = d1.getYear()+"/"+d1.getMonth()+"/"+d1.getDate();
String day2 = d2.getYear()+"/"+d2.getMonth()+"/"+d2.getDate();
if(day1.equals(day2)){
System.out.println("Same day");
}

Dates in java are moments in time, with a resolution of "to the millisecond". To compare two dates effectively, you need to first set both dates to the "same time" in hours, minutes, seconds, and milliseconds. All of the "setTime" methods in a java.util.Date are depricated, because they don't function correctly for the internationalization and localization concerns.
To "fix" this, a new class was introduced GregorianCalendar
GregorianCalendar cal1 = new GregorianCalendar(2012, 11, 17);
GregorianCalendar cal2 = new GregorianCalendar(2012, 11, 17);
return cal1.equals(cal2); // will return true
The reason that GregorianCalendar works is related to the hours, minutes, seconds, and milliseconds being initialized to zero in the year, month, day constructor. You can attempt to approximate such with java.util.Date by using deprecated methods like setHours(0); however, eventually this will fail due to a lack of setMillis(0). This means that to use the Date format, you need to grab the milliseconds and perform some integer math to set the milliseconds to zero.
date1.setHours(0);
date1.setMinutes(0);
date1.setSeconds(0);
date1.setTime((date1.getTime() / 1000L) * 1000L);
date2.setHours(0);
date2.setMinutes(0);
date2.setSeconds(0);
date2.setTime((date2.getTime() / 1000L) * 1000L);
return date1.equals(date2); // now should do a calendar date only match
Trust me, just use the Calendar / GregorianCalendar class, it's the way forward (until Java adopts something more sophisticated, like joda time.

There is two way you can do it. first one is format both the date in same date format or handle date in string format.
SimpleDateFormat sdf = new SimpleDateFormat("dd/MM/yyyy");
String date1 = sdf.format(selectedDate);
String date2 = sdf.format(compareDate);
if(date1.equals(date2)){
}else{
}
Or
Calendar toDate = Calendar.getInstance();
Calendar nowDate = Calendar.getInstance();
toDate.set(<set-year>,<set-month>,<set-date->);
if(!toDate.before(nowDate))
//display your report
else
// don't display the report

Above answers are correct but consider using JodaTime - its much simpler and intuitive API.
You could set DateTime using with* methods and compare them.
Look at this answer

Problems with the Date 31-12-9999 in java

My program needs to represent this date as a java.sql.date object , but it seems that when I create a new date (using the calendar) and set it to '9999-12-31' and finally convert this java.util.date object to a java.sql.date object, this date is converted to something like '000-01-31'.
Calendar calendar = Calendar.getInstance();
calendar.set(9999, 12, 31);
infinityDate = new java.sql.Date(normalizeDate(calendar.getTime()).getTime());
infinityDate should be 31-12-9999
but when my code reaches here :
if(otherDate.equals(infinityDate))
{// Do stuff}
It never goes into the if condition as the infinityDate has for some reason been changed to 31-01-000, even though otherDate is infact '31-12-9999'.
The fact that otherDate is 31-12-9999 tells me that java can represent this dates , but for some reason , when I construct it using a calendar it changes the date. (otherDate comes from a jdbc statement which fetches data from a database)
This reference date '31-12-9999' has been fixed by some client , so it cannot be changed and my program has to be able to compare some incoming date values with this.
Does anyone know why this is happening , I realize that http://en.wikipedia.org/wiki/Year_10,000_problem may be a problem for dates after year 9999 , but I should be safe by a day.
EDIT : The Normalize date method only "normalizes the given date to midnight of that day"
private static java.util.Date normalizeDate(java.util.Date date) {
Calendar calendar = Calendar.getInstance();
calendar.setTime(date);
calendar.set(Calendar.HOUR_OF_DAY, 0);
calendar.set(Calendar.MINUTE, 0);
calendar.set(Calendar.SECOND, 0);
calendar.set(Calendar.MILLISECOND, 0);
date = calendar.getTime();
return date;
}
But , this issue was appearing before I was normalizing the date , I normalized it in an attempt to fix this.

Months are zero indexed. Use 11 for December, not 12. This is why you are rolling over the year.

Calendar.MONTH is zero-based. The call
calendar.set(9999, 12, 31);
sets the date to "the 31st day in the 13th month of the year 9999", which is then implicitly converted to the 1st month of the year 10000. It would result in an exception if you first called
calendar.setLenient(false);

Check hours, minutes, seconds and milliseconds that are held into these 2 date objects. I believe they are different.
If your want to compare the date (year, month, day) only you should probably create your custom Comparator and use it.