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how to compare two calendar objects?

I want to compare two date objects like this:
String format = "MM/dd/yyyy hh:mm a";
SimpleDateFormat sdf = new SimpleDateFormat(format);
fakeDate = sdf.parse("15/07/2013 11:00 AM");
fakeDate2 = sdf.parse("15/07/2013 12:00 AM");
int diff = date2.getHours() - date1.getHours();
but then I see getHours is deprecated.
So i have used:
Calendar calendar1 = Calendar.getInstance();
calendar.setTime(new Date("15/7/2013 11:00AM"));
Calendar calendar2 = Calendar.getInstance();
calendar.setTime(new Date("11/7/2013 12:00AM"));
calendar1.get(Calendar.HOUR_OF_DAY) - calendar2.get(Calendar.HOUR_OF_DAY)
but then I see the diff is zero. I guess i change the same calendar instance all the time and compare it to itself. no?
how would you write this?

new Date("15/7/2013 11:00AM")
is not the correct way to construct a Date object. It's deprecated also. Use proper SimpleDateFormat as you are doing well in your first example.
The format MM/dd/yyyy hh:mm a is not correct as per the input date string.
It should be like this.
String format = "dd/MM/yyyy hh:mm a";
SimpleDateFormat sdf = new SimpleDateFormat(format);
Date fakeDate = sdf.parse("15/07/2013 11:00 AM");
Date fakeDate2 = sdf.parse("15/07/2013 12:00 AM");
Calendar cal = Calendar.getInstance();
cal.setTime(fakeDate); //set time to first date
int hours1 = cal.get(Calendar.HOUR); // get the hours
cal.setTime(fakeDate2); // set time to second date
int hours2 = cal.get(Calendar.HOUR); // get the hours
System.out.println(hours1 - hours2); // 11 hours

You seemed really confused though it is depreciated this code shall give you a difference of 11 hours which makes sense
SimpleDateFormat dateFormat = new SimpleDateFormat("MM/dd/yyyy hh:mm a");
Date fakeDate = dateFormat.parse("15/07/2013 11:00 AM");
Date fakeDate2 = dateFormat.parse("15/07/2013 12:00 AM");
System.out.println(fakeDate.getHours()-fakeDate2.getHours());
Or if you are so interested to use Calendar do it this way
Calendar calendar = Calendar.getInstance();
calendar.setTime(fakeDate);
Calendar calendar2 = Calendar.getInstance();
calendar2.setTime(fakeDate2);
System.out.println(calendar.get(Calendar.HOUR_OF_DAY) -
calendar2.get(Calendar.HOUR_OF_DAY));
You will get the same 11 hours difference or simply do it this way
long millisForNewDate = toDate.getTime();
long millisForOldDate = fromDate.getTime();
long difference = millisForNewDate - millisForOldDate;
int hoursDifferenceBetweenDates = (int)(difference/(1000*60*60));

How to subtract X day from a Date object in Java?

I want to do something like:
Date date = new Date(); // current date
date = date - 300; // substract 300 days from current date and I want to use this "date"
How to do it?

Java 8 and later
With Java 8's date time API change, Use LocalDate
LocalDate date = LocalDate.now().minusDays(300);
Similarly you can have
LocalDate date = someLocalDateInstance.minusDays(300);
Refer to https://stackoverflow.com/a/23885950/260990 for translation between java.util.Date <--> java.time.LocalDateTime
Date in = new Date();
LocalDateTime ldt = LocalDateTime.ofInstant(in.toInstant(), ZoneId.systemDefault());
Date out = Date.from(ldt.atZone(ZoneId.systemDefault()).toInstant());
Java 7 and earlier
Use Calendar's add() method
Calendar cal = Calendar.getInstance();
cal.setTime(dateInstance);
cal.add(Calendar.DATE, -30);
Date dateBefore30Days = cal.getTime();

#JigarJoshi it's the good answer, and of course also #Tim recommendation to use .joda-time.
I only want to add more possibilities to subtract days from a java.util.Date.
Apache-commons
One possibility is to use apache-commons-lang. You can do it using DateUtils as follows:
Date dateBefore30Days = DateUtils.addDays(new Date(),-30);
Of course add the commons-lang dependency to do only date subtract it's probably not a good options, however if you're already using commons-lang it's a good choice. There is also convenient methods to addYears,addMonths,addWeeks and so on, take a look at the api here.
Java 8
Another possibility is to take advantage of new LocalDate from Java 8 using minusDays(long days) method:
LocalDate dateBefore30Days = LocalDate.now(ZoneId.of("Europe/Paris")).minusDays(30);

Simply use this to get date before 300 days, replace 300 with your days:
Date date = new Date(); // Or where ever you get it from
Date daysAgo = new DateTime(date).minusDays(300).toDate();
Here,
DateTime is org.joda.time.DateTime;
Date is java.util.Date

Java 8 Time API:
Instant now = Instant.now(); //current date
Instant before = now.minus(Duration.ofDays(300));
Date dateBefore = Date.from(before);

As you can see HERE there is a lot of manipulation you can do. Here an example showing what you could do!
DateFormat dateFormat = new SimpleDateFormat("yyyy/MM/dd HH:mm:ss");
Calendar cal = Calendar.getInstance();
//Add one day to current date.
cal.add(Calendar.DATE, 1);
System.out.println(dateFormat.format(cal.getTime()));
//Substract one day to current date.
cal = Calendar.getInstance();
cal.add(Calendar.DATE, -1);
System.out.println(dateFormat.format(cal.getTime()));
/* Can be Calendar.DATE or
* Calendar.MONTH, Calendar.YEAR, Calendar.HOUR, Calendar.SECOND
*/

With Java 8 it's really simple now:
LocalDate date = LocalDate.now().minusDays(300);
A great guide to the new api can be found here.

In Java 8 you can do this:
Instant inst = Instant.parse("2018-12-30T19:34:50.63Z");
// subtract 10 Days to Instant
Instant value = inst.minus(Period.ofDays(10));
// print result
System.out.println("Instant after subtracting Days: " + value);

I have created a function to make the task easier.
For 7 days after dateString: dateCalculate(dateString,"yyyy-MM-dd",7);
To get 7 days upto dateString: dateCalculate(dateString,"yyyy-MM-dd",-7);
public static String dateCalculate(String dateString, String dateFormat, int days) {
Calendar cal = Calendar.getInstance();
SimpleDateFormat s = new SimpleDateFormat(dateFormat);
try {
cal.setTime(s.parse(dateString));
} catch (ParseException e) {
e.printStackTrace();
}
cal.add(Calendar.DATE, days);
return s.format(cal.getTime());
}

You may also be able to use the Duration class. E.g.
Date currentDate = new Date();
Date oneDayFromCurrentDate = new Date(currentDate.getTime() - Duration.ofDays(1).toMillis());

You can easily subtract with calendar with SimpleDateFormat
public static String subtractDate(String time,int subtractDay) throws ParseException {
Calendar cal = Calendar.getInstance();
SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd HH:mm", Locale.ENGLISH);
cal.setTime(sdf.parse(time));
cal.add(Calendar.DATE,-subtractDay);
String wantedDate = sdf.format(cal.getTime());
Log.d("tag",wantedDate);
return wantedDate;
}

Date comparison in android

I'm facing some problems in comparing the current date and the date which is retrieved from Database.I just retrieved date from DataBase and Stored in a Date variable like this
String due_date_task = cursor.getString(cursor.getColumnIndex(dueDateOfTask));
SimpleDateFormat currentFormater = new SimpleDateFormat("dd/MM/yyyy");
Date date1 = currentFormater.parse(due_date_task);
Now,what i want is should check whether date which is retrieved from DataBase is Equivalent to CurrentDate or not.
Calendar currentDate = Calendar.getInstance();
Date date2 = currentDate.getTime();
if(date1.equals(date2))
System.out.println("Today Task");
i just want to check like this.Thanks in advance

For exact match including milliseconds, use getTime:
if(date.getTime() == date1.getTime()){
//do something
}

You can use this function:
private boolean compareDates(Calendar objCal1, Calendar objCal2) {
return ((objCal1.get(Calendar.YEAR) == objCal2.get(Calendar.YEAR))
&& (objCal1.get(Calendar.DAY_OF_YEAR) == objCal2.get(Calendar.DAY_OF_YEAR)));
}
creating the calendar objects:
Calendar objCal1 = new GregorianCalendar().setTime(date);
Calendar objCal2 = new GregorianCalendar().setTime(date1);

Try this way to get the current date,
Calendar calCurr = Calendar.getInstance();
Log.i("Time in mili of Current - Normal", ""+calCurr.getTimeInMillis()); // see what it gives? dont know why?
Date date = new Date();
calCurr.set(date.getYear()+1900, date.getMonth()+1, date.getDate(), date.getHours(), date.getMinutes(), date.getSeconds());// so added one month to it
Log.i("Time in mili of Current - after update", ""+calCurr.getTimeInMillis()); // now get correct
now create Calendar object for database value,
String due_date_task = cursor.getString(cursor.getColumnIndex(dueDateOfTask));
SimpleDateFormat currentFormater = new SimpleDateFormat("dd/MM/yyyy");
Date date = currentFormater.parse(due_date_task);
Calendar start = Calendar.getInstance();
start.set(date.getYear(), date.getMonth(), date.getDate(), date.getHours(), date.getMinutes(), date.getSeconds());
and now Compare both the Calendar objects
if(calCurr.equals(start))

Android : date format in a String

I have a problem to sort date because of the format of these dates.
I obtain the date :
final Calendar c = Calendar.getInstance();
mYear = c.get(Calendar.YEAR);
mMonth = c.get(Calendar.MONTH);
mDay = c.get(Calendar.DAY_OF_MONTH);
And I build a String with these values.
dateRappDB = (new StringBuilder()
.append(mYear).append(".")
.append(mMonth + 1).append(".")
.append(mDay).append(" ")).toString();
The problem is that if the month is for example February, mMonth value is 2. So dates with months like October (10) comes before in my list.
What I need is that month and day are formated like MM and dd. But I don't know how to do it in my case.
EDIT :
I solved the problem by using a DateFormat like said above.
I replaced this :
dateRappDB = (new StringBuilder()
.append(mYear).append(".")
.append(mMonth + 1).append(".")
.append(mDay).append(" ")).toString();
By this :
Date date = new Date(mYear - 1900, mMonth, mDay);
dateFacDB = DateFormat.format("yyyy.MM.dd", date).toString();
And it works.
Thanks to all of you for your help :)

here is a simple way to convert Date to String :
SimpleDateFormat simpleDate = new SimpleDateFormat("dd/MM/yyyy");
String strDt = simpleDate.format(dt);

Calendar calendar = Calendar.getInstance();
Date now = calendar.getTime();
String timestamp = simpleDateFormat.format(now);
These might come in handy
SimpleDateFormat simpleDateFormat =
new SimpleDateFormat("yyyy-MM-dd'T'HH:mm:ss.SSSSSSZZZZZ");
this format is equal to --> "2016-01-01T09:30:00.000000+01:00"
SimpleDateFormat simpleDateFormat =
new SimpleDateFormat("yyyy-MM-dd'T'HH:mm:ssZZZZZ");
this format is equal to --> "2016-06-01T09:30:00+01:00"

here is the example for date format
SimpleDateFormat sdf = new SimpleDateFormat("dd-MM-yyyy");
Date date = new Date();
System.out.println("format 1 " + sdf.format(date));
sdf.applyPattern("E MMM dd yyyy");
System.out.println("format 2 " + sdf.format(date));

You need to sort dates, not strings. Also, have you heared about DateFormat? It makes all that appends for you.

If I understand your issue, you create a list of dates and since they're strings, they get arranged in a dictionary-order number-wise, which means you get october before february (10 before 2).
If I were you, I would store my results in a container where I control the insertion point (like an array list) or where I can control the sorting algorithm.

Date text box should be always 1st date current month.?

I want a javascript or java program should always give date 1st of current month.
Is there any tech?

You can use Calendar for Java
Date date = new Date(System.currentTimeMillis());
cal = Calendar.getInstance();
cal.setTime(date);
cal.set(Calendar.DAY_OF_MONTH, 1);
Now you do what every you want to do with this Calendar object like to get the Day of the Week (Sat, Sun, .... )
int weekday = cal.get(Calendar.DAY_OF_WEEK);
And for JavaScript you can use:
var theFirst = new Date();
theFirst.setDate(1);
setDate sets the day of the month for the Date object (from 1 to 31). Then you can do whatever you want with theFirst, like get the day of the week.

Calendar ans = Calendar.getInstance();
ans.set(ans.get(Calendar.YEAR),
ans.get(Calendar.MONTH),
1,
0,
0,
0
);
System.out.println(ans.getTime());