将日历对象转换为java格式的字符串，格式为“yyyy-mm-dd hh：mm：ss”

Question

I'm converting a date stored in a calendar object in a string for a query in MySQL. I need the string in format "yyyy-MM-dd HH:mm:ss", ie: "2010-01-01 15:30:00". I'm using a code like:

    Calendar TimeStop = Calendar.getInstance();
    TimeStop.set(2010, 01, 01, 15, 30, 0);
    SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
    String TimeStop_Str = sdf.format(TimeStop.getTime());

now, the string, instead of being "2010- 01 -01 15:30:00" like I would expect is "2010- 02 -01 15:30:00". I have already checked on http://download.oracle.com/javase/1.4.2/docs/api/java/text/SimpleDateFormat.html for possible errors in the parser formula (for example the capital MM for month or capital HH for hours) but it didn't worked.

I guess that there is some other field that I should set, or maybe there is another method... any idea?

Answer 1

Calendar.JANUARY is actually 0 , not 1 .

When you provide 01 for the month field in set , you're actually setting the month to February, which is why you get 02 when SimpleDateFormat renders it as MM .

When you use any of the Calendar.get/set methods, you must take extra precautions to make sure that you are aware of this discrepancy between the "natural" 1-based indexing, and the more "awkward" Calendar 's 0-based indexing. Any time you're getting/setting the month of a Calendar , there's always this potential to cause a serious bug.

This is just one of those really awkward design in Calendar that leaves a lot to be desired. One of the better, more pleasant date/time API library available out there is Joda Time , so if at all possible, you may consider a switch to that library instead.

API links

• Calendar.MONTH - "Field number for get and set indicating the month. This is a calendar-specific value. The first month of the year [...] is JANUARY which is 0".
• Calendar.set(…, int month, …) - " month - the value used to set the MONTH calendar field. Month value is 0-based. eg, 0 for January."

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On octal literals

Also, note that 01 is actually an octal literal (ie base 8 ). You shouldn't make a habit of prepending 0 to integer literals ( §3.10.1 ), because they can cause subtle bugs/errors unless you're very careful.

For example, int i = 09; is an illegal Java code.

    System.out.println(010); // prints 8, not 10

See also

• Can I customize syntax highlighting in Eclipse to show octal literals differently?
• Octal number literals: when? why? ever?

Answer 2

It's just that months start from 0 (not 1 )

That said, you should not convert to string in order to insert a date in the DB. You can either use the timestamp, or java.sql.Date together with PreparedStatement