Month is not printed from a date - Java DateFormat

问题

How to get month from a date in java :

        DateFormat inputDF  = new SimpleDateFormat("mm/dd/yy");
        Date date1 = inputDF.parse("9/30/11");

        Calendar cal = Calendar.getInstance();
        cal.setTime(date1);

        int month = cal.get(Calendar.MONTH);
        int day = cal.get(Calendar.DAY_OF_MONTH);
        int year = cal.get(Calendar.YEAR);

        System.out.println(month+" - "+day+" - "+year);

This code return day and year but not month.

output :

0 - 30 - 2011

回答1:

This is because your format is incorrect: you need "MM/dd/yy" for the month, because "mm" is for minutes:

DateFormat inputDF  = new SimpleDateFormat("MM/dd/yy");
Date date1 = inputDF.parse("9/30/11");

Calendar cal = Calendar.getInstance();
cal.setTime(date1);

int month = cal.get(Calendar.MONTH);
int day = cal.get(Calendar.DAY_OF_MONTH);
int year = cal.get(Calendar.YEAR);

System.out.println(month+" - "+day+" - "+year);

Prints 8 - 30 - 2011 (because months are zero-based; demo)

回答2:

First, you used mm in your date format, which is "minutes" according to the Javadocs. You set the minutes to 9, not the month. It looks like the month defaults to 0 (January).

Use MM (capital 'M's) to parse the month. Then, you will see 8, because in Calendar months start with 0, not 1. Add 1 to get back the desired 9.

The first month of the year in the Gregorian and Julian calendars is JANUARY which is 0

// MM is month, mm is minutes
DateFormat inputDF  = new SimpleDateFormat("MM/dd/yy");  

and later

int month = cal.get(Calendar.MONTH) + 1; // To shift range from 0-11 to 1-12

回答3:

If you read the SimpleDateFormat javadoc, you'll notice that mm is for minutes. You need MM for month.

DateFormat inputDF  = new SimpleDateFormat("MM/dd/yy");

Otherwise the format doesn't read a month field and assumes a value of 0.

回答4:

Month format should be MM instead of mm

 DateFormat inputDF  = new SimpleDateFormat("MM/dd/yy");

回答5:

Time for someone to provide the modern answer. The other answers were good answers when the question was asked in 2013 and are still correct. Today there is no reason why you should struggle with the old, outdated and simultaneously notoriously troublesome SimpleDateFormat class. java.time, the modern Java date and time API, is so much nicer to work with:

    DateTimeFormatter inputFormatter = DateTimeFormatter.ofPattern("M/d/yy");
    LocalDate date1 = LocalDate.parse("9/30/11", inputFormatter);
    System.out.println(date1);

This prints

2011-09-30

The LocalDate class represents a date without time-of-day, exactly what you need, it matches your requirement much more precisely than the old classes Date and Calendar.

The format pattern strings used with DateTimeFormatter resemble those from SimpleDateFormat, there are a few differences. You may use uppercase MM to require two-digit month (like 09 for September) or a single M to allow the month to be written with one or two digits. Similarly dd or d for day of month. yy denotes two-digit year and is interpreted with base 2000, that is, from 2000 to 2099 inclusive (wouldn’t work for my birthday).

Link: Oracle tutorial Trail: Date Time explaining how to use java.time.

回答6:

Try like this using MM instead of mm:-

    DateFormat inputDF  = new SimpleDateFormat("MM/dd/yy");
    Date date1 = inputDF.parse("9/30/11");

    Calendar cal = Calendar.getInstance();
    cal.setTime(date1);

    int month = cal.get(Calendar.MONTH);
    int day = cal.get(Calendar.DAY_OF_MONTH);
    int year = cal.get(Calendar.YEAR);

    System.out.println(month+" - "+day+" - "+year);

The month printed will be 8 as index starts from 0

or try with:-

int month = cal.get(Calendar.MONTH) + 1;

回答7:

mmis for minutes, use MM while specifying format.

Calendar cal = Calendar.getInstance();
cal.setTime(date1);

int month = cal.get(Calendar.MONTH);// returns month value index starts from 0

来源：https://stackoverflow.com/questions/19036033/month-is-not-printed-from-a-date-java-dateformat