How to get previous 7 dates from a particular date

问题:

//explain
public class DateLoop {
    static String finalDate; 
    static String particularDate;

    public static void main(String[] args) {
        // TODO Auto-generated method stub

        SimpleDateFormat sdf = new SimpleDateFormat("d-M-yyyy ");
        Calendar cal = Calendar.getInstance();
        particularDate = "2-1-2018";
        // get starting date
        cal.add(Calendar.DAY_OF_YEAR, -7);

        // loop adding one day in each iteration
        for(int i = 0; i< 7; i++){
            cal.add(Calendar.DAY_OF_YEAR, 1);
            finalDate =sdf.format(cal.getTime());
            System.out.println(finalDate);
            //ie, its giving previous 7 dates from present date, but I want
            //particular date... thanks in advance
        }
    }

}

ie, its giving previous 7 dates from present date, but I want previous 7 dates from particular date.

回答1:

tl;dr

LocalDate.of( 2018 , Month.JANUARY , 23 )
         .minusDays( … )

java.time

You are using troublesome old date-time classes that are now legacy, supplanted by the java.time classes.

Use LocalDate for a date-only without time-of-day.

Using the Month enum.

LocalDate start = LocalDate.of( 2018 , Month.JANUARY , 23 ) ;  // 2018-01-23.

Using month numbers, 1-12 for January-December.

LocalDate start = LocalDate.of( 2018 , 1 , 23 ) ;  // 2018-01-23.

Collect a sequence of dates.

List<LocalDate> dates = new ArrayList<>( 7 ) ;
for( int i = 1 ; i <= 7 ; i ++ ) {
    LocalDate ld = start.minusDays( i ) ;  // Determine previous date.
    dates.add( ld ) ;  // Add that date object to the list. 
}

For earlier Android, use the ThreeTen-Backport and ThreeTenABP projects.

回答2:

As Uta Alexandru and Basil Bourque have said already, don’t use the long outmoted classes SimpleDateFormat and Calendar. java.time, the modern Java date and time API also known as JSR-310, is so much nicer to work with:

    DateTimeFormatter dtf = DateTimeFormatter.ofPattern("d-M-uuuu");
    LocalDate date = LocalDate.parse("2-1-2018", dtf)
            .minusDays(7);

    for(int i = 0; i < 7; i++) {
        date = date.plusDays(1);
        String finalDate = date.format(dtf);
        System.out.println(finalDate);
    }

This prints:

27-12-2017
28-12-2017
29-12-2017
30-12-2017
31-12-2017
1-1-2018
2-1-2018

Not only is the code slightly simpler and shorter, more importantly, it is clearer and more natural to read.

Question: Can I use java.time on Android?

You certainly can. It just requires at least Java 6.

• In Java 8 and later the new API comes built-in.
• In Java 6 and 7 get the ThreeTen Backport, the backport of the new classes (ThreeTen for JSR 310).
• On Android, use the Android edition of ThreeTen Backport. It’s called ThreeTenABP.

Links

• Oracle tutorial: Date Time, explaining how to use java.time.
• ThreeTen Backport project
• ThreeTenABP, Android edition of ThreeTen Backport
• Question: How to use ThreeTenABP in Android Project, with a very thorough explanation.
• Java Specification Request (JSR) 310, where the modern date and time API was first described.

回答3:

if you want to have some date from some data do something like below.

public void dateFromRandomDate(String date){
    SimpleDateFormat formatter2=new SimpleDateFormat("dd-MMM-yyyy");  
    Date date2=formatter2.parse(date); 
    Calendar calendar = Calendar.getInstance();
    //this sets the date to given date
    calendar.calendar.setTime(date2);
    //now call getTime() or add ,subtract date from here
    //this will add 1 year to given one,similarlly others will work.
    calendar.add(Calendar.YEAR,1);
}