You can use Calendar class :

Calendar cal = Calendar.getInstance();
cal.add(Calendar.DATE, -7);
System.out.println("Date = "+ cal.getTime());

But as @Sean Patrick Floyd mentioned , Joda-time is the best Java library for Date.

add(Calendar.DAY_OF_MONTH, 7);

From Calendar JavaDoc

From exactly now:

long DAY_IN_MS = 1000 * 60 * 60 * 24;
new Date(System.currentTimeMillis() - (7 * DAY_IN_MS))

From arbitrary Date date:

new Date(date.getTime() - (7 * DAY_IN_MS))

Edit: As pointed out in the other answers, does not account for daylight savings time, if that's a factor.

Just to clarify that limitation I was talking about:

For people affected by daylight savings time, if by 7 days earlier, you mean that if right now is 12pm noon on 14 Mar 2010, you want the calculation of 7 days earlier to result in 12pm on 7 Mar 2010, then be careful.

This solution finds the date/time exactly 24 hours * 7 days= 168 hours earlier.

However, some people are surprised when this solution finds that, for example, (14 Mar 2010 1:00pm) - 7 * DAY_IN_MS may return a result in(7 Mar 2010 12:00pm) where the wall-clock time in your timezone isn't the same between the 2 date/times (1pm vs 12pm). This is due to daylight savings time starting or ending that night and the "wall-clock time" losing or gaining an hour.

If DST isn't a factor for you or if you really do want (168 hours) exactly (regardless of the shift in wall-clock time), then this solution works fine.

Otherwise, you may need to compensate for when your 7 days earlier doesn't really mean exactly 168 hours (due to DST starting or ending within that timeframe).

LocalDate is perfect for this job:

LocalDate.now().plusDays(7);

You can get your string representation with

.format(DateTimeFormatter.ISO_DATE);

If you're not able to use Java 8, then you have a few options:

• Use the ThreeTen-Backport, which backports most functionality of the Java 8 JSR-310 API, normally available in the java.time package. See here for details. This package is available in Maven Central.

• You can also use Joda Time. The peculiar thing is that these two projects have almost the same layout of their websites.

• If you're otherwise not able to use ThreeTen-Backport or Joda Time, you can use this:

  Calendar c = Gregorian​Calendar.getInstance();
  c.add(Calendar.DATE, 7);
  
  String s = new Simple​Date​Format("yyyy-MM-dd")
      .format(c.getTime());
  

  Warning
  Many things are wrong with the old Date and Time API, see here. Use this only if you have no other option.

Using the java.time classes built into Java 8 and later:

LocalDate from = LocalDate.parse("2015-05-01");
LocalDate to = LocalDate.parse("2015-05-07");

long days = ChronoUnit.DAYS.between(from, to);    // 6 days
long weeks = ChronoUnit.WEEKS.between(from, to);  // 0 weeks

String dt = "2008-01-01";  // Start date
SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd");
Calendar c = Calendar.getInstance();
c.setTime(sdf.parse(dt));
c.add(Calendar.DATE, 7);  // number of days to add
dt = sdf.format(c.getTime());  // dt is now the new date

took from this thread

How can I increment a date by one day in Java?

Parse the date:

Date myDate = dateFormat.parse(dateString);

And then either figure out how many milliseconds you need to subtract:

Date newDate = new Date(myDate.getTime() - 604800000L); // 7 * 24 * 60 * 60 * 1000

Or use the API provided by the java.util.Calendar class:

Calendar calendar = Calendar.getInstance();
calendar.setTime(myDate);
calendar.add(Calendar.DAY_OF_YEAR, -7);
Date newDate = calendar.getTime();

Then, if you need to, convert it back to a String:

String date = dateFormat.format(newDate);

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();