The DateFormat
class introduced in Section 6.3 has some additional methods, notably
parse( )
, which tries to parse a string
according to the format stored in the given
DateFormat
object.
// DateParse1.java SimpleDateFormat formatter = new SimpleDateFormat ("yyyy-MM-dd"); String input = args.length == 0 ? "1818-11-11" : args[0]; System.out.print(input + " parses as "); Date t; try { t = formatter.parse(input); System.out.println(t); } catch (ParseException e) { System.out.println("unparseable using " + formatter); }
This will parse any date back to Year Zero and well beyond Year 2000.
What if the date is embedded in an input string? You could, of
course, use the string’s substring( )
method to extract it, but there is an
easier way. The ParsePosition
object from
java.text
is designed to represent (and track) the
position of an imaginary cursor in a
string. Suppose we have genealogical data with input strings
representing the times of a person’s life:
BD: 1913-10-01 Vancouver, B.C. DD: 1983-06-06 Toronto, ON
This lists one person’s birth date (BD) and place, and death
date (DD) and place. We can parse these using
String.indexOf('
')
to find the space
after the :
character,
DateFormat
parse()
to parse the
date, and String.substring( )
to get the city
and other geographic information.
Here’s how:
// DateParse2.java SimpleDateFormat formatter = new SimpleDateFormat ("yyyy-MM-dd"); String input[] = { "BD: 1913-10-01 Vancouver, B.C.", "MD: 1948-03-01 Ottawa, ON", "DD: 1983-06-06 Toronto, ON" }; for (int i=0; i<input.length; i++) { String aLine = input[i]; String action; switch(aLine.charAt(0)) { case 'B': action = "Born"; break; case 'M': action = "Married"; break; case 'D': action = "Died"; break; // others... default: System.err.println("Invalid code in " + aLine); continue; } int p = aLine.indexOf(' '), ParsePosition pp = new ParsePosition(p); Date d = formatter.parse(aLine, pp); if (d == null) { System.err.println("Invalid date in " + aLine); continue; } String location = aLine.substring(pp.getIndex( )); System.out.println( action + " on " + d + " in " + location); }
This works like I said it would:
Born on Wed Oct 01 00:00:00 PDT 1913 in Vancouver, B.C. Married on Mon Mar 01 00:00:00 PST 1948 in Ottawa, ON Died on Mon Jun 06 00:00:00 PDT 1983 in Toronto, ON
Note that the polymorphic form of parse( )
that
takes one argument throws a
ParseException
if the input cannot be
parsed, while the form that takes a ParsePosition
as its second argument returns null
to indicate
failure.
3.142.166.31