You could use
<http://java.sun.com/javase/6/docs/api/java/util/Calendar.html#set(int,%20int)>

Use input.substring( 4, 7 ) to extract the Julian day.  Use substring( 0, 4 )
to get the year.  Set the year with

 set( Calendar.YEAR, year );

and the day of the year with

 set( Calendar.DAY_OF_YEAR, julianDay );

You should consider making the API Javadocs a primary source.

That should be "e.g.," (with a comma), not "i.e.".

 If the String would be

1234

 , is this day 234 of year 1, day 34 of year 12, or day 4 of
 year 123, or another day of another year, or not an element
 of the input notation?

 To write a parser and interpreter for a language, you need a
 specification for the

     - syntax and the
     - semantics of this language.

 The syntax is the set of all valid representations, the
 semantics gives the year and day of the year for each element
 of this set.

 One example or even several examples do not provide this.

 This newsgroup comp.lang.java.help is intended to give help
 for the programming language Java. So if you have specified
 your syntax and semantics, then you can get help on how to
 implement it in Java here.

 But if you do not provide syntax or semantics, one does not
 know, what it /is/ that you want to implement.

     »Hi, I have invented a new number system, that represents
     values in a more compact and convenient way
     i.e. (423@f4)

     Is there a way to read this with Java?«

On 05 Jul 2008 01:53:30 GMT, "I_Am_The_Walrus"
<thewalrus@will_f_u_upINvalid> wrote, quoted or indirectly quoted
someone who said :

see http://mindprod.com/jgloss/calendar.html#PARSING

All you have to do is change the SimpleDateFormat mask.

So 2005365 would represent December 31st of 2005?

Are there always 4 digits for the year and 3 digits for the day of the
year? Then you could use a SimpleDateFormat with a pattern containing
"D" for a "Day in year" field. For example:

import java.text.*;
public class YearAndDayOfYear {
   public static void main(String[] args) throws Exception {
      DateFormat dateParser = new SimpleDateFormat("yyyyDDD");
      System.out.println(dateParser.parse("2005001"));
      System.out.println(dateParser.parse("2005365"));
   }
}