FULL PRODUCT VERSION :
java version "1.6.0_16"
Java(TM) SE Runtime Environment (build 1.6.0_16-b01)
Java HotSpot(TM) Client VM (build 14.2-b01, mixed mode, sharing)

same issue happens with JDK5

ADDITIONAL OS VERSION INFORMATION :
Microsoft Windows XP [Version 5.1.2600]

A DESCRIPTION OF THE PROBLEM :
Run the attach code and see the difference.
Both parts are exactly the same except for the call : c1.get(Calendar.WEEK_OF_YEAR)

Whatever a Java object is, Doing a get on it should NEVER affects its state.


STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
run the attached program

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
The output is:
Day 27
Date 27/Dec/2009 12:00:00
Day 27
Date 27/Dec/2009 12:00:00


ACTUAL -
Day 31
Date 31/Dec/2009 12:00:00
Day 27
Date 27/Dec/2009 12:00:00


REPRODUCIBILITY :
This bug can be reproduced always.

---------- BEGIN SOURCE ----------

import java.util.Calendar;
import java.text.*;
import java.util.*;


public class DateBug {
private static SimpleDateFormat sdf = new SimpleDateFormat();
private static FieldPosition fieldPos = new FieldPosition(0);
private static String DEFAULT_DATE_TIME_PATTERN = "dd/MMM/yyyy hh:mm:ss";

public DateBug(){
}


public static void main(String[] args) {
Calendar c1 = Calendar.getInstance();
c1.clear();
c1.set(Calendar.YEAR, 2009);
System.out.println(c1.get(Calendar.WEEK_OF_YEAR)); // <--- This statments affect the outcome.

c1.set(Calendar.WEEK_OF_YEAR, 53);
System.out.println("Day " + c1.get(Calendar.DAY_OF_MONTH));//<-- It Correct answer is 27 but it prints 31
System.out.println("Date " + formatDateTime(c1.getTime(),DEFAULT_DATE_TIME_PATTERN));


c1 = Calendar.getInstance();
c1.clear();
c1.set(Calendar.YEAR, 2009);
// System.out.println(c1.get(Calendar.WEEK_OF_YEAR));

c1.set(Calendar.WEEK_OF_YEAR, 53);
System.out.println("Day " + c1.get(Calendar.DAY_OF_MONTH)); // <-- It Correct answer is 27
System.out.println("Date " + formatDateTime(c1.getTime(),DEFAULT_DATE_TIME_PATTERN)); // Correct
}

public static String formatDateTime(Date date, String pattern) {
StringBuffer buffer = new StringBuffer(20);
try {
if (date == null)
return "";
sdf.applyPattern(pattern);
buffer = sdf.format(date, buffer, fieldPos);
} catch (Exception ex) {
buffer.append("Error");
}

return buffer.toString();
}

}
---------- END SOURCE ----------