Calendar c = Calendar.getInstance();
DateFormat sdf = new SimpleDateFormat("dd/MM/yyyy");
c.setTime( sdf.parse("31/12/2010"));
out.println( c.get( Calendar.WEEK_OF_YEAR ) );  

---

---

---

---

---

---

---

---

---

---

---

---

---

---

---

---

---

---

---

---