About synchronized dead lock

Question

I tried to use while instead of wait() and notify() to let two thread communicate. Whether the loop will continue depends on the change of the flag----isFull & isEmpty, how ever it locks during the run . but if I remove the synchronized modifier before the add() and eat() method , it works . Who knows why synchronized will cause the lock , they are modifying two different methods!!!
The who programe i s shown below
Help if who knows thanks a lot to tell me
I want to know the inner mechanism of synchronized
email: [email protected]
class SoupTest {
public static void main(String args[]) {
     Soup s = new Soup();
     Producer p1 = new Producer(s);
     Consumer c1 = new Consumer(s);
     p1.setPriority(10);
     c1.setPriority(1);
p1.start();
c1.start();
class Consumer extends Thread {
private Soup soup;
public Consumer(Soup s) {
     soup = s;
public void run() {
     char c;
     for (int i = 0; i < 10; i++) {
     c = soup.eat();
     System.out.println("Ate a letter: " + c);
     try {
          sleep((int)Math.random() * 2000000000);
     } catch (InterruptedException e) { }
class Producer extends Thread {
private Soup soup;
private String alphabet = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";
public Producer(Soup s) {
     soup = s;
public void run() {
char c;
System.out.println("I am running");
for (int i = 0; i < 10; i++) {
c = alphabet.charAt((int)(Math.random() * 26));
soup.add(c);
System.out.println("Added " + c + " to the soup.");
System.out.print("mimi");
try {
sleep((int)(Math.random() * 100));
} catch (InterruptedException e) { }
System.out.print("kiki");
class Soup {
private char buffer[] = new char[6];
private int eatnext = 0;
private int addnext = 0;
// flags to keep track of our buffer status
public boolean isFull = false;
private boolean isEmpty = true;
public synchronized char eat() {
     char toReturn;
// we can't eat if there isn't anything in the buffer
System.out.println("Empty " + isEmpty) ;
     while (isEmpty) {
          //try {
     //wait();
     //} catch (InterruptedException e) {
     toReturn = buffer[eatnext];
     eatnext = (eatnext + 1) % 6;
// did we eat the last letter ��
     if (eatnext == addnext ) {
     isEmpty = true;
// we know the buffer can't be full, because we just ate
     isFull=false;
//      notify();
// return the letter to the thread that is eating
     return(toReturn);
public synchronized void add(char c) {
// wait around until there's room to add another letter
System.out.print("pipi");
while(isFull) {
     //try {
     //     wait(); // this will exit when isFull turns false
     // } catch (InterruptedException e) {
     // add the letter to the next available spot
System.out.println("add" + addnext) ;
     buffer[addnext] = c;
// change the next available spot
     addnext = (addnext + 1) % 6;
// are we full ?
     if (addnext == eatnext) {
     isFull = true;
     isEmpty = false;
//     notify();
}

Answer 1

Please use code formatting when you post code. No one likes to read code without indentation. (See formating tips just above the text field where you enter your text)
/Kaj