IllegalMonitorStateException

[Paul Michael]

i created these classes and got that exception. i tried using the synchronized(this) statement instead and it worked... it's a bit confusing because letters and numbers are two diff objects and i'm wondering why my first aproach didn't work... hewp!

import java.util.*;
public class Mailbox {
private boolean letterAvailable;
private boolean numberAvailable;
private Stack numbers = new Stack();
private Stack letters = new Stack();
public void storeNumber(char ch) {
synchronized(numbers) {
System.out.println("storeNumber(): "+ch);
numbers.push(new Character(ch));
numberAvailable = true;
notifyAll();
}
}
public void retrieveNumber() {
synchronized(numbers) {
while (!numberAvailable) {
try {
wait();
} catch(Exception e) {}
}
numberAvailable = false;
System.out.println("retrieveNumber(): "+((Character) numbers.pop()).charValue());
}
}
public void storeLetter(char ch) {
synchronized(letters) {
System.out.println("storeLetter(): "+ch);
letters.push(new Character(ch));
letterAvailable = true;
notifyAll();
}
}
public void retrieveLetter() {
synchronized(letters) {
while (!letterAvailable) {
try {
wait();
} catch(Exception e) {}
}
letterAvailable = false;
System.out.println("retrieveLetter(): "+((Character) letters.pop()).charValue());
}
}
}

class MailboxTest {
private Mailbox mb;
MailboxTest(Mailbox mb) {
this.mb=mb;
}
class LetterWriter extends Thread {
public void run() {
while (true) {
char c = (char) (Math.random()*100);
if (Character.isLetter(c)) {
mb.storeLetter(c);
}
}
}
}
class LetterReader extends Thread {
public void run() {
while (true) {
mb.retrieveLetter();
}
}
}
class NumberWriter extends Thread {
public void run() {
while (true) {
char c = (char) (Math.random()*100);
if (Character.isDigit(c)) {
mb.storeNumber(c);
}
}
}
}
class NumberReader extends Thread {
public void run() {
while (true) {
mb.retrieveNumber();
}
}
}
public static void main(String args[]) {
Mailbox mb = new Mailbox();
MailboxTest mbt = new MailboxTest(mb);
mbt.new LetterReader().start();
mbt.new LetterWriter().start();
mbt.new NumberReader().start();
mbt.new NumberWriter().start();
}

}

[Marcela Blei]

if you look at the API doc. at the Object.notify method for example, you can find the following:
A thread becomes the owner of the object's monitor in one of three ways:
-By executing a synchronized instance method of that object.
-By executing the body of a synchronized statement that synchronizes on the object.
-For objects of type Class, by executing a synchronized static method of that class.
So, what does this mean: That you have to own the thread�s monitor for using notify, for example. Read the API doc carefully for a deep detail of the following methods from Object: notify, wait, notifyAll.

[Paul Michael]

i see!!! i should have specifically used the letters.notify()/letters.notifyAll() and numbers.notify()/numbers.notifyAll() instead!
thanks a lot Marcela Blei!!!