Notify and Wait

Two things to be noticed:
1) notify() and wait() need to be used inside a synchronized block. They are used for release/acquire object lock.

2) In getMessage() method, notify() is called at first beginning. It doesn't actually release the lock atm, as stated in API "Wakes up a single thread that is waiting on this object's monitor...The awakened thread will not be able to proceed until the current thread relinquishes the lock on this object."

import java.util.Vector; 
 
class Producer extends Thread { 
    static final int MAXQUEUE = 5; 
    private Vector messages = new Vector(); 
  
    public void run() { 
        try { 
            while ( true ) { 
                putMessage(); 
                sleep( 1000 ); 
            } 
        }  
        catch( InterruptedException e ) { } 
    } 
 
    private synchronized void putMessage() 
        throws InterruptedException { 
        
        while ( messages.size() == MAXQUEUE ) 
            wait(); 
        messages.addElement( new java.util.Date().toString() ); 
        notify(); 
    } 
 
    // Called by Consumer 
    public synchronized String getMessage() 
        throws InterruptedException { 
        notify(); 
        while ( messages.size() == 0 ) 
            wait(); 
        String message = (String)messages.firstElement(); 
        messages.removeElement( message ); 
        return message; 
    } 
} 
 
class Consumer extends Thread { 
    Producer producer; 
     
    Consumer(Producer p) { 
        producer = p; 
    } 
  
    public void run() { 
        try { 
            while ( true ) { 
                String message = producer.getMessage(); 
                System.out.println("Got message: " + message); 
                sleep( 2000 ); 
            } 
        }  
        catch( InterruptedException e ) { } 
    } 
  
    public static void main(String args[]) { 
        Producer producer = new Producer(); 
        producer.start(); 
        new Consumer( producer ).start(); 
    } 
} 

See: Exploring Java: Threads