Classic Producer/Consumer question

Question

All,
I've a straightforward (hopefully) question regarding the producer consumer thread example. While I get the entire picture - I have a question about the lines in the code :
               sleep((int)(Math.random() 100));*
in the Producer. WITHOUT this line, the output goes haywire (with Producer and Consumers putting/getting values randomly)- while with it, the producers and consumers start behaving normally (ie Producer Puts and Consumer Gets like clockwork)
Why is it necessary to put this line in? What's it trying to put to sleep anyways and for what reason?
public class DerTest {
     public static void main(String...argv) throws Exception {
          Holder holder = new Holder();
          Producer producer = new Producer(holder);
          Consumer consumer = new Consumer(holder);
          producer.start();
          consumer.start();
class Holder {
     private int i;
     private boolean hasPut;
     public synchronized void put(int i) throws Exception {
          while(this.hasPut == true) {
               wait();
          this.i = i;
          this.hasPut = true;
          notifyAll();
     public synchronized int get() throws Exception {
          while(this.hasPut == false) {
               wait();
          this.hasPut = false;
          notifyAll();
          return this.i;
class Producer extends Thread {
     private Holder holder;
     public Producer(Holder holder) {
          this.holder = holder;
     public void run() {
          int i = 0;
          try {
               while(true) {
                    this.holder.put(i);
                    System.out.println("Putting : "+i);
                    i++;
                    sleep((int)(Math.random() * 100));
          } catch(Exception e) {
               e.printStackTrace();
class Consumer extends Thread {
     private Holder holder;
     public Consumer(Holder holder) {
          this.holder = holder;
     public void run() {
          try {
               while(true) {
                    int i = this.holder.get();
                    System.out.println("Getting : "+i);
          } catch(Exception e) {
               e.printStackTrace();
}

Answer 1

because the printlns are not synchronized with the get/put calls. put those printlns inside of the get/put methods and you will see the output you expect.