Are this threads running concurrently?

Question

This is an example I reproduced from the Kathy&Bert's SCJP book.
public class Machinery implements Runnable{
     Operator operator;
     public Machinery(Operator operator){
          this.operator = operator;
     public static void main(String[] args){
          Operator o = new Operator();
          Thread operator = new Thread(o);
          operator.setName("Operator");
          Machinery m = new Machinery(o);
          Thread machine = new Thread(m);          
          machine.setName("Machine");
          machine.start();
          operator.start();                    
     public void run(){     
          for(int c=0; c<2; ++c){
               System.out.println("Machine...");
               synchronized(operator){
                    try{
                         System.out.println("Machine waiting...");
                         operator.wait(5000);                         
                         for(int i=0; i<3; ++i){                    
                              System.out.println("Machine processing...");
                              Thread.sleep(2000);
                    }catch(InterruptedException ex){
                         ex.printStackTrace();
class Operator implements Runnable{
     String name = Thread.currentThread().getName();
     public void run(){
          for(int c=0; c<2; ++c){
               // Line 44...
               System.out.println("Operator...");
               synchronized(this){                    
                    try{
                    for(int i=0; i<3; ++i){                         
                         System.out.println("Operator operating...");
                         Thread.sleep(500);
                    notify();
                    wait(100);
                    }catch(InterruptedException ie){ie.printStackTrace();}
}I don't know why after Operator notifies Machinery and get out of the synchronized code, the operator doesn't print the statement "Operator" on line 45 (there's a comment on line 44), I mean, this code is out of the synchronized block and since both threads are executing concurrently I thought it should print. I tried changing the Machinery's loop counter to 33 (to see if this was because of the scheduler "time slicing") so the "operator" line would print at least in between the machinery's loop print statements but I had no luck. Right now is as if both threads are running one at a time... I know I must be missing something here so if anyone can tell me what is I would really appreciate it.

Answer 1

In your Operator class you do this.
notify();
wait(100);So it notifies machinery and then waits 100 milliseconds. The wait releases the lock allowing machinery to do its thing. The key thing to note is that Operator is still in the synchronized block of code.
So basically, you don't see "Operator..." among the "Machine processing..." because the operator has to wait for Machinery to finish so that it may obtain the lock and exit the synchronized block.
This is assuming the program is running as you wrote it. It's completely legal for the System.out statements to be moved into the synchronized blocks without your knowledge.