Explain me this output (notify)

[Shivit Agarwal]

Hello,
I didn't get the output ...Can anybody explain me ??
I have bolden the problem in output.

The code is from THE COMPLETE REFERENCE - HERBERT SCHILDT


My second question is what happens when a notify() is encountered before a wait() statement as in the program below ??

class Q
{
   int n;
   boolean valueSet=false;
 
   synchronized void put(int i)
   {
      if(valueSet)
      {
   
         try
         {
            System.out.println(" Entry of Put :"+i); 
 
            wait();
         }
         catch(InterruptedException ieo)
         {
            System.out.println("\n Interrupt Caught ");
         }
      }
 
      this.n=i; 
      valueSet=true;
      System.out.println(" Put : "+n);
      notify();
      System.out.println(" What will be the next line");
   }
 
   synchronized int get()
   {
      if(!valueSet)
      {
         try
         {
            System.out.println("Entry of get:"+n);
            wait();
            System.out.println(" How many times"); 
         }
         catch(InterruptedException ieo)
         {
            System.out.println("\n Interrupt Caught ");
         }
      }
 
      System.out.println(" Got : "+n);
      valueSet=false;
      notify();
      return n;
   }
 
}
 
class Producer implements Runnable
{
   Q q;
  
   Producer(Q obj)
   {
      q=obj;
      new Thread(this,"Producer").start();
      System.out.println("Producer hai ");
   }
  
   public void run()
   {
      int i=0;
 
      while(true)
         q.put(i++);
   }
}
 
class Consumer implements Runnable
{
   Q q;
  
   Consumer(Q obj)
   {
      q=obj;
      new Thread(this,"Consumer").start();
      System.out.println("Consumer hai ");
   }
  
   public void run()
   {
      while(true)
         q.get();
   }
}
 
class withnotify
{
   public static void main(String[] args)
   {
      Q q=new Q();
      new Producer(q);
      new Consumer(q);
      System.exit(1);
   }
}
       
 
   

class Q
{
   int n;
   boolean valueSet=false;
 
   synchronized void put(int i)
   {
      if(valueSet)
      {
   
         try
         {
            System.out.println(" Entry of Put :"+i); 
 
            wait();
         }
         catch(InterruptedException ieo)
         {
            System.out.println("\n Interrupt Caught ");
         }
      }
 
      this.n=i; 
      valueSet=true;
      System.out.println(" Put : "+n);
      notify();
      System.out.println(" What will be the next line");
   }
 
   synchronized int get()
   {
      if(!valueSet)
      {
         try
         {
            System.out.println("Entry of get:"+n);
            wait();
            System.out.println(" How many times"); 
         }
         catch(InterruptedException ieo)
         {
            System.out.println("\n Interrupt Caught ");
         }
      }
 
      System.out.println(" Got : "+n);
      valueSet=false;
      notify();
      return n;
   }
 
}
 
class Producer implements Runnable
{
   Q q;
  
   Producer(Q obj)
   {
      q=obj;
      new Thread(this,"Producer").start();
      System.out.println("Producer hai ");
   }
  
   public void run()
   {
      int i=0;
 
      while(true)
         q.put(i++);
   }
}
 
class Consumer implements Runnable
{
   Q q;
  
   Consumer(Q obj)
   {
      q=obj;
      new Thread(this,"Consumer").start();
      System.out.println("Consumer hai ");
   }
  
   public void run()
   {
      while(true)
         q.get();
   }
}
 
class withnotify
{
   public static void main(String[] args)
   {
      Q q=new Q();
      new Producer(q);
      new Consumer(q);
 
      System.exit(1);
   }
}
    

Output
Producer hai
Put : 0
What will be the next line
Entry of Put :1 Why and how this line gets printed
Consumer hai
Got : 0
Entry in get:0
Put : 1
What will be the next line
How many times
Got : 1
Entry in get:1
Put : 2
What will be the next line
How many times
Got : 2
Entry in get:2
Put : 3
What will be the next line
How many times
Got : 3
Entry in get:3
Put : 4
What will be the next line
How many times
Got : 4
Entry in get:4
Put : 5
What will be the next line
Entry of Put :6
How many times
Got : 5
Entry in get:5
Put : 6
What will be the next line
How many times
Got : 6
Entry in get:6
Put : 7
What will be the next line
How many times
Got : 7
Entry in get:7


[HENRY: Fixed Indentation in Code]
[ April 19, 2008: Message edited by: Henry Wong ]

[Henry Wong]

My second question is what happens when a notify() is encountered before a wait() statement as in the program below ??

If a notify() is called, when there are no threads waiting on the monitor, the notification is lost.

Henry

[Shivit Agarwal]

@Henry
Thanks mate. But can yoyu tell me the use of notify(). In the above code what notify is doing I cannot see any purpose behind above ??

[Keith Flo]

Shivit,

without trying to debug the above example ... here is how notify() works.

Notify() is a method of class object that informs one thread and one thread only (notifyAll() informs all waiting threads) that it can stop waiting and continue with program execution.

For example: thread A acquires the lock [aka 'monitor'] on object Foo which is running in thread B. Thread A calls the wait method() of the Foo instance and releases its lock. This tells Foo instance .. I'm gonna wait until you notify me that you are finished whatever it it is you are doing. Foo instance finishes and calls notify(). Thread A goes back to runnable, reaquires the lock on foo instance and will complete.

Both wait and notify must be called from a synchronized method (or block). Wait throws an InterruptedException.

hope this helps.