Threads: synchronized method (K&B 1.5, page 733)

[Mea Cook]

Hello all,

I have some problems with SELF TEST, exercise 1 (K&B, Java 1.5, Chapter 9, page 733)

According to the book the answer is Wallace-1 Gromit-1 Gromit-2 Wallace-2

I compiled the code and got another result. Wallace-1 Wallace-2 Gromit-1 Gromit-2

public class Messager implements Runnable{
    
    private String name;
    
    public void run(){
        message(1);
        message(2);
    }
    
    public Messager(String param1){
       this.name = param1;
    }
       
    private synchronized void message(int param1) {
        System.out.println(name + "-" + param1 + " ");
    } 
    
    public static void main(String [] args) {
        new Thread(new Messager("Wallace")).start();
        new Thread(new Messager("Gromit")).start();
    }
   
}

Thank you all in advance

[Ankit Garg]

Is this a self test question (basically did you create the program yourself or was it given in the book)?? In my K&B for SCJP6 book, there's no question like this. I would say that the output is unpredictable. It can be the output given in the book, or what you got or a few other possible outputs...

[Ralph Jaus]

K&B (Java 5) doesn't say the output is "Wallace-1 Gromit-1 Gromit-2 Wallace-2". Instead they ask "Which of the follwing is a possible result ?" and give you 7 options. Of these "Wallace-1 Gromit-1 Gromit-2 Wallace-2" is the only one possible.

Of course there are other possible outputs, as Ankit already mentioned. But these are not part of the options given in the question.

Hope this helps.

[Abimaran Kugathasan]

We can't predict the answer, can be any combination. But the values should be there!

[KrishnaPrasad raghavan]

Guys, the synchronized metod synchronize the thread execution. I am also tempted to say the result is unpredictable, but for the synchronized keyword.
I did try running the code and I got the following results

First try
-----------
Wallace-1
Wallace-2
Gromit-1
Gromit-2

Second Try
------------
Gromit-1
Gromit-2
Wallace-1
Wallace-2

Third Try
----------
Gromit-1
Gromit-2
Wallace-1
Wallace-2

The point is it really depends which thread is running first. So the answer is unpredicatable.

[Weiheng Zhang]

        new Thread(new Messager("Wallace")).start();   
        new Thread(new Messager("Gromit")).start();   

There are two instances of Thread and two Messengers: (The thread objects overide their run() with that of Messenger)
Synchronization applies to individual instance, inside each instance, execution comes by the order of 1, 2. It is guaranteed. But JVM doesn't guarantee which code segment of specific thread come first and second and so forth. Say, it may execute like:

Thread A, Thread B

-> A, 1st statment
-> put A in a runnable status
-> B, 1st statement
-> B, 2nd statement
-> put B in a runnalbe status
-> A, 2nd statement
-> B, 3rd statement
.....

Therefore, many results of combination. But inside each thread, statements run through by the order of source code.
In this case, whatever combination, Wallace-2 won't come before Wallace-1, and Gromit-1 must come before Gromit-2.

[Mea Cook]

Really speaking I don't understand te role of synchronized in this context. Can anybordy help?

private synchronized void message(int i) {
...
}

this is the same as

private  void message(int i) {
synchronized(this)
...
}

What object will be locked: from class Thread or Messager? And how long?
I thought, that if 1 Thread is locked, then runs to the end the 2 Thread and then the 1 Thread runs to the end.
But the answer shows, it is not correct...

Thank you

[Abimaran Kugathasan]

Lock is for Objects. In your case it's for Messager object.

The period is up to the end of your run() method or block.

We can't predict which thread will get the first chance to be executed, but if one thread got a chance, the it'll finish the job, then only the other thread will have a chance to be executed.

[Bert Bates]

Hi Mea,

Welcome to the ranch!

You tackled an interesting question here, and the only 'Javaranch-ish' recommendation I have is that you should show ranchers not only the code from the mock question you're studying, but also the entire question - in this case (and in many cases), the parts of the question outside of the code are also essential to a valid discussion.

Thanks,

Bert

[Mea Cook]

Thank you Bert! I am only few days on ranch, but I like it very much.

I'll do my best and next time write the full questions not only code.

This question is resolved for me and I close it