Niko's mock exam question

[jayalakshmi charugundla]

Hi all,

I have a doubt in this program.It is compiling successfully. According to the below statement the answer is false. I think it should be true.

Will you explain me ?

Just before the main method exits, the account’s number field is guaranteed to have value 2000.

    

class Account {
    private Integer number = 0;
    public synchronized void setNumber(Integer number) {
        this.number = number;
    }
    public synchronized Integer getNumber() {
        return number;
    }
}

class Client extends Thread {
    Account account;
    public Client(Account account) {
        this.account = account;
    }
    public void run() {
        for (int i = 1; i <= 1000; i++) {
            account.setNumber(account.getNumber() + 1);

        }
    }
}

public class Run {
    public static void main(String[] args) throws Exception {
        Account account = new Account();
        Client one = new Client(account);
        Client two = new Client(account);
        one.start();
        two.start();
        one.join();
        two.join();
        // here
    }
}

[Ankit Garg]

Did you try to run the program?? The output will clear your doubt I guess. Also try this modified version of the program

    

class Account {
    private Integer number = 0;
    public synchronized void setNumber(Integer number) {
        this.number = number;
    }
    public synchronized Integer getNumber() {
        return number;
    }
}

class Client extends Thread {
    Account account;
    public Client(Account account) {
        this.account = account;
    }
    public void run() {
	synchronized(account) {
          for (int i = 1; i <= 1000; i++) {
            account.setNumber(account.getNumber() + 1);

          }
	}
    }
}

public class Run {
    public static void main(String[] args) throws Exception {
        Account account = new Account();
        Client one = new Client(account);
        Client two = new Client(account);
        one.start();
        two.start();
        one.join();
        two.join();
        System.out.println(account.getNumber());
    }
}

[jayalakshmi charugundla]

Ankit I ran the program what you have given there. But now it is displaying something like this,

run:
Exception in thread "main" java.lang.VerifyError: (class: Account, method: <init> signature: ()V) Constructor must call super() or this()
at Job.main(Job.java:38)
Java Result: 1
BUILD SUCCESSFUL (total time: 0 seconds)

I m sorry I couldnt find out any difference btw the two programs. How is it possible?



thanks in advance,

jaya

[Siva Masilamani]

OK here is the correct answer.

Since because get and set are not atomic the value is not gauranteed.

E.g Lets consider number of iteration is 2.

First Iteration :

Thread 1 executes the getNumber and its value is 0 but before setNumber is executed for this thread,Thread 2 gets the lock and executes the getNumber and retrieve its value as 0 again and also execute setNumber and set the value as 1. Note:Thread 1 did not execute setNumber yet.

Now again Thread 1 gets the lock and execute the setNumber method ,But the value retrieved by this thread was 0 (when its getNumber was executed) so it again update the value to 1.

Iteration 2:

Thread 1 executes getNumber and get the value as 1 and then execute the setNumber method and set the value as 2.

Thread 2 execure the getNumber and get the value as 2 and then execute the setNumber and set the value as 3.

So value 4 is never reached in this scenario.


Correct me if i am wrong?

[Siva Masilamani]

Let me know if oyu have any question.

[Raghavan Muthu]

jayalakshmi charugundla wrote:Ankit I ran the program what you have given there. But now it is displaying something like this,

run:
Exception in thread "main" java.lang.VerifyError: (class: Account, method: <init> signature: ()V) Constructor must call super() or this()
at Job.main(Job.java:38)

I did NOT get any errors. It ran successfully by showing the result as '2000'.

[Raghavan Muthu]

jayalakshmi charugundla wrote:
I m sorry I couldnt find out any difference btw the two programs. How is it possible?

Please look at the code carefully. The modified version does 'synchronize' on the 'account' object before getting into the loop!

This is one such incident to show how we generally overlook and give a different or possibly wrong answer in the exams!

[Siva Masilamani]

OK here is the correct answer.

Since because get and set are not atomic the value is not gauranteed.

E.g Lets consider number of iteration is 2.

First Iteration :

Thread 1 executes the getNumber and its value is 0 but before setNumber is executed for this thread,Thread 2 gets the lock and executes the getNumber and retrieve its value as 0 again and also execute setNumber and set the value as 1. Note:Thread 1 did not execute setNumber yet.

Now again Thread 1 gets the lock and execute the setNumber method ,But the value retrieved by this thread was 0 (when its getNumber was executed) so it again update the value to 1.

Iteration 2:

Thread 1 executes getNumber and get the value as 1 and then execute the setNumber method and set the value as 2.

Thread 2 execure the getNumber and get the value as 2 and then execute the setNumber and set the value as 3.

So value 4 is never reached in this scenario.


Correct me if i am wrong?

[jayalakshmi charugundla]

Hi,

I ran the program again n now it is showing the correct output. I have replaced the value 2 where actually it was in the for loop . But it is displaying 4 as the
output.

As I know, join() stops the current loop iteration(ie. thread1) and will be joined to the thread2 after finishing its (thread2) work. That means first thread 2 finished its work. Somewhere I have seen second thread1 finisned its job before the first one.


Am I right?

Siva I didnt understand the program.

If you dont mind could you please explain it easily about join() and this program.?

[Siva Masilamani]

Did you look at my recent post???


I will edit my old post

I explained it clearly in the recent post..If you still have doubt i could explain it again.

[Siva Masilamani]

jayalakshmi charugundla wrote:Hi,

I ran the program again n now it is showing the correct output. I have replaced the value 2 where actually it was in the for loop . But it is displaying 4 as the
output.

As I know, join() stops the current loop iteration(ie. thread1) and will be joined to the thread2 after finishing its (thread2) work. That means first thread 2 finished its work. Somewhere I have seen second thread1 finisned its job before the first one.


Am I right?

Siva I didnt understand the program.



If you dont mind could you please explain it easily about join() and this program.?

No you are wrong about join method.

Join method will cause the currently running thread to join at the completion of the thread reference by it.

In the above program join is called from main thread.So in this case main thread will wait until thread one completes and it has nothing to do with thread two.

currently running thread corresponds to where the join call is present,if it is in main then it is the main thread that will execute the join call and hence main thread will wait if call from another method lets sat getNumber in this case then the currently running thread could be one or two.


My example program

 class Account {

    private int number = 0;

    public synchronized void setNumber(Integer number) {
        System.out.println("SetNumber by Thread "+Thread.currentThread().getId()+" and no is "+number);
        this.number = number;
    }

    public synchronized Integer getNumber() {
        System.out.println("getNumber by Thread "+Thread.currentThread().getId()+"and no is "+number);
        return number;
    }
}

class Client extends Thread {

    Account account;

    public Client(Account account) {
        this.account = account;
    }

    public void run() {
        for (int i = 1; i <=2; i++) {
            System.out.println("Count is "+i+" for thread "+Thread.currentThread().getId());
            account.setNumber(account.getNumber() + 1);
        }
        System.out.println("Thread " + Thread.currentThread().getId() + " is completed");
    }
}

public class Run {

    public static void main(String[] args) throws Exception {
        Account account = new Account();
        Client one = new Client(account);
        Client two = new Client(account);
        one.start();
        two.start();
        System.out.println("Main is Waiting for One");
        one.join();
        System.out.println("Main is Waiting for Two");
        two.join();
        System.out.println("Main is Completed");
        System.out.println(account.getNumber());
        // here
    }
} 

Output:

Main is Waiting for One
Count is 1 for thread 9
Count is 1 for thread 8
getNumber by Thread 8and no is 0
getNumber by Thread 9and no is 0
SetNumber by Thread 9 and no is 1
Count is 2 for thread 9
getNumber by Thread 9and no is 1
SetNumber by Thread 9 and no is 2
Thread 9 is completed
SetNumber by Thread 8 and no is 1 //look at here thread 8 again set the value to 1 not to 3 because its getNumber return 0 for the 1st iteration
Count is 2 for thread 8
getNumber by Thread 8and no is 1
SetNumber by Thread 8 and no is 2
Thread 8 is completed
Main is Waiting for Two
Main is Completed
getNumber by Thread 1and no is 2
2
   

Hope it helps you.

[jayalakshmi charugundla]

yes, I have seen it. It is showing the output as 4.

I am really unable to understand? Here I am defining other different program and is similar to the previous . The output for the progrm is false.

  

class Account {
    private Integer number = 0;
    public void increase() {
        number++;
    }
    public synchronized Integer getNumber() {
        return number;
    }
}

class Client extends Thread {
    Account account;
    public Client(Account account) {
        this.account = account;
    }
    public void run() {
        for (int i = 1; i <= 1000; i++) {
            account.increase();
        }
    }
}

public class Run {
    public static void main(String[] args) throws Exception {
        Account account = new Account();
        Client one = new Client(account);
        Client two = new Client(account);
        one.start();
        two.start();
        one.join();
        two.join();
        // here
    }
}

  

How ?

If you dont feel pain, you can tell me?

[Siva Masilamani]

Did you see my example program with its output?

That will answer both of your question

let me know if you still have doubt onthis.

[G Venu]

I dont think we can arrive at the same output all the time. the final value may or may not be 2000.

 

 public class Run {  
     public static void main(String[] args) throws Exception {  
         Account account = new Account();  
         Client one = new Client(account);  
         Client two = new Client(account);  
         one.start();  
         two.start();  //after this statement is executed, both thread one and two are competing for CPU and JVM allocates CPU according to the time slicing mechanism that the OS provides.
         one.join();  //this method invocation  pauses main method until one is completed. but at this point thread one and two are still competing for the CPU.
         two.join();  //this point will be reached only after one is completed. but chances are thread two has completed its execution already.
         //only thing that can be guaranteed here is that since account methods are synchronized, the account.getNumber() after two.join() will surely return a number greater than 1000
     }  
 } 

[jayalakshmi charugundla]

Thank you all