Threads

[Dishi Jain]

 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
   }
}
 

What will be account number field's maximum value?

[Sandra Bachan]

Dishi Jain wrote:

 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
   }
}
 

What will be account number field's maximum value?

Please tell us what output you get when running the code. Is this output different than what you think the output should be?

[Dishi Jain]

Sometimes the loop ends with account number value = 2000 but sometimes it ends with value less than 2000 for eg. 1999.
According to me every time the last running thread should stop with account number value = 2000 but no less.

[Henry Wong]

Dishi Jain wrote: Sometimes the loop ends with account number value = 2000 but sometimes it ends with value less than 2000 for eg. 1999.
According to me every time the last running thread should stop with account number value = 2000 but no less.

The place to focus on here is this line...

account.setNumber(account.getNumber() + 1); 

Basically, the getNumber() method is synchronized, and hence, the call to the method is atomic; the setNumber() method is synchronized, and hence, the call to the method is atomic.

However, there is no synchronization that spans both calls simultaneously, and hence, the complete expression is not guaranteed to be atomic. It is possible for one thread to read a number (say number 42), increment it, but before it can set the new number (43), the other thread reads the same number 42.

Henry

[Dishi Jain]

Henry Wong wrote:

Dishi Jain wrote: Sometimes the loop ends with account number value = 2000 but sometimes it ends with value less than 2000 for eg. 1999.
According to me every time the last running thread should stop with account number value = 2000 but no less.

The place to focus on here is this line...

account.setNumber(account.getNumber() + 1); 

Basically, the getNumber() method is synchronized, and hence, the call to the method is atomic; the setNumber() method is synchronized, and hence, the call to the method is atomic.

However, there is no synchronization that spans both calls simultaneously, and hence, the complete expression is not guaranteed to be atomic. It is possible for one thread to read a number (say number 42), increment it, but before it can set the new number (43), the other thread reads the same number 42.

Henry

Thanks Henry,

I get it that why the value might be less than 2000
But now when the previous thread will come back, will it set the account number value which it gets last time..??
And everything what the other thread has done will be lost?

[Henry Wong]

Dishi Jain wrote:
But now when the previous thread will come back, will it set the account number value which it gets last time..??
And everything what the other thread has done will be lost?

Not sure what you are asking? Are you asking if there are cases where it might be more than 2000?

Henry

[Dishi Jain]

Henry Wong wrote:

Dishi Jain wrote:
But now when the previous thread will come back, will it set the account number value which it gets last time..??
And everything what the other thread has done will be lost?

Not sure what you are asking? Are you asking if there are cases where it might be more than 2000?

Henry

No. I mean if :

1. Thread_0 upon getNumber() retrieves 42 and then leaves the lock.
2. Then Thread_1 comes which also retrieves 42, and sets it 43 and runs for next 7 times increasing number value upto let say 50 and now leaves the lock.
3. Now Thread_0 is back.

In above scenario will the number field will again be set as 43 by Thread_0, since what it read lastly was 42, making the other thread losing its changes. ??

[Henry Wong]

Dishi Jain wrote:
In above scenario will the number field will again be set as 43 by Thread_0, since what it read lastly was 42

yes

[Dishi Jain]

Henry Wong wrote:

Dishi Jain wrote:
In above scenario will the number field will again be set as 43 by Thread_0, since what it read lastly was 42

yes

Thanks Henry