Make sure 2 threads share the same object

Question

I have been thinking about this for quite some time. However, I really couldn't figure it out due lack of experience and knowledge.
I just needed to know how can I make sure that 2 or more threads (after certain checkings that they should share a same common object) will really share the same object.
For example (to UlrikaJ if you are reading this):
How can I achieve the following?? Thanks!
"When James and Ada separately want to access their shared account,two transactions are started that accesses the object associated with that account number."

Answer 1

Thanks for the answer. You are welcome :-)
public class Account
   public Account getAccount(String userInput)
     // Some processes to verify the userInput go here.
     return account;
}//End of Account class
public UserThread implements Runnable
   private String name;
   private Semaphore s;
   private Account userAccount; //Reference for the Account class
   public UserThread(String name, Semaphore s)
      this.name = name;
      this.s = s;
   }//End of constructor
   public void run()
      for(;;)
          getUserInput(); //Ofcourse the implementation will depend on what you are doing
          s.lock(); //obtain a lock on Semaphore object
          //Entering critical section of the code
          this.userAccount = accountObject.getAccount(userInput);
          //End of critical section of the code.
          s.signal(); //Release the lock so that other thread
          //can access the getAccount() method...irrespective of the
          //variable or the object being used or referred to.
      }//End of for() loop
   }//End of run() method
} //End of UserThread class
public class AccountApplication
   public static void main(String args[])
      Semaphore sem = new Semaphore(0);
      //Always initialize the Semaphore with 0 when you create it.
      UserThread user1 = new UserThread("User Number 1",sem);
      UserThread user2 = new UserThread("User Number 2",sem);
      //All threads share the same Semaphore object
      Thread userThread1 = new Thread(user1);
      Thread userThread2 = new Thread(user2);
      userThread1.start();
      userThread2.start();
   }//End of main() method
}//End of classWell, I have given the outline of the code. I have not compiled or run the program so it might have errors in its syntax.
Obviously, you will definitely have to change it to suit your requirements. I have just shown you how to protect yur codes' critical section.
Hope this clears your doubts.
Vijay :-)