Assume two threads share a BankAccount object with balance of zero (0), and that the BankAccount class provides deposit and withdraw methods and has a ReentrantLock named myLock, as shown below.Note that only the deposit method uses the lock.Thread one deposits $10 ten times and, concurrently, thread two withdraws $10 ten times.Which statement regarding the balance after all thread calls is definitely true?
A)The balance could be zero or positive.
B)The balance is zero.
C)The balance could be zero or negative.
D)The balance is positive.

Question

Assume two threads share a BankAccount object with balance of zero (0), and that the BankAccount class provides deposit and withdraw methods and has a ReentrantLock named myLock, as shown below.Note that only the deposit method uses the lock.Thread one deposits $10 ten times and, concurrently, thread two withdraws $10 ten times.Which statement regarding the balance after all thread calls is definitely true?  
A)The balance could be zero or positive. 
B)The balance is zero. 
C)The balance could be zero or negative. 
D)The balance is positive.

Quizplus · Accepted Answer

Since the two threads are accessing the BankAccount object concurrently, it is possible for thread two to attempt a withdrawal before thread one has finished depositing all ten times. This could result in the balance being negative if the withdrawal occurs before all deposits have been made.

For example, thread one could deposit $10 five times, resulting in a balance of $50. If thread two then attempts a withdrawal, it would withdraw $10, leaving a balance of $40. However, thread one could then deposit another $10, bringing the balance back up to $50. If thread two then attempts another withdrawal, it would again withdraw $10, leaving a balance of $40. This cycle could continue until all ten deposits and withdrawals have been made, resulting in a final balance of zero. However, if thread two makes a withdrawal before thread one has made any deposits, then the balance would immediately become negative.

Assume Two Threads Share a BankAccount Object with Balance of Zero