Why do we need to synchronize deposit method?

Question about the task

Under discussion

I understand that withdraw method will be used by more than one thread so it's compulsory to use synchronize on it. Why do we have to synchronize the deposit method while it will be used by only one thread (addMoney thread)?

Figure out how the program works.

During testing, the log was found to contain the following:
.....
Deposit 100, account balance 1100
Deposit 100, account balance 1200
Spend 1000, account balance 100
Insufficient money
.....

There's a bug:
Money is lost when debiting an account.

Find and fix the bug.

Requirements:

The BankAccount class must contain a private BigDecimal field called balance.
The BankAccount class's deposit(String money) method must delegate behavior to the deposit(BigDecimal money) method.
The BankAccount class's withdraw(String money) method must delegate behavior to the withdraw(BigDecimal money) method.
Use synchronized in the withdraw(BigDecimal money) method, if necessary.
Use synchronized in the deposit(BigDecimal money) method, if necessary.

package com.codegym.task.task17.task1720; public class ATM { static BankAccount account = new BankAccount("Amigo"); public static volatile boolean isStopped; public static void main(String[] args) throws InterruptedException { addMoney.start(); SpendThread spendThread = new SpendThread(); SpendThread spendThread1 = new SpendThread(); SpendThread spendThread2 = new SpendThread(); spendThread.start(); spendThread1.start(); spendThread2.start(); Thread.sleep(4000); isStopped = true; } private static Thread addMoney = new Thread() { @Override public void run() { while (!isStopped) { account.deposit("1000"); // Make a deposit try { Thread.sleep(1000); } catch (InterruptedException e) { break; } } } }; public static class SpendThread extends Thread { @Override public void run() { while (!isStopped) { try { account.withdraw("100"); // Make a withdrawal } catch (NotEnoughMoneyException e) { System.out.println("Insufficient money"); } try { Thread.sleep(100); } catch (InterruptedException e) { break; } } } } }

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Guadalupe Gagnon

Level 37 , Tampa, United States

18 December 2020, 17:35

useful

Because it is accessing line 6 of the BankAccount class, the balance variable. All 4 threads have unfettered access to that variable and, without synchronization, would be interfering with each other. When a thread enters a synchronized method this means that all of the fields that are part of that object become synchronized to that thread. It's important to remember that the method is not "synchronized", it is the object's fields that are the actual synchronized part, any thread in any method that is synchronized would have per turn access to the fields. For example if three threads were in the withdraw method and 1 thread were in deposit, they would all process in order 1 at a time to use the balance variable. The way that the deposit and withdraw methods work in this code is that they first grab what the balance is in a temp variable, make the necessary changes to that temp variable, then overwrite the old balance with the temp amount. By not having the deposit method synchronized, that means that any thread that enters that method (the one thread that you notice does) can access the balance variable in between the read/writes from the threads that are using that variable in the withdraw method.

Isma

Level 41 , Madrid, Spain

18 December 2020, 18:02

Great answer! It's not about the method itself but the variable that is modified by that method. Thank you!