Bank Account

Instructions

Simulate a bank account supporting opening/closing, withdrawals, and deposits of money. Watch out for concurrent transactions!

A bank account can be accessed in multiple ways. Clients can make deposits and withdrawals using the internet, mobile phones, etc. Shops can charge against the account.

Create an account that can be accessed from multiple threads/processes (terminology depends on your programming language).

It should be possible to close an account; operations against a closed account must fail.

This exercise introduces concurrency. To pass the last test you might find the synchronized keyword or locks useful.

Problems arising from running code concurrently are often intermittent because they depend on the order the code is executed. Therefore the last test runs many threads several times to increase the chances of catching a bug. That means this test should fail if your implementation is not thread safe, but there is a chance it will pass just because there was no concurrent modification attempt. It is unlikely that this will occur several times in a row since the order the code is executed should vary every time you run the test. So if you run the last test a couple of times and it passes every time then you can be reasonably sure that your implementation is correct.