I applied for a Senior Backend Developer role in Kraków, Poland. I received a message from the recruiter, and then a phone interview was scheduled. During the interview, the recruiter described the company, asked about my experience and expected salary, and verified my English speaking skills. The interview was conducted in Polish, as it is my mother tongue and the offer was for a position in Poland.

Following the interview, I received a coding task. I was required to write an application in Java or Scala with a REST API for money transfers between bank accounts. The application needed to be simple, with high-quality code, and its correct behavior verified by tests. Data was to be stored in memory, and no authorization mechanisms were required for this task. I also could not use the Spring framework. The final application had to be executable as a single *.jar file.

There wasn't a strict deadline, but candidates usually complete the task within 7 days. I completed it in 4 days, fulfilling all explicit and implicit requirements. They rejected my application because I did not handle concurrent transactions in my app.

Design and write an app in Java or Scala used for money transfer between bank accounts with a REST API.