JavaBank

The case study

Problem

Handling money with no rounding errors and rules that cannot be bypassed, demonstrating clean architecture with no frameworks, database or libraries.

Solution

Only Java 21 and its standard library, with dependencies in a single direction; money always in BigDecimal (a Money value object) and a balance that only changes through a method which logs the transaction in the same step.

Outcome

Runnable via a run.sh that compiles with javac, no install needed; code written to read like production. Especially relevant for fintech.

Process

Development process

The reasoning behind the technical decisions: why each technology was chosen, what challenges came up and how they were solved.

1. 01

   Zero dependencies, on purpose

   I built the bank with only Java 21 and its standard library —no frameworks, no database. The constraint is deliberate: it forces the clean architecture to stand on its own, with dependencies pointing in a single direction, and demonstrates the fundamentals without a library doing the hard part.

2. 02

   Money, modeled correctly

   Money is never represented with float: always BigDecimal, wrapped in a Money value object that centralizes rounding. It is the most important decision in the project, because a cent-level error in a bank is not a detail.

3. 03

   Invariants you cannot bypass

   The balance only changes through a method that logs the transaction in the same step: it is impossible to move money without leaving a trace. Modeling the rule inside the object —rather than in the code that uses it— is what makes this exercise especially relevant for fintech.

Repositories

Commits

1. 
   Cambios en el Readme

   ceb5e4c·Eduardo·3 weeks ago

2. 
   Initial commit

   307508c·Eduardo·3 weeks ago