Banking System (Console)

About this project

A console banking system: open accounts, deposit, withdraw, transfer, print a statement, credit interest and audit the books. Plain Java — `javac *.java` and `java Bank`, with no build tool, no library and no database.

The idea it is built around is that money is never a double. 0.1 + 0.2 is not 0.3 in Java any more than anywhere else, and menu option 9 computes the whole argument live: ten deposits of ten paise leaving 0.9999999999999999, a customer refused their last ten paise because the balance is really 0.09999999999999998, and four standard-library ways of rounding one charge of Rs 1.005 that do not agree with each other. Every figure on that screen is computed as the program runs, so none of it can quietly go stale.

The second theme is encapsulation as a safety property. The balance is private and changes only through methods that enforce the rules, so no code path can set an account negative. A transfer either completes on both sides or happens on neither, and the audit checks every balance against its own ledger — the property a bank would actually care about.

What you will learn

• Use BigDecimal with an explicit scale and rounding mode for money, and know why double fails
• Treat encapsulation as a safety property rather than ceremony
• Keep a transfer atomic, so it can never debit one account without crediting the other
• Reconcile a balance against its own ledger, which is what an audit actually is
• Model refusals as exceptions carrying enough detail to explain themselves

Before you start

• Java basics — classes, methods and exceptions
• JDK 17 or newer. No build tool, no library, no database.

New to Java? The free Java course covers everything this project assumes.

Features

• BigDecimal money with a fixed scale and rounding policy in one place
• A live demonstration of double precision loss, computed at runtime
• Atomic transfers, with the bank total printed before and after
• An audit that reconciles every balance against its own transaction ledger
• Refusals that explain themselves — overdrawing tells you how short you are
• Indian digit grouping and ASCII-only output, so a Windows console renders it correctly

Technology stack

Track: Java · Level: Starter

What is included

• Money.java, Transaction.java, Account.java and Bank.java — commented throughout
• A README on BigDecimal, encapsulation and transfer atomicity
• Suggested extensions, including replaying the ledger from a file and a test class

Delivered as a 31 KB ZIP. SHA-256 published under Changelog so you can verify it.

System requirements

Operating system

Any

Runtime

JDK 17 or newer. javac and java only — no Maven, no Gradle, no library.

Licence

Frequently asked

Changelog