Bank Transaction System

Course: Advanced Programming — Assignment I
Topic: ADT Design — Inheritance, Polymorphism & Exception Handling
Language: Java (JDK 11+)

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Overview

A minimal bank-transaction system that demonstrates core OOP principles:

Concept	Where it appears
Interface implementation	TransactionInterface → BaseTransaction
Concrete vs Abstract class	BaseTransaction (concrete) vs a possible abstract layer
Polymorphism / method overriding	apply() in Deposit and Withdrawal subclasses
Early vs Late binding	Main.java — Q4 upcasting demonstration
Custom checked exception	InsufficientFundsException extends Exception
throws declaration	WithdrawalTransaction.apply(BankAccount)
try…catch…finally	WithdrawalTransaction.apply(BankAccount, boolean)
Reversible transactions	WithdrawalTransaction.reverse()

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Project Structure

BankTransactionSystem/
├── README.md
├── .gitignore
└── src/
    └── com/
        └── transactions/
            ├── interfaces/
            │   └── TransactionInterface.java      # Root contract
            ├── models/
            │   └── BankAccount.java               # Account value object
            ├── exceptions/
            │   └── InsufficientFundsException.java # Checked exception
            ├── transactions/
            │   ├── BaseTransaction.java            # Q1 — concrete base
            │   ├── DepositTransaction.java         # Q2 — irreversible credit
            │   └── WithdrawalTransaction.java      # Q2/Q3 — reversible debit
            └── Main.java                           # Q4 — client / driver code

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Class Diagram

«interface»
TransactionInterface
  + getAmount(): double
  + getDate(): Calendar
  + getTransactionID(): String
  + printTransactionDetails(): void
  + apply(BankAccount): void throws InsufficientFundsException
         ▲
         │ implements
         │
  BaseTransaction  (concrete)
  - amount: double
  - date: Calendar
  - transactionID: String
  + apply(BankAccount)   ← no-op / logs only
         ▲
         │ extends
         ├──────────────────────────────────────┐
         │                                      │
  DepositTransaction                  WithdrawalTransaction
  + apply(BankAccount)                + apply(BankAccount) throws InsufficientFundsException
    ↳ credits account                 + apply(BankAccount, boolean)  ← overloaded
    ↳ irreversible                      ↳ try-catch-finally, partial withdrawal
                                      + reverse(): boolean
                                        ↳ restores debited amount

BankAccount
  - accountNumber: String
  - accountHolder: String
  - balance: double
  + credit(double): void
  + debit(double): void

«exception»
InsufficientFundsException extends Exception
  - withdrawalAmount: double
  - availableBalance: double
  + getShortfall(): double

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Running the Project

Command-line (no build tool)

# 1. Compile — from the project root
javac -d out $(find src -name "*.java")

# 2. Run
java -cp out com.transactions.Main

IntelliJ IDEA

1. Open the project root as a new project.
2. Mark src as the Sources Root (Right-click → Mark Directory as → Sources Root).
3. Run Main.java.

VS Code

Install the Extension Pack for Java, open the folder, then click ▶ on Main.java.

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Expected Output (excerpt)

▓▓▓ Q1 | BaseTransaction — accessors & base apply() ▓▓▓
getAmount()        : 250.0
getTransactionID() : 3F9A...
...
[BaseTransaction] apply() called — no mutation performed.

▓▓▓ Q2b | WithdrawalTransaction — reversible debit & reverse() ▓▓▓
╔══ WITHDRAWAL APPLIED ═══╗
║  Balance: 15000 -> 12000 ║
╚═════════════════════════╝
╔══ WITHDRAWAL REVERSED ══╗
║  Restored: KES +3000.00  ║
╚═════════════════════════╝

▓▓▓ Q3a | Exception via 'throws' ▓▓▓
[CAUGHT InsufficientFundsException]
  Message   : Insufficient funds: attempted to withdraw KES 2000.00 ...
  Shortfall : KES 1500.00

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Key Design Decisions

Decision	Rationale
InsufficientFundsException extends Exception	Checked exception — the compiler forces every caller to acknowledge the risk of a failed withdrawal at compile time, which is the correct choice for a recoverable business-rule violation.
apply() declared with throws in the interface	Allows WithdrawalTransaction to propagate the checked exception while DepositTransaction silently narrows the throws clause (valid in Java overriding rules).
BaseTransaction is concrete, not abstract	Required by the assignment to enable direct instantiation for Q4 type-casting demonstration.
reverse() credits amountDebited, not getAmount()	Correctly handles the partial-withdrawal case where only a fraction of the requested amount was actually debited.

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Author

Advanced Programming — Assignment I
October 2024, Semester: Sept – Dec 2024