ATM Simulator

A console-based ATM (Automated Teller Machine) simulator built in Java that demonstrates object-oriented programming principles and clean code practices.

🏧 Features

• Card Selection: Choose from multiple pre-loaded bank cards with interactive menu
• PIN Authentication: Secure 3-attempt PIN verification system with attempt tracking
• Card Expiration Check: Automatic validation of card expiration dates
• Withdrawal Operations:
  • Quick amounts (CHF 10, 20, 50, 100)
  • Custom amount withdrawal with validation
  • Balance verification and overdraft prevention
  • Transaction cancellation option
• Deposit Operations: Add money to your account with amount validation
• Balance Display: Real-time balance updates after transactions
• Error Handling: Comprehensive input validation and graceful error recovery
• Clean UI: Professional bordered console display with consistent formatting
• Session Management: Proper card insertion/ejection and session cleanup

🚀 Getting Started

Prerequisites

• Java 11 or higher
• IDE (IntelliJ IDEA, Eclipse, or VS Code)
• Terminal/Command prompt

Running the Application

1. Clone the repository:

   git clone <repository-url>
   cd atmSimulator
   

2. Compile and run:

   javac -d out src/main/java/ch/javamilesiii/*.java
   java -cp out main.java.ch.javamilesiii.Main
   

   Or run directly from your IDE by executing the Main.java file.

🎮 How to Use

1. Start the application - You'll see the welcome screen
2. Insert a card - Choose from 5 available test cards
3. Enter PIN - Each card has a specific PIN (see Test Data section)
4. Select operation:
   • 1 - Withdraw money
   • 2 - Deposit money
   • 3 - Eject card and exit
5. Follow the prompts for your selected operation

Example Session

######################################################################
#                                                                    #
#    Welcome! Insert your card:                                     #
#                                                                    #
######################################################################
1 - Insert card, 0 - Exit
Choice: 1

######################################################################
#                                                                    #
#    Select your card:                                              #
#                                                                    #
######################################################################
1 - Aubrey Thomas
2 - Layla Roberts
3 - Abigail Peterson
4 - Avery Phillips
5 - Sarah Taylor
Choice: 1

######################################################################
#                                                                    #
#    Enter your PIN:                                                #
#                                                                    #
######################################################################
PIN: 738143

######################################################################
#                                                                    #
#    Hello Aubrey Thomas!                                           #
#    What can I do for you today?                                   #
#                                                                    #
#    Balance: 1000.0 CHF                                            #
#                                                                    #
#                                                                    #
#                                                                    #
#                                                                    #
#                                                                    #
#                                                                    #
#                                                                    #
#                                                                    #
#    1 - Withdraw                                                    #
#    2 - Deposit                                                     #
#    3 - Eject Card                                                  #
#                                                                    #
######################################################################

🧪 Test Data

The application comes with 5 pre-configured cards:

Card Owner	PIN	Initial Balance
Aubrey Thomas	738143	CHF 1000.0
Layla Roberts	612543	CHF 5100.0
Abigail Peterson	867826	CHF 1250.0
Avery Phillips	874256	CHF 250.0
Sarah Taylor	989631	CHF 1000.0

🏗️ Architecture

The project follows SOLID principles and demonstrates clean code practices with improved method extraction and separation of concerns:

UML Class Diagram

Class Structure

src/main/java/ch/javamilesiii/
├── Main.java              # Application entry point and test data setup
├── ATM.java               # Main controller with extracted methods
├── BankAccount.java       # Account data and transaction operations  
├── Card.java              # Card information and validation logic
├── IOHandler.java         # User input handling with error recovery
└── MessageDisplay.java    # Console UI formatting and text wrapping

Design Principles Applied

• Single Responsibility: Each class and method has one clear, focused purpose
• Method Extraction: Large methods broken down into smaller, testable units
• Error Handling: Comprehensive input validation with graceful recovery
• DRY Principle: Common functionality extracted into reusable helper methods
• Encapsulation: Private helper methods with clear public interfaces
• Defensive Programming: Null checks, boundary validation, and safe defaults

Key Architectural Improvements

• ATM Controller Refactoring: Main workflow split into focused methods

  • selectCard() - Handles card selection process
  • authenticateUser() - Manages PIN verification with attempt limits
  • runMainLoop() - Controls main menu and transaction flow
  • handleWithdrawal() / handleDeposit() - Process specific transaction types

• Enhanced MessageDisplay:

  • Text wrapping with multi-line support
  • Consistent bordered layout system
  • Reusable menu layout functionality

• Robust IOHandler: Input validation with retry mechanisms

• Improved BankAccount: Transaction validation and error messaging

Relationship Overview

• Main creates and configures the ATM system with test data
• ATM serves as the central controller, coordinating all operations
• MessageDisplay handles all user interface formatting and display
• IOHandler manages all user input with validation
• Card links to BankAccount and provides secure authentication
• BankAccount manages financial data and transaction processing

🔧 Key Components

ATM Class

• Session Management: Orchestrates complete ATM workflow from card insertion to ejection
• Authentication: Handles secure PIN verification with attempt tracking and lockout
• Transaction Processing: Manages withdrawal and deposit operations with validation
• User Interface: Coordinates with MessageDisplay for consistent user experience
• Error Recovery: Graceful handling of invalid inputs and transaction failures
• Method Organization: Well-structured with single-responsibility methods

BankAccount Class

• Data Management: Stores account information (IBAN, owner, balance) securely
• Transaction Logic: Handles withdrawal and deposit operations with business rules
• Validation: Ensures transaction amounts are valid (positive, within balance)
• Error Messaging: Provides clear feedback on transaction failures
• Balance Protection: Prevents overdrafts and invalid operations

Card Class

• Secure Storage: Contains card details and expiration information safely
• Authentication: Provides PIN validation functionality
• Expiration Checking: Validates card is not expired before use
• Account Linking: Connects card to associated bank account
• Data Integrity: Ensures card information consistency

MessageDisplay Class

• Professional UI: Creates consistent bordered console output with proper spacing
• Text Processing: Handles text wrapping for long messages across multiple lines
• Menu Systems: Provides reusable menu layouts for different screens
• Dynamic Content: Adapts display based on content length and requirements
• Layout Management: Maintains consistent formatting across all user interactions

IOHandler Class

• Input Management: Handles all user input with comprehensive error checking
• Type Safety: Validates numeric inputs with automatic retry on errors
• Buffer Management: Properly cleans input buffers to prevent carry-over issues
• User Experience: Provides clear prompts and error messages for invalid input
• Recovery Mechanisms: Allows users to retry after input errors

🛡️ Security & Error Handling

Input Security:

• All user inputs are sanitized and validated before processing
• Buffer clearing prevents input carry-over between operations
• Type checking ensures only valid data types are accepted
• Range validation for monetary amounts and menu choices

Authentication Security:

• Maximum 3 PIN attempts with account lockout
• PIN validation without storing plaintext comparisons
• Session timeout and automatic card ejection on failures
• Card expiration date verification before transactions

Transaction Safety:

• Overdraft protection with balance verification
• Positive amount validation for all transactions
• Atomic transaction processing (all-or-nothing approach)
• Clear success/failure messaging with balance updates

Error Recovery:

• Graceful handling of invalid inputs with retry prompts
• Comprehensive exception catching with user-friendly messages
• Automatic cleanup on session termination or errors
• Consistent error messaging across all operations

System Robustness:

• Null pointer protection throughout the application
• Boundary condition checking for all numeric inputs
• Defensive programming practices in all critical operations
• Comprehensive logging of errors for debugging purposes

🔜 Potential Enhancements

• Transaction history logging
• Different account types (savings, checking)
• Transfer between accounts
• Receipt generation
• Admin mode for card management
• Configurable withdrawal limits
• Multi-language support

📝 Technical Details

• Java Version: 11+
• Architecture: Object-oriented with clear separation of concerns
• UI: Console-based with formatted output
• Data Storage: In-memory (resets on restart)
• Input Handling: Scanner-based with comprehensive error recovery

📄 License

This project is licensed under the Apache License 2.0 - see the LICENCE file for details.

🎓 Learning Objectives

This project demonstrates:

Core Programming Concepts:

• Object-oriented programming principles and encapsulation
• Method extraction and code organization techniques
• Error handling and defensive programming practices
• Input validation and user experience design

Software Design Principles:

• SOLID principles applied in practice
• Single Responsibility Principle through focused classes and methods
• DRY (Don't Repeat Yourself) principle with reusable components
• Separation of concerns in software architecture
• Clean code practices and readable method naming

Advanced Techniques:

• Session state management and lifecycle control
• Robust error recovery and graceful degradation
• Text processing and dynamic UI formatting
• Security considerations (PIN attempts, validation)
• Comprehensive testing scenarios with realistic data

Professional Development:

• Project documentation and README creation
• Code refactoring and continuous improvement
• Git version control and project organization
• UML diagrams and technical documentation

👨‍💻 Author

Created as a learning project to demonstrate Java programming skills and software design principles.

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Note: This is a simulation for educational purposes. No real financial transactions are performed.