Implement comprehensive REST API backend for Java Fundamentals applications

.gitignore

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 .vscode/
 
 ### Mac OS ###
-.DS_Store
+.DS_Store
+
+### Java compiled classes ###
+*.class
+
+### Maven ###
+target/
+pom.xml.tag
+pom.xml.releaseBackup
+pom.xml.versionsBackup
+pom.xml.next
+release.properties
+dependency-reduced-pom.xml
+buildNumber.properties
+.mvn/timing.properties
+.mvn/wrapper/maven-wrapper.jar

README.md

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+# Java Fundamentals Backend API
+
+A comprehensive REST API backend for Java fundamental programming applications built with Spring Boot.
+
+## 🚀 Features
+
+- **6 Java Application Services** - Population projection, financial calculations, loan computations, lottery games, geometric calculations, and ticket pricing
+- **REST API Endpoints** - Complete RESTful API with JSON request/response
+- **Database Integration** - H2 in-memory database for execution history storage
+- **Input Validation** - Comprehensive validation for all application inputs
+- **Interactive Frontend** - Web-based demo interface for testing applications
+- **Execution History** - Track and retrieve past application executions
+- **Error Handling** - Robust error handling with meaningful error messages
+
+## 📋 API Endpoints
+
+### Base URL: `http://localhost:8080/api/v1`
+
+| Endpoint | Method | Description |
+|----------|--------|-------------|
+| `/` | GET | API information and endpoint list |
+| `/applications` | GET | Get list of available applications |
+| `/execute` | POST | Execute an application |
+| `/history` | GET | Get execution history |
+| `/result/{id}` | GET | Get specific execution result by ID |
+| `/health` | GET | Health check endpoint |
+
+## 🔧 Available Applications
+
+### 1. Population Projection (`population-projection`)
+Calculate population growth over time based on birth, death, and immigration rates.
+
+**Input Parameters:**
+- `years` (required): Number of years to project (1-50)
+- `currentPopulation` (optional): Starting population (default: 312,032,486)
+
+**Example Request:**
+```json
+{
+  "applicationType": "population-projection",
+  "inputData": {
+    "years": 5,
+    "currentPopulation": 100000000
+  }
+}
+```
+
+### 2. Financial Application (`financial-application`)
+Calculate compound interest with monthly deposits.
+
+**Input Parameters:**
+- `months` (required): Number of months (1-600)
+- `monthlyDeposit` (optional): Monthly deposit amount (default: $100)
+- `annualInterestRate` (optional): Annual interest rate percentage (default: 3.75%)
+
+**Example Request:**
+```json
+{
+  "applicationType": "financial-application",
+  "inputData": {
+    "months": 12,
+    "monthlyDeposit": 150,
+    "annualInterestRate": 4.0
+  }
+}
+```
+
+### 3. Ticket Price (`ticket-price`)
+Calculate ticket price based on age.
+
+**Input Parameters:**
+- `age` (required): Age in years (0-150)
+
+**Example Request:**
+```json
+{
+  "applicationType": "ticket-price",
+  "inputData": {
+    "age": 25
+  }
+}
+```
+
+### 4. Compute Loan (`compute-loan`)
+Calculate loan payment details.
+
+**Input Parameters:**
+- `annualInterestRate` (required): Annual interest rate percentage (0-100)
+- `numberOfYears` (required): Loan term in years (1-50)
+- `loanAmount` (required): Principal loan amount
+
+**Example Request:**
+```json
+{
+  "applicationType": "compute-loan",
+  "inputData": {
+    "annualInterestRate": 5.5,
+    "numberOfYears": 30,
+    "loanAmount": 250000
+  }
+}
+```
+
+### 5. Lottery (`lottery`)
+Play a two-digit lottery game.
+
+**Input Parameters:**
+- `guess` (required): Your two-digit guess (0-99)
+
+**Example Request:**
+```json
+{
+  "applicationType": "lottery",
+  "inputData": {
+    "guess": 42
+  }
+}
+```
+
+### 6. Compute Angles (`compute-angles`)
+Calculate triangle angles from three points.
+
+**Input Parameters:**
+- `x1`, `y1` (required): First point coordinates
+- `x2`, `y2` (required): Second point coordinates
+- `x3`, `y3` (required): Third point coordinates
+
+**Example Request:**
+```json
+{
+  "applicationType": "compute-angles",
+  "inputData": {
+    "x1": 0, "y1": 0,
+    "x2": 3, "y2": 0,
+    "x3": 0, "y3": 4
+  }
+}
+```
+
+## 📦 Response Format
+
+All successful executions return a response in this format:
+
+```json
+{
+  "applicationType": "application-name",
+  "inputData": { /* input parameters */ },
+  "outputData": { /* calculation results */ },
+  "timestamp": "2025-08-21T18:06:31.123456789",
+  "status": "SUCCESS",
+  "errorMessage": null,
+  "resultId": 1
+}
+```
+
+Error responses:
+
+```json
+{
+  "applicationType": "application-name",
+  "inputData": null,
+  "outputData": null,
+  "timestamp": "2025-08-21T18:06:31.123456789",
+  "status": "ERROR",
+  "errorMessage": "Detailed error description",
+  "resultId": null
+}
+```
+
+## 🛠️ Technology Stack
+
+- **Spring Boot 3.2.0** - Main framework
+- **Java 17** - Programming language
+- **Spring Data JPA** - Database integration
+- **H2 Database** - In-memory database
+- **Jackson** - JSON serialization
+- **Maven** - Dependency management
+- **JUnit 5** - Testing framework
+
+## 🏃‍♂️ Running the Application
+
+### Prerequisites
+- Java 17 or higher
+- Maven 3.6 or higher
+
+### Steps
+1. **Clone the repository**
+   ```bash
+   git clone <repository-url>
+   cd Ivipulshrivastava
+   ```
+
+2. **Build the application**
+   ```bash
+   mvn clean compile
+   ```
+
+3. **Run tests**
+   ```bash
+   mvn test
+   ```
+
+4. **Start the application**
+   ```bash
+   mvn spring-boot:run
+   ```
+
+5. **Access the application**
+   - API: http://localhost:8080/api/v1/
+   - Frontend Demo: http://localhost:8080/api/
+   - H2 Console: http://localhost:8080/api/h2-console
+
+## 🧪 Testing the API
+
+### Using curl
+
+```bash
+# Get available applications
+curl -X GET http://localhost:8080/api/v1/applications
+
+# Execute population projection
+curl -X POST http://localhost:8080/api/v1/execute \
+  -H "Content-Type: application/json" \
+  -d '{
+    "applicationType": "population-projection",
+    "inputData": {"years": 5}
+  }'
+
+# Get execution history
+curl -X GET http://localhost:8080/api/v1/history
+```
+
+### Using the Web Interface
+
+1. Open http://localhost:8080/api/ in your browser
+2. Select an application from the available cards
+3. Fill in the required parameters
+4. Click "Execute Application"
+5. View results and execution history
+
+## 📁 Project Structure
+
+```
+src/
+├── main/
+│   ├── java/com/ivipulshrivastava/backend/
+│   │   ├── FundamentalsBackendApplication.java
+│   │   ├── controller/
+│   │   │   └── ApplicationController.java
+│   │   ├── service/
+│   │   │   ├── ApplicationService.java
+│   │   │   ├── ApplicationRegistry.java
+│   │   │   ├── ApplicationExecutor.java
+│   │   │   ├── PopulationProjectionExecutor.java
+│   │   │   ├── FinancialApplicationExecutor.java
+│   │   │   ├── TicketPriceExecutor.java
+│   │   │   ├── ComputeLoanExecutor.java
+│   │   │   ├── LotteryExecutor.java
+│   │   │   └── ComputeAnglesExecutor.java
+│   │   ├── model/
+│   │   │   └── CalculationResult.java
+│   │   ├── repository/
+│   │   │   └── CalculationResultRepository.java
+│   │   ├── dto/
+│   │   │   ├── ApplicationRequest.java
+│   │   │   └── ApplicationResponse.java
+│   │   └── config/
+│   │       └── ApplicationConfig.java
+│   └── resources/
+│       ├── application.properties
+│       └── static/
+│           └── index.html
+└── test/
+    └── java/com/ivipulshrivastava/backend/
+        └── service/
+            └── PopulationProjectionExecutorTest.java
+```
+
+## 🔍 Database Schema
+
+### calculation_results table
+- `id` (BIGINT) - Primary key
+- `application_type` (VARCHAR) - Type of application executed
+- `input_data` (TEXT) - JSON input parameters
+- `output_data` (TEXT) - JSON output results
+- `timestamp` (TIMESTAMP) - Execution timestamp
+- `client_ip` (VARCHAR) - Client IP address
+
+## 🎯 Key Features
+
+### Input Validation
+- Range validation for numerical inputs
+- Required field validation
+- Type validation
+- Custom business rule validation
+
+### Error Handling
+- Comprehensive exception handling
+- Meaningful error messages
+- HTTP status code mapping
+- Logging for debugging
+
+### Extensibility
+- Easy to add new applications
+- Modular executor pattern
+- Configurable through Spring properties
+- Database-agnostic design
+
+## 📈 Monitoring & Logging
+
+- Application logs available in console
+- Database queries logged (configurable)
+- Execution history stored persistently
+- Health check endpoint for monitoring
+
+## 🔮 Future Enhancements
+
+1. **Authentication & Authorization**
+2. **Rate Limiting**
+3. **API Documentation with Swagger**
+4. **Docker Support**
+5. **PostgreSQL/MySQL Support**
+6. **Caching Layer**
+7. **Metrics & Monitoring**
+8. **API Versioning**
+
+## 📄 License
+
+This project is part of the Java Fundamentals learning repository.