Java Static Members, Nested Classes & Enums Masterclass

// Enum Definition
enum OrderStatus {
    PENDING("Order received, awaiting processing"),
    PROCESSING("Order is being prepared"),
    SHIPPED("Order dispatched from warehouse"),
    DELIVERED("Order successfully delivered"),
    CANCELLED("Order cancelled by customer");

    private final String description;

    OrderStatus(String description) {
        this.description = description;
    }

    public String getDescription() {
        return description;
    }
}

class ShoppingCart {
    // Static field: shared across all cart instances
    static int totalCartsCreated = 0;
    static final double TAX_RATE  = 0.18; // 18% GST

    // Static initializer block
    static {
        System.out.println("  [CLASS LOADED] ShoppingCart initialized. Tax Rate: " + (TAX_RATE * 100) + "%");
    }

    String customerId;
    double subtotal;
    OrderStatus status;

    ShoppingCart(String customerId, double subtotal) {
        this.customerId = customerId;
        this.subtotal   = subtotal;
        this.status     = OrderStatus.PENDING;
        ShoppingCart.totalCartsCreated++;
    }

    double calculateTotalWithTax() {
        return subtotal * (1 + TAX_RATE);
    }

    void updateStatus(OrderStatus newStatus) {
        this.status = newStatus;
    }

    void displayOrderSummary() {
        System.out.printf("  Customer: %-10s | Subtotal: $%7.2f | Total+Tax: $%7.2f | Status: %s%n",
                customerId, subtotal, calculateTotalWithTax(), status.name());
        System.out.println("    -> " + status.getDescription());
    }

    // Static Nested Class (does not need outer instance)
    static class TaxCalculator {
        static double computeGST(double amount) {
            return amount * TAX_RATE;
        }
    }
}

public class Main {
    public static void main(String[] args) {
        System.out.println("=== 1. Static Initializer & Object Creation ===");
        ShoppingCart cart1 = new ShoppingCart("CUST-001", 499.99);
        ShoppingCart cart2 = new ShoppingCart("CUST-002", 1200.00);

        System.out.println("Total carts created: " + ShoppingCart.totalCartsCreated);

        System.out.println("
=== 2. Order Status Enum Lifecycle ===");
        cart1.displayOrderSummary();
        cart1.updateStatus(OrderStatus.PROCESSING);
        cart1.displayOrderSummary();
        cart1.updateStatus(OrderStatus.SHIPPED);
        cart1.displayOrderSummary();

        System.out.println("
=== 3. Enum Iteration via values() ===");
        System.out.println("All Order Statuses:");
        for (OrderStatus s : OrderStatus.values()) {
            System.out.printf("  %-12s [#%d] -> %s%n", s.name(), s.ordinal(), s.getDescription());
        }

        System.out.println("
=== 4. Static Nested Class Usage ===");
        double gst = ShoppingCart.TaxCalculator.computeGST(cart2.subtotal);
        System.out.printf("  GST on $%.2f = $%.2f%n", cart2.subtotal, gst);

        System.out.println("
=== 5. Enum in switch expression ===");
        OrderStatus current = OrderStatus.DELIVERED;
        String message = switch (current) {
            case PENDING    -> "Your order is in queue.";
            case PROCESSING -> "We are packing your items!";
            case SHIPPED    -> "Out for delivery!";
            case DELIVERED  -> "Enjoy your purchase!";
            case CANCELLED  -> "Sorry to see you go.";
        };
        System.out.println("  Status message: " + message);
    }
}

enum UserRole { ADMIN, MANAGER, EMPLOYEE, GUEST }

class Employee {
    static int headcount = 0;
    String name;
    double salary;
    UserRole role;

    Employee(String name, double salary, UserRole role) {
        this.name   = name;
        this.salary = salary;
        this.role   = role;
        headcount++;
    }

    void displayInfo() {
        System.out.printf("  %-14s | Role: %-8s | Salary: $%,.2f%n", name, role, salary);
    }
}

public class PracticalApplication {
    public static void main(String[] args) {
        Employee e1 = new Employee("Ravi Kumar",   85000, UserRole.MANAGER);
        Employee e2 = new Employee("Priya Devi",   62000, UserRole.EMPLOYEE);
        Employee e3 = new Employee("Admin Singh", 110000, UserRole.ADMIN);

        System.out.println("=== HR Portal — Employee Directory ===");
        e1.displayInfo(); e2.displayInfo(); e3.displayInfo();
        System.out.println("Total Headcount : " + Employee.headcount);
    }
}