TelecomBillingGUI.java

/*📱 Telecom Billing System – Java Swing GUI
This project is a Java Swing–based GUI application that calculates the monthly telecom bill for a user based on package type (A–E) and total call duration in seconds.
⭐ Features
User-friendly GUI built with JFrame, JPanel, and GridLayout
Input fields for:
Package type (A–E)
Seconds called
Built-in input validation for incorrect or missing values
Automatic bill calculation based on:
Monthly rental
Per–second call rates
Discounts & surcharges:
10% discount if call charges > LKR 5000
5% surcharge if call charges < LKR 1000
Displays final bill in a non-editable text box
Error messages shown in real-time

🧮 Billing Logic
Each package (A–E) has:
Different monthly rental
Different per-second charge
Final bill = monthly rental + (seconds × per-second rate)
Then discount/surcharge rules are applied.

🧰 Technologies Used
Java Swing
Event Handling (ActionListener)
GUI Layout Management (GridLayout) */

package project;

import javax.swing.*;
import java.awt.*;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;

public class TelecomBillingGUI {

    public static void main(String[] args) {
        // Create the frame
        JFrame frame = new JFrame("Telecom Billing System");
        frame.setSize(400, 300);
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

        // Create panels
        JPanel panel = new JPanel();
        panel.setLayout(new GridLayout(5, 2, 10, 10));
        frame.add(panel);

        // Package type input
        JLabel packageLabel = new JLabel("Package Type (A-E):");
        JTextField packageField = new JTextField();
        panel.add(packageLabel);
        panel.add(packageField);

        // Seconds called input
        JLabel secondsLabel = new JLabel("Seconds Called:");
        JTextField secondsField = new JTextField();
        panel.add(secondsLabel);
        panel.add(secondsField);

        // Button to calculate
        JButton calculateButton = new JButton("Calculate Bill");
        panel.add(new JLabel()); // Spacer
        panel.add(calculateButton);

        // Result label
        JLabel resultLabel = new JLabel("Final Bill: ");
        panel.add(resultLabel);

        JTextField resultField = new JTextField();
        resultField.setEditable(false);
        panel.add(resultField);

        // Error message
        JLabel errorLabel = new JLabel("");
        errorLabel.setForeground(Color.RED);
        panel.add(errorLabel);

        // Billing logic
        calculateButton.addActionListener(new ActionListener() {
            @Override
            public void actionPerformed(ActionEvent e) {
                String packageType = packageField.getText().toUpperCase();
                String secondsText = secondsField.getText();

                // Validate input
                if (!secondsText.matches("\\d+") || packageType.length() != 1 || !"ABCDE".contains(packageType)) {
                    errorLabel.setText("Invalid input! Check package type or seconds.");
                    resultField.setText("");
                    return;
                }

                errorLabel.setText(""); // Clear previous error message
                int secondsCalled = Integer.parseInt(secondsText);

                // Define package details
                String[] packages = {"A", "B", "C", "D", "E"};
                double[] monthlyRentals = {1000, 1500, 2000, 2500, 3000};
                double[] perSecondCharges = {2, 1.5, 1, 0.75, 0.5};

                int packageIndex = "ABCDE".indexOf(packageType);
                double monthlyRental = monthlyRentals[packageIndex];
                double perSecondCharge = perSecondCharges[packageIndex];

                // Calculate call charges
                double callCharges = secondsCalled * perSecondCharge;
                double totalCharges = monthlyRental + callCharges;

                // Apply discount or surcharge
                if (callCharges > 5000) {
                    totalCharges *= 0.9; // 10% discount
                } else if (callCharges < 1000) {
                    totalCharges *= 1.05; // 5% surcharge
                }

                // Display the result
                resultField.setText(String.format("LKR %.2f", totalCharges));
            }
        });

        // Set the frame visible
        frame.setVisible(true);
    }
}