Main.java

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

class Addition {
    double calculate(double num1, double num2) {
        return num1 + num2;
    }
}

class Subtraction {
    double calculate(double num1, double num2) {
        return num1 - num2;
    }
}

class Multiplication {
    double calculate(double num1, double num2) {
        return num1 * num2;
    }
}

class Division {
    double calculate(double num1, double num2) {
        return num1 / num2;
    }
}

public class Main {
    public static void main(String[] args) {
        JFrame frame = new JFrame("Simple Calculator");
        frame.setSize(350, 450);
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setLayout(new BorderLayout());

        JTextField display = new JTextField();
        display.setFont(new Font("Arial", Font.BOLD, 28));
        display.setHorizontalAlignment(JTextField.RIGHT);
        display.setEditable(false);

        frame.add(display, BorderLayout.NORTH);

        JPanel panel = new JPanel();
        panel.setLayout(new GridLayout(4, 4, 10, 10));

        String[] buttons = {
                "7", "8", "9", "+",
                "4", "5", "6", "-",
                "1", "2", "3", "*",
                "C", "0", "=", "/"
        };

        final double[] firstNumber = new double[1];
        final String[] operator = new String[1];

        for (String text : buttons) {
            JButton button = new JButton(text);
            button.setFont(new Font("Arial", Font.BOLD, 24));

            button.addActionListener(new ActionListener() {
                public void actionPerformed(ActionEvent e) {
                    String value = button.getText();

                    if (value.matches("[0-9]")) {
                        display.setText(display.getText() + value);
                    } 
                    else if (value.equals("+") || value.equals("-") || value.equals("*") || value.equals("/")) {
                        firstNumber[0] = Double.parseDouble(display.getText());
                        operator[0] = value;
                        display.setText("");
                    } 
                    else if (value.equals("=")) {
                        double secondNumber = Double.parseDouble(display.getText());
                        double result = 0;

                        if (operator[0].equals("+")) {
                            Addition add = new Addition();
                            result = add.calculate(firstNumber[0], secondNumber);
                        } 
                        else if (operator[0].equals("-")) {
                            Subtraction subtract = new Subtraction();
                            result = subtract.calculate(firstNumber[0], secondNumber);
                        } 
                        else if (operator[0].equals("*")) {
                            Multiplication multiply = new Multiplication();
                            result = multiply.calculate(firstNumber[0], secondNumber);
                        } 
                        else if (operator[0].equals("/")) {
                            if (secondNumber == 0) {
                                display.setText("Error");
                                return;
                            }

                            Division divide = new Division();
                            result = divide.calculate(firstNumber[0], secondNumber);
                        }

                        display.setText(String.valueOf(result));
                    } 
                    else if (value.equals("C")) {
                        display.setText("");
                        firstNumber[0] = 0;
                        operator[0] = "";
                    }
                }
            });

            panel.add(button);
        }

        frame.add(panel, BorderLayout.CENTER);
        frame.setVisible(true);
    }
}