Calcu.java

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

public class Calcu extends JFrame implements ActionListener {
    JTextField display;
    double num1 = 0, num2 = 0, result = 0;
    char operator;

    public Calcu() {
        setTitle("Calculator");
        setSize(400, 500);
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        setLocationRelativeTo(null); // center the window

        // Display
        display = new JTextField();
        display.setEditable(false);
        display.setFont(new Font("Segoe UI", Font.BOLD, 30));
        display.setBackground(Color.BLACK);
        display.setForeground(Color.GREEN);
        display.setHorizontalAlignment(JTextField.RIGHT);
        display.setBorder(BorderFactory.createEmptyBorder(10, 10, 10, 10));
        add(display, BorderLayout.NORTH);

        // Button Panel
        JPanel panel = new JPanel(new GridLayout(4, 4, 10, 10));
        panel.setBackground(Color.DARK_GRAY);
        panel.setBorder(BorderFactory.createEmptyBorder(20, 20, 20, 20));

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

        for (String text : buttons) {
            JButton button = new JButton(text);
            button.setFont(new Font("Segoe UI", Font.BOLD, 22));
            button.setFocusPainted(false);
            button.setBackground(isOperator(text) ? new Color(255, 140, 0) : Color.LIGHT_GRAY);
            button.setForeground(Color.BLACK);
            button.setBorder(BorderFactory.createLineBorder(Color.WHITE, 2));
            button.addActionListener(this);
            panel.add(button);
        }

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

    private boolean isOperator(String text) {
        return "+-*/=".contains(text);
    }

    @Override
    public void actionPerformed(ActionEvent e) {
        String command = e.getActionCommand();

        if (command.charAt(0) >= '0' && command.charAt(0) <= '9') {
            display.setText(display.getText() + command);
        } else if (command.equals("C")) {
            display.setText("");
            num1 = num2 = result = 0;
        } else if (command.equals("=")) {
            num2 = Double.parseDouble(display.getText());

            switch (operator) {
                case '+': result = num1 + num2; break;
                case '-': result = num1 - num2; break;
                case '*': result = num1 * num2; break;
                case '/': 
                    if (num2 != 0) result = num1 / num2;
                    else {
                        display.setText("Error");
                        return;
                    }
                    break;
            }
            display.setText(String.valueOf(result));
        } else {
            num1 = Double.parseDouble(display.getText());
            operator = command.charAt(0);
            display.setText("");
        }
    }

    public static void main(String[] args) {
        new Calcu();
    }
}