Create a file named Calculator.java and copy the following complete program into it.

import javax.swing.*;
import java.awt.*;
import java.math.BigDecimal;

public class Calculator extends JFrame {

    private final JTextField display;

    private double firstNumber = 0;
    private String operator = "";
    private boolean startNewNumber = true;

    public Calculator() {
        setTitle("Java Calculator");
        setSize(350, 450);
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        setLocationRelativeTo(null);
        setResizable(false);

        display = new JTextField("0");
        display.setEditable(false);
        display.setHorizontalAlignment(JTextField.RIGHT);
        display.setFont(new Font("Arial", Font.BOLD, 28));
        display.setMargin(new Insets(10, 10, 10, 10));

        JPanel buttonPanel = new JPanel(new GridLayout(4, 4, 5, 5));

        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("Arial", Font.BOLD, 20));

            button.addActionListener(e -> handleButton(text));

            buttonPanel.add(button);
        }

        JButton equalsButton = new JButton("=");
        equalsButton.setFont(new Font("Arial", Font.BOLD, 22));
        equalsButton.addActionListener(e -> calculateResult());

        JPanel bottomPanel = new JPanel(new BorderLayout());
        bottomPanel.add(equalsButton, BorderLayout.CENTER);

        setLayout(new BorderLayout(8, 8));
        add(display, BorderLayout.NORTH);
        add(buttonPanel, BorderLayout.CENTER);
        add(bottomPanel, BorderLayout.SOUTH);
    }

    private void handleButton(String text) {
        if (text.equals("C")) {
            clearCalculator();
        } else if ("+-*/".contains(text)) {
            selectOperator(text);
        } else {
            enterDigit(text);
        }
    }

    private void enterDigit(String digit) {
        if (startNewNumber) {
            display.setText(digit.equals(".") ? "0." : digit);
            startNewNumber = false;
            return;
        }

        String current = display.getText();

        if (digit.equals(".")) {
            if (!current.contains(".")) {
                display.setText(current + ".");
            }
        } else if (current.equals("0")) {
            display.setText(digit);
        } else {
            display.setText(current + digit);
        }
    }

    private void selectOperator(String selectedOperator) {
        try {
            firstNumber = Double.parseDouble(display.getText());
            operator = selectedOperator;
            startNewNumber = true;
        } catch (NumberFormatException ex) {
            showError("Invalid number.");
        }
    }

    private void calculateResult() {
        if (operator.isEmpty()) {
            return;
        }

        try {
            double secondNumber =
                Double.parseDouble(display.getText());

            double result;

            switch (operator) {
                case "+":
                    result = firstNumber + secondNumber;
                    break;

                case "-":
                    result = firstNumber - secondNumber;
                    break;

                case "*":
                    result = firstNumber * secondNumber;
                    break;

                case "/":
                    if (secondNumber == 0) {
                        showError("Cannot divide by zero.");
                        clearCalculator();
                        return;
                    }

                    result = firstNumber / secondNumber;
                    break;

                default:
                    return;
            }

            if (!Double.isFinite(result)) {
                showError("Result is outside the supported range.");
                clearCalculator();
                return;
            }

            display.setText(
                BigDecimal.valueOf(result)
                    .stripTrailingZeros()
                    .toPlainString()
            );

            operator = "";
            startNewNumber = true;

        } catch (NumberFormatException ex) {
            showError("Invalid number.");
            clearCalculator();
        }
    }

    private void clearCalculator() {
        display.setText("0");
        firstNumber = 0;
        operator = "";
        startNewNumber = true;
    }

    private void showError(String message) {
        JOptionPane.showMessageDialog(
            this,
            message,
            "Calculator Error",
            JOptionPane.ERROR_MESSAGE
        );
    }

    public static void main(String[] args) {
        SwingUtilities.invokeLater(() -> {
            Calculator calculator = new Calculator();
            calculator.setVisible(true);
        });
    }
}

After completing the basic project, try adding these features:

1. Add percentage calculation.
2. Add positive/negative number support using a +/- button.
3. Add a backspace button to delete the last digit.
4. Add keyboard support for numbers and operators.
5. Improve the user interface with custom colors and fonts.
6. Add square and square-root operations.
7. Support repeated calculations.
8. Add a calculation history panel.