MIT-Curricular/OOP/Java/Lab/Week13/Calculator.java

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

public class Calculator extends JFrame implements ActionListener {

    private JTextField display;
    private double result = 0;
    private String operator = "=";
    private boolean calculating = true;

    public Calculator() {
        display = new JTextField("0", 20);
        display.setHorizontalAlignment(JTextField.RIGHT);
        display.setEditable(false);

        // Create panel with grid layout for buttons
        JPanel buttonPanel = new JPanel();
        buttonPanel.setLayout(new GridLayout(4, 4, 5, 5));

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

        for (String label : buttonLabels) {
            JButton button = new JButton(label);
            buttonPanel.add(button);
            button.addActionListener(this);
        }

        JButton clearButton = new JButton("C");
        clearButton.addActionListener(e -> {
            result = 0;
            operator = "=";
            calculating = true;
            display.setText("0");
        });

        // Layout
        setLayout(new BorderLayout(5, 5));
        add(display, BorderLayout.NORTH);
        add(buttonPanel, BorderLayout.CENTER);
        add(clearButton, BorderLayout.SOUTH);

        setTitle("Simple Calculator");
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        pack();
        setLocationRelativeTo(null);
    }

    public void actionPerformed(ActionEvent event) {
        String command = event.getActionCommand();

        if (
            (command.charAt(0) >= '0' && command.charAt(0) <= '9') ||
            command.equals(".")
        ) {
            if (calculating) {
                display.setText(command);
            } else {
                display.setText(display.getText() + command);
            }
            calculating = false;
        } else {
            if (calculating) {
                if (command.equals("-")) {
                    display.setText(command);
                    calculating = false;
                } else {
                    operator = command;
                }
            } else {
                double x = Double.parseDouble(display.getText());
                calculate(x);
                operator = command;
                calculating = true;
            }
        }
    }

    private void calculate(double n) {
        switch (operator) {
            case "+":
                result += n;
                break;
            case "-":
                result -= n;
                break;
            case "*":
                result *= n;
                break;
            case "/":
                result /= n;
                break;
            case "=":
                result = n;
                break;
        }
        display.setText("" + result);
    }

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