Simplest Java Calculator GUI

SimpleCalculator.java

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

public class SimpleCalculator extends JFrame implements ActionListener {

	private JTextField displayField;
	private double currentValue = 0;
	private String currentOperator = "";
	private boolean start = true;

	public SimpleCalculator() {
		setTitle("Simple Calculator");
		setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
		setResizable(false);
		setLocationRelativeTo(null);

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

		displayField = new JTextField("0");
		displayField.setEditable(false);
		add(displayField, BorderLayout.NORTH);

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

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

		add(panel, BorderLayout.CENTER);
		pack();
	}

	public void actionPerformed(ActionEvent e) {
		String action = e.getActionCommand();

		switch (action) {
			case "C":
				currentValue = 0;
				currentOperator = "";
				displayField.setText("0");
				start = true;
				break;
			case "+":
			case "-":
			case "*":
			case "/":
				if (!currentOperator.isEmpty()) {
					performCalculation();
				}
				currentOperator = action;
				currentValue = Double.parseDouble(displayField.getText());
				start = true;
				break;
			case "=":
				performCalculation();
				currentOperator = "";
				start = true;
				break;
			default:
				if (start) {
					displayField.setText(action);
					start = false;
				} else {
					displayField.setText(displayField.getText() + action);
				}
				break;
		}
	}

	private void performCalculation() {
		double newValue = Double.parseDouble(displayField.getText());
		switch (currentOperator) {
			case "+":
				currentValue += newValue;
				break;
			case "-":
				currentValue -= newValue;
				break;
			case "*":
				currentValue *= newValue;
				break;
			case "/":
				if (newValue != 0) {
					currentValue /= newValue;
				} else {
					currentValue = Double.NaN; // Division by zero error
				}
				break;
		}
		displayField.setText(String.valueOf(currentValue));
	}

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