import javax.swing.JFrame;
import javax.swing.JButton;
import javax.swing.JLabel;
import javax.swing.JTextField;
import java.awt.event.ActionListener;
import java.awt.event.ActionEvent;
import java.awt.FlowLayout;

class MainWindow extends JFrame {
	private JButton calcButton = new JButton("Számít");
	public JTextField baseField = new JTextField(10);
	public JTextField heightField = new JTextField(10);
	public JTextField resultField = new JTextField(10);
	private JLabel baseLabel = new JLabel("alap:");
	private JLabel heightLabel = new JLabel("magasság:");
	MainWindow() {
		resultField.setVisible(false);
		add(baseLabel);
		add(baseField);
		add(heightLabel);
		add(heightField);
		add(resultField);
		add(calcButton);
		
		setLayout(new FlowLayout());
		setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
		pack();
		centerWindow(this);
		setVisible(true);
	}
	public void addCalcButtonActionListener(ActionListener listener) {
		calcButton.addActionListener(listener);
	}	
	public static void centerWindow(java.awt.Window frame) {
		java.awt.Dimension dimension =
		    java.awt.Toolkit.getDefaultToolkit().getScreenSize();
		int x = (int)((dimension.getWidth() - frame.getWidth()) / 2);
		int y = (int)((dimension.getHeight() - frame.getHeight()) / 2);
		frame.setLocation(x, y);
	}	
}

class Controller {
	MainWindow mainWindow = new MainWindow();
	Model model = new Model();
	Controller() {
		mainWindow.addCalcButtonActionListener(new ActionListener(){
			public void actionPerformed(ActionEvent event) {
				String baseStr = mainWindow.baseField.getText();
				String heightStr = mainWindow.heightField.getText();
				double base = Double.parseDouble(baseStr);
				double height = Double.parseDouble(heightStr);
				Double area = model.calcArea(base, height);
				mainWindow.resultField.setText(area.toString());
				mainWindow.resultField.setVisible(true);
				mainWindow.pack();
			}
		});
	}
}

class Model {
	public double calcArea(double base, double height) {
		if(base <= 0 || height <=0) {
			throw new IllegalArgumentException();
		}
		return (base * height) /2;
	}
}

class Program01 {
	public static void main(String[]args) {
		new Controller();
	}
}