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

class Program01 extends JFrame {
	JPanel topPanel = new JPanel();
	JPanel bottomPanel = new JPanel();	
	JLabel taskLabel = new JLabel("Háromszög területszámítás");	
	JLabel baseLabel = new JLabel("Alap");
	JLabel heightLabel = new JLabel("Magasság");
	JLabel resultLabel = new JLabel("Eredmény");	
	JTextField baseField = new JTextField(10);
	JTextField heightField = new JTextField(10);
	JTextField resultField = new JTextField(10);	
	JButton calcButton = new JButton("Számít");
	
	Program01() {
		calcButton.addActionListener(new ActionListener() {
			public void actionPerformed(ActionEvent event) {
				calcButtonActionListener(event);
			}
		});
		resultLabel.setVisible(false);
		resultField.setVisible(false);
		
		topPanel.add(taskLabel);
		bottomPanel.setLayout(new FlowLayout());
		bottomPanel.add(baseLabel);
		bottomPanel.add(baseField);
		bottomPanel.add(heightLabel);
		bottomPanel.add(heightField);
		bottomPanel.add(resultLabel);
		bottomPanel.add(resultField);
		bottomPanel.add(calcButton);
		
		setLayout(new BoxLayout(getContentPane(), BoxLayout.Y_AXIS));
		add(topPanel);
		add(bottomPanel);
		setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
		pack();
		centerWindow(this);
		setVisible(true);
	}
	public void calcButtonActionListener(ActionEvent event) {
		String baseStr = baseField.getText();
		String heightStr = heightField.getText();
		double base = Double.parseDouble(baseStr);
		double height = Double.parseDouble(heightStr);
		Double result = calcArea(base, height);
		resultField.setText(result.toString());
		resultLabel.setVisible(true);
		resultField.setVisible(true);
		pack();
	}
	public double calcArea(double base, double height) {
		if(base<=0 || height<=0) {
			throw new IllegalArgumentException();
		}
		return (base * height) / 2.0;
	}
	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);
	}
	public static void main(String[]args) {
		new Program01();
	}
}