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

public class Main extends JFrame{
	private JTextField txtf1;
	private JTextField txtf2;
	private JTextField txtt;
	private double a;
	private double b;
	private double sum;

	public static void main(String[] args) {
		Main go = new Main();
		go.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
		go.setSize(500,400);
		go.setVisible(true);

	}

	public Main(){
		super("Title window");
		txtf1 = new JTextField(10);
		txtf2 = new JTextField(10);
		txtt = new JTextField(10);
		setLayout(new FlowLayout());

		add(txtf1);
		add(txtf2);
		add(txtt);

		txtf2.addActionListener(
				new ActionListener() {
					public void actionPerformed(ActionEvent event) {
						a = Double.parseDouble(txtf1.getText());
						b = Double.parseDouble(txtf2.getText());
						sum = a + b;
						txtt.setText(Double.toString(sum));
					}
				}

				);

	}

}