Ligar button

import java.awt.Dimension;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;

import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JPanel;
import javax.swing.JTextField;

public class button extends JPanel {
	private JButton cal;
	private JTextField n1;
	private JLabel jcomp3;
	private JLabel jcomp4;
	private JTextField n2;
	private JLabel res;
	private JTextField resul;
	private ActionListener calcular;

	public button() {
		
		calcular = new ActionListener() {
			public void actionPerformed(ActionEvent e) {
				double resultado = (Double.parseDouble(n1.getText()) * Double.parseDouble(n2.getText())) / 10;
				resul.setText(String.valueOf(resultado));
			}
		};
		
		// construct components
		cal = new JButton("Calcular");
		cal.addActionListener(calcular);

		n1 = new JTextField(5);
		jcomp3 = new JLabel("nota1");
		jcomp4 = new JLabel("nota2");
		n2 = new JTextField(5);
		res = new JLabel("Resultado");
		resul = new JTextField(5);

		// adjust size and set layout
		setPreferredSize(new Dimension(624, 531));
		setLayout(null);

		// add components
		add(cal);
		add(n1);
		add(jcomp3);
		add(jcomp4);
		add(n2);
		add(res);
		add(resul);

		// set component bounds (only needed by Absolute Positioning)
		cal.setBounds(155, 125, 140, 20);
		n1.setBounds(40, 75, 100, 25);
		jcomp3.setBounds(70, 40, 100, 25);
		jcomp4.setBounds(195, 40, 100, 25);
		n2.setBounds(160, 75, 100, 25);
		res.setBounds(290, 40, 100, 25);
		resul.setBounds(280, 75, 100, 25);
	}

	public static void main(String[] args) {
		JFrame frame = new JFrame("Botao");
		frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
		frame.getContentPane().add(new button());
		frame.pack();
		frame.setVisible(true);
	}
}