title: Where is the error in this Java code?
body: <p>I am trying to use window builder to code this simple task, however, eclipse always says its wrong and doesn't execute. Please what is the mistake on this code? *all the packages are imported. Hello, I am trying to use window builder to code this simple task, however, eclipse always says its wrong and doesn't execute. Please what is the mistake on this code? *all the packages are imported. </p>
  
  <pre><code>private JPanel contentPane;
  private JTextField nota_periodo1;
  private JTextField nota_periodo2;
  private JTextField nota_periodo3;
  
  /**
   * Launch the application.
   */
  public static void main(String[] args) {
      EventQueue.invokeLater(new Runnable() {
          public void run() {
              try {
                  Ventana1 frame = new Ventana1();
                  frame.setVisible(true);
              } catch (Exception e) {
                  e.printStackTrace();
              }
          }
      });
  }
  
  /**
   * Create the frame.
   * @return 
   */
  
  
  public Ventana1() {
      setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
      setBounds(100, 100, 450, 300);
      contentPane = new JPanel();
      contentPane.setBackground(new Color(176, 224, 230));
      contentPane.setForeground(new Color(176, 224, 230));
      contentPane.setBorder(new EmptyBorder(5, 5, 5, 5));
      setContentPane(contentPane);
      contentPane.setLayout(null);
  
      JLabel lblCalculadoraDeNotas = new JLabel("CALCULADORA DE NOTAS SGS");
      lblCalculadoraDeNotas.setBounds(146, 20, 200, 16);
      contentPane.add(lblCalculadoraDeNotas);
  
      JLabel lblNewLabel = new JLabel("Nota Periodo I:");
      lblNewLabel.setBounds(20, 60, 104, 16);
      contentPane.add(lblNewLabel);
  
      nota_periodo1 = new JTextField();
      nota_periodo1.setBounds(118, 55, 130, 26);
      contentPane.add(nota_periodo1);
      nota_periodo1.setColumns(10);
  
      JLabel lblNewLabel_1 = new JLabel("Nota Periodo 2:");
      lblNewLabel_1.setBounds(20, 88, 104, 16);
      contentPane.add(lblNewLabel_1);
  
      JLabel lblNotaPeriodo = new JLabel("Nota Periodo 3:");
      lblNotaPeriodo.setBounds(20, 113, 104, 16);
      contentPane.add(lblNotaPeriodo);
  
      nota_periodo2 = new JTextField();
      nota_periodo2.setBounds(128, 83, 130, 26);
      contentPane.add(nota_periodo2);
      nota_periodo2.setColumns(10);
  
      nota_periodo3 = new JTextField();
      nota_periodo3.setBounds(118, 108, 130, 26);
      contentPane.add(nota_periodo3);
      nota_periodo3.setColumns(10);
  
      // OBJETO DE INICIO
      Ventana1 start = new Ventana1();
      String principio = start.Inicio();
  
      JButton calcular = new JButton("Calcular");
      calcular.addActionListener(new ActionListener() {
          public void actionPerformed(ActionEvent e) {
              System.out.println(principio);
          }
      });
      calcular.setBounds(40, 159, 104, 29);
      contentPane.add(calcular);
  
      JLabel lblUstedTieneQue = new JLabel("Usted Tiene que Sacar: ");
      lblUstedTieneQue.setBounds(23, 200, 157, 16);
      contentPane.add(lblUstedTieneQue);
  
      JLabel nota_examen_final = new JLabel("");
      nota_examen_final.setBounds(172, 200, 61, 16);
      contentPane.add(nota_examen_final);
  
      JLabel lblEnElExamen = new JLabel("en el Examen Final");
      lblEnElExamen.setBounds(245, 200, 124, 16);
      contentPane.add(lblEnElExamen);
  
  
      }
  
  //METODO INICIO
  public String Inicio (){
      int nota1 = Integer.parseInt(nota_periodo1.getText());
      int nota2 = Integer.parseInt(nota_periodo2.getText());
      int nota3 = Integer.parseInt(nota_periodo3.getText());
      double nota1p = nota1*0.35;
      double nota2p = nota2*0.25;
      double nota3p = nota3*0.2;
      double notapass = 60-(nota1p+nota2p+nota3p);
      double notaex = notapass/0.2;
      JOptionPane.showMessageDialog(null, notaex);
      return null;
  
  }
  </code></pre>