Netbeans 8 y encontrar fallos.

[REHome]

Hola:

Código algo corregido.

Código Java:

import java.io.OutputStream;
import gnu.io.CommPortIdentifier;
import gnu.io.SerialPort;
import java.util.Enumeration;
import javax.swing.JOptionPane;
 
/*
 * To change this license header, choose License Headers in Project Properties.
 * To change this template file, choose Tools | Templates
 * and open the template in the editor.
 */
 
/**
 *
 * @author Meta
 */
public class JAVADUINO_JFrame extends javax.swing.JFrame {
 
    /**
     * Creates new form JAVADUINO_JFrame
     */
    
    private static final String L8ON = "Led_8_ON";
    private static final String L8OFF = "Led_8_OFF";
    
    // Variables de conexión.
    private final OutputStream output = null;
    SerialPort serialPort;
    private final String PUERTO = "COM4";
    
    private static final int TIMEOUT = 2000;
    
    private static final int DATA_RATE = 115200;
    
    public JAVADUINO_JFrame() {
        initComponents();
        inicializarConexion();
    }
 
    public final void inicializarConexion(){
        CommPortIdentifier puertoID = null;
        Enumeration puertoEnum = CommPortIdentifier.getPortIdentifiers();
        
        while(puertoEnum.hasMoreElements()){
            CommPortIdentifier actualPuertoID = (CommPortIdentifier) puertoEnum.nextElement();
            if (PUERTO.equals(actualPuertoID.getName())){
                puertoID = actualPuertoID;
                break;
            }
        }
        
            if (puertoID == null){
    mostrarError("No se puede conectar al puerto.");
    System.exit(ERROR);
}
    
    try{
        serialPort = (SerialPort) puertoID.open(this.getClass().getName(), TIMEOUT);
        // Parámetros puerto serie.
        serialPort.setSerialPortParams(DATA_RATE, SerialPort.DATABITS_8, SerialPort.STOPBITS_2, SerialPort.PARITY_NONE);
        output = serialPort.getOutputStream();
} 
    catch (Exception e){
    mostrarError(e.getMessage());
    System.exit(ERROR);
}
    }
   
 
    private void enviarDatos(String datos){
        try{
            output.write(datos.getBytes());
        }
        catch(Exception e){
            mostrarError("ERROR");
            System.exit(ERROR);
        }
    }
    
    public void mostrarError(String mensaje){
        JOptionPane.showMessageDialog(this, mensaje, "ERROR", JOptionPane.ERROR_MESSAGE);
        
    }
    /**
     * This method is called from within the constructor to initialize the form.
     * WARNING: Do NOT modify this code. The content of this method is always
     * regenerated by the Form Editor.
     */
    @SuppressWarnings("unchecked")
    // <editor-fold defaultstate="collapsed" desc="Generated Code">                          
    private void initComponents() {
 
        jButton1 = new javax.swing.JButton();
        jButton2 = new javax.swing.JButton();
 
        setDefaultCloseOperation(javax.swing.WindowConstants.EXIT_ON_CLOSE);
 
        jButton1.setText("ON");
 
        jButton2.setText("OFF");
 
        javax.swing.GroupLayout layout = new javax.swing.GroupLayout(getContentPane());
        getContentPane().setLayout(layout);
        layout.setHorizontalGroup(
            layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
            .addGroup(layout.createSequentialGroup()
                .addGap(21, 21, 21)
                .addComponent(jButton1)
                .addPreferredGap(javax.swing.LayoutStyle.ComponentPlacement.RELATED, 101, Short.MAX_VALUE)
                .addComponent(jButton2)
                .addGap(45, 45, 45))
        );
        layout.setVerticalGroup(
            layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
            .addGroup(javax.swing.GroupLayout.Alignment.TRAILING, layout.createSequentialGroup()
                .addContainerGap(250, Short.MAX_VALUE)
                .addGroup(layout.createParallelGroup(javax.swing.GroupLayout.Alignment.BASELINE)
                    .addComponent(jButton1)
                    .addComponent(jButton2))
                .addGap(27, 27, 27))
        );
 
        pack();
    }// </editor-fold>                        
 
    /**
     * @param args the command line arguments
     */
    public static void main(String args[]) {
        /* Set the Nimbus look and feel */
        //<editor-fold defaultstate="collapsed" desc=" Look and feel setting code (optional) ">
        /* If Nimbus (introduced in Java SE 6) is not available, stay with the default look and feel.
         * For details see http://download.oracle.com/javase/tutorial/uiswing/lookandfeel/plaf.html 
         */
        try {
            for (javax.swing.UIManager.LookAndFeelInfo info : javax.swing.UIManager.getInstalledLookAndFeels()) {
                if ("Nimbus".equals(info.getName())) {
                    javax.swing.UIManager.setLookAndFeel(info.getClassName());
                    break;
                }
            }
        } catch (ClassNotFoundException ex) {
            java.util.logging.Logger.getLogger(JAVADUINO_JFrame.class.getName()).log(java.util.logging.Level.SEVERE, null, ex);
        } catch (InstantiationException ex) {
            java.util.logging.Logger.getLogger(JAVADUINO_JFrame.class.getName()).log(java.util.logging.Level.SEVERE, null, ex);
        } catch (IllegalAccessException ex) {
            java.util.logging.Logger.getLogger(JAVADUINO_JFrame.class.getName()).log(java.util.logging.Level.SEVERE, null, ex);
        } catch (javax.swing.UnsupportedLookAndFeelException ex) {
            java.util.logging.Logger.getLogger(JAVADUINO_JFrame.class.getName()).log(java.util.logging.Level.SEVERE, null, ex);
        }
        //</editor-fold>
 
        /* Create and display the form */
        java.awt.EventQueue.invokeLater(new Runnable() {
            public void run() {
                new JAVADUINO_JFrame().setVisible(true);
            }
        });
    }
 
    // Variables declaration - do not modify                     
    private javax.swing.JButton jButton1;
    private javax.swing.JButton jButton2;
    // End of variables declaration                   
 
    private void mostrarError(String no_se_puede_conectar_al_puerto) {
        throw new UnsupportedOperationException("Not supported yet."); //To change body of generated methods, choose Tools | Templates.
    }
}
 

Me sale este error.

run:
java.lang.ExceptionInInitializerError
Caused by: java.lang.RuntimeException: Uncompilable source code - method mostrarError(java.lang.String) is already defined in class JAVADUINO_JFrame
at JAVADUINO_JFrame.<clinit>(JAVADUINO_JFrame.java:17 4)
Exception in thread "main" Java Result: 1
BUILD SUCCESSFUL (total time: 0 seconds)