Error from java NetBeans IDE

I'm trying to create an application test (since I'm starting in java) and I have an error:

main.java

package aplicaciondejava;

public class main {
    public static void main(String[] args) {
       Ventana1 window=new Ventana1();
       window.setTitle("Ventana1");
       window.setVisible(true);
    }
}

Ventana1.java

package aplicaciondejava;

public class Ventana1 extends javax.swing.JFrame {

    public Ventana1() {
        initComponents();
    }

    @SuppressWarnings("unchecked")
    // <editor-fold defaultstate="collapsed" desc="Generated Code">                          
    private void initComponents() {

        jButton1 = new javax.swing.JButton();

        setDefaultCloseOperation(javax.swing.WindowConstants.EXIT_ON_CLOSE);

        jButton1.setText("Salir");
        jButton1.addActionListener(new java.awt.event.ActionListener() {
            public void actionPerformed(java.awt.event.ActionEvent evt) {
                jButton1ActionPerformed(evt);
            }
        });

        javax.swing.GroupLayout layout = new javax.swing.GroupLayout(getContentPane());
        getContentPane().setLayout(layout);
        layout.setHorizontalGroup(
            layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
            .addGroup(javax.swing.GroupLayout.Alignment.TRAILING, layout.createSequentialGroup()
                .addContainerGap(337, Short.MAX_VALUE)
                .addComponent(jButton1)
                .addContainerGap())
        );
        layout.setVerticalGroup(
            layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
            .addGroup(javax.swing.GroupLayout.Alignment.TRAILING, layout.createSequentialGroup()
                .addContainerGap(266, Short.MAX_VALUE)
                .addComponent(jButton1)
                .addContainerGap())
        );

        pack();
    }// </editor-fold>                        

    private void jButton1ActionPerformed(java.awt.event.ActionEvent evt) {                                         
        System.exit(0);
    }                                        


    public static void main(String args[]) {

        java.awt.EventQueue.invokeLater(new Runnable() {
            public void run() {
                new Ventana1().setVisible(true);
            }
        });
    }

    // Variables declaration - do not modify                     
    private javax.swing.JButton jButton1;
    // End of variables declaration                   
}

When I try to compile it, or even without compiling it, it comes out

cannot find symbol
  symbol:   variable System
  location: class Ventana1
----
(Alt-Enter shows hints)  

I'm using jdk: /

NetBeans IDE 8.2

If I press build and open the java it works correctly, but in the IDE I get redrawn in red and it does not give me writing suggestions, it would be normal when I write System.o I would have to leave a list of suggestions, and It makes me uncomfortable and not pleasant to see an error that is not an error