NullPointerException error, where is the error?

Question

NullPointerException error, where is the error?

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#1 by (1 votes)
#2 by (-3 votes)

1

I wanted to know where the error is. I read that the NullPointerException error is when we try to access a variable that was not initialized, I just did not find this error in my code.

import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.PreparedStatement;
import java.sql.ResultSet;
import java.sql.SQLException;
import java.sql.Statement;
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 User
 */
public  class NovoJFrame extends javax.swing.JFrame {

     PreparedStatement stl;
     ResultSet rs;
     Connection con;



    public NovoJFrame() {
        initComponents();

        IniciarBD();

        this.PrencherComboBox();

        jPanel3.setVisible(false);
    }


    public void PrencherComboBox(){
        String sql = "Select * from ALUNO";
        try {

               stl = con.prepareStatement(sql);
               rs = stl.executeQuery();

               while(rs.next()){
                   jComboBox1.addItem(rs.getString("NOME"));
               }
           } catch(SQLException ex){
               JOptionPane.showMessageDialog(null,ex);
           }

    }





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

        jPanel1 = new javax.swing.JPanel();
        jLabel6 = new javax.swing.JLabel();
        jPanel2 = new javax.swing.JPanel();
        jButton1 = new javax.swing.JButton();
        jLabel7 = new javax.swing.JLabel();
        jComboBox1 = new javax.swing.JComboBox<>();
        jLabel1 = new javax.swing.JLabel();
        jLabel3 = new javax.swing.JLabel();
        jComboBox2 = new javax.swing.JComboBox<>();
        jButton2 = new javax.swing.JButton();
        jPanel3 = new javax.swing.JPanel();
        jLabel5 = new javax.swing.JLabel();
        jLabel4 = new javax.swing.JLabel();
        jLabel8 = new javax.swing.JLabel();
        jButton3 = new javax.swing.JButton();

        setDefaultCloseOperation(javax.swing.WindowConstants.EXIT_ON_CLOSE);
        setBackground(java.awt.Color.blue);

        jPanel1.setBackground(new java.awt.Color(0, 51, 255));
        jPanel1.setForeground(new java.awt.Color(204, 255, 51));

        jLabel6.setIcon(new javax.swing.ImageIcon(getClass().getResource("/liceu terras.png"))); // NOI18N

        javax.swing.GroupLayout jPanel1Layout = new javax.swing.GroupLayout(jPanel1);
        jPanel1.setLayout(jPanel1Layout);
        jPanel1Layout.setHorizontalGroup(
            jPanel1Layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
            .addGroup(jPanel1Layout.createSequentialGroup()
                .addGap(265, 265, 265)
                .addComponent(jLabel6)
                .addContainerGap(javax.swing.GroupLayout.DEFAULT_SIZE, Short.MAX_VALUE))
        );
        jPanel1Layout.setVerticalGroup(
            jPanel1Layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
            .addGroup(javax.swing.GroupLayout.Alignment.TRAILING, jPanel1Layout.createSequentialGroup()
                .addContainerGap(33, Short.MAX_VALUE)
                .addComponent(jLabel6)
                .addGap(31, 31, 31))
        );

        jPanel2.setBackground(new java.awt.Color(255, 255, 0));

        jButton1.setFont(new java.awt.Font("Tahoma", 0, 36)); // NOI18N
        jButton1.setText("Visualizar Tempo");
        jButton1.addActionListener(new java.awt.event.ActionListener() {
            public void actionPerformed(java.awt.event.ActionEvent evt) {
                jButton1ActionPerformed(evt);
            }
        });

        jLabel7.setIcon(new javax.swing.ImageIcon(getClass().getResource("/med grande.png"))); // NOI18N

        javax.swing.GroupLayout jPanel2Layout = new javax.swing.GroupLayout(jPanel2);
        jPanel2.setLayout(jPanel2Layout);
        jPanel2Layout.setHorizontalGroup(
            jPanel2Layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
            .addGroup(jPanel2Layout.createSequentialGroup()
                .addGap(19, 19, 19)
                .addComponent(jLabel7)
                .addGap(40, 40, 40)
                .addComponent(jButton1)
                .addContainerGap(javax.swing.GroupLayout.DEFAULT_SIZE, Short.MAX_VALUE))
        );
        jPanel2Layout.setVerticalGroup(
            jPanel2Layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
            .addGroup(javax.swing.GroupLayout.Alignment.TRAILING, jPanel2Layout.createSequentialGroup()
                .addContainerGap(100, Short.MAX_VALUE)
                .addGroup(jPanel2Layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
                    .addComponent(jButton1, javax.swing.GroupLayout.PREFERRED_SIZE, 77, javax.swing.GroupLayout.PREFERRED_SIZE)
                    .addComponent(jLabel7, javax.swing.GroupLayout.PREFERRED_SIZE, 79, javax.swing.GroupLayout.PREFERRED_SIZE))
                .addGap(61, 61, 61))
        );

        jComboBox1.setFont(new java.awt.Font("Tahoma", 0, 24)); // NOI18N

        jLabel1.setFont(new java.awt.Font("Tahoma", 0, 24)); // NOI18N
        jLabel1.setText("Aluno:");

        jLabel3.setFont(new java.awt.Font("Tahoma", 0, 24)); // NOI18N
        jLabel3.setText("Quantidade:");

        jComboBox2.setFont(new java.awt.Font("Tahoma", 0, 24)); // NOI18N
        jComboBox2.setModel(new javax.swing.DefaultComboBoxModel<>(new String[] { "Item 1", "Item 2", "Item 3", "Item 4" }));

        jButton2.setFont(new java.awt.Font("Tahoma", 0, 36)); // NOI18N
        jButton2.setText("Adicionar");
        jButton2.addActionListener(new java.awt.event.ActionListener() {
            public void actionPerformed(java.awt.event.ActionEvent evt) {
                jButton2ActionPerformed(evt);
            }
        });

        jLabel5.setFont(new java.awt.Font("Tahoma", 0, 36)); // NOI18N
        jLabel5.setForeground(new java.awt.Color(51, 255, 0));
        jLabel5.setText("Com Sucesso");

        jLabel4.setFont(new java.awt.Font("Tahoma", 0, 36)); // NOI18N
        jLabel4.setForeground(new java.awt.Color(0, 255, 0));
        jLabel4.setText("Operação Realizada");

        jLabel8.setIcon(new javax.swing.ImageIcon(getClass().getResource("/simbolo dinheiro.png"))); // NOI18N

        javax.swing.GroupLayout jPanel3Layout = new javax.swing.GroupLayout(jPanel3);
        jPanel3.setLayout(jPanel3Layout);
        jPanel3Layout.setHorizontalGroup(
            jPanel3Layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
            .addGroup(jPanel3Layout.createSequentialGroup()
                .addContainerGap()
                .addGroup(jPanel3Layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
                    .addGroup(jPanel3Layout.createSequentialGroup()
                        .addComponent(jLabel4)
                        .addContainerGap(javax.swing.GroupLayout.DEFAULT_SIZE, Short.MAX_VALUE))
                    .addGroup(javax.swing.GroupLayout.Alignment.TRAILING, jPanel3Layout.createSequentialGroup()
                        .addGap(0, 0, Short.MAX_VALUE)
                        .addGroup(jPanel3Layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
                            .addGroup(javax.swing.GroupLayout.Alignment.TRAILING, jPanel3Layout.createSequentialGroup()
                                .addComponent(jLabel8)
                                .addGap(115, 115, 115))
                            .addGroup(javax.swing.GroupLayout.Alignment.TRAILING, jPanel3Layout.createSequentialGroup()
                                .addComponent(jLabel5)
                                .addGap(51, 51, 51))))))
        );
        jPanel3Layout.setVerticalGroup(
            jPanel3Layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
            .addGroup(jPanel3Layout.createSequentialGroup()
                .addContainerGap()
                .addComponent(jLabel8)
                .addPreferredGap(javax.swing.LayoutStyle.ComponentPlacement.RELATED)
                .addComponent(jLabel4)
                .addPreferredGap(javax.swing.LayoutStyle.ComponentPlacement.UNRELATED)
                .addComponent(jLabel5)
                .addContainerGap(30, Short.MAX_VALUE))
        );

        jButton3.setFont(new java.awt.Font("Tahoma", 0, 36)); // NOI18N
        jButton3.setText("Status");

        javax.swing.GroupLayout layout = new javax.swing.GroupLayout(getContentPane());
        getContentPane().setLayout(layout);
        layout.setHorizontalGroup(
            layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
            .addComponent(jPanel1, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, Short.MAX_VALUE)
            .addGroup(layout.createSequentialGroup()
                .addComponent(jPanel2, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, Short.MAX_VALUE)
                .addContainerGap())
            .addGroup(layout.createSequentialGroup()
                .addGroup(layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
                    .addGroup(layout.createSequentialGroup()
                        .addComponent(jLabel1)
                        .addPreferredGap(javax.swing.LayoutStyle.ComponentPlacement.RELATED)
                        .addComponent(jComboBox1, javax.swing.GroupLayout.PREFERRED_SIZE, 173, javax.swing.GroupLayout.PREFERRED_SIZE))
                    .addGroup(layout.createSequentialGroup()
                        .addComponent(jLabel3)
                        .addPreferredGap(javax.swing.LayoutStyle.ComponentPlacement.RELATED)
                        .addComponent(jComboBox2, javax.swing.GroupLayout.PREFERRED_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.PREFERRED_SIZE))
                    .addGroup(layout.createSequentialGroup()
                        .addComponent(jButton2)
                        .addPreferredGap(javax.swing.LayoutStyle.ComponentPlacement.UNRELATED)
                        .addComponent(jButton3)))
                .addPreferredGap(javax.swing.LayoutStyle.ComponentPlacement.RELATED, javax.swing.GroupLayout.DEFAULT_SIZE, Short.MAX_VALUE)
                .addComponent(jPanel3, javax.swing.GroupLayout.PREFERRED_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.PREFERRED_SIZE)
                .addGap(116, 116, 116))
        );
        layout.setVerticalGroup(
            layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
            .addGroup(layout.createSequentialGroup()
                .addComponent(jPanel1, javax.swing.GroupLayout.PREFERRED_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.PREFERRED_SIZE)
                .addGroup(layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING, false)
                    .addGroup(layout.createSequentialGroup()
                        .addPreferredGap(javax.swing.LayoutStyle.ComponentPlacement.RELATED)
                        .addComponent(jPanel3, javax.swing.GroupLayout.PREFERRED_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.PREFERRED_SIZE)
                        .addGap(6, 6, 6))
                    .addGroup(layout.createSequentialGroup()
                        .addGap(26, 26, 26)
                        .addGroup(layout.createParallelGroup(javax.swing.GroupLayout.Alignment.BASELINE)
                            .addComponent(jLabel1)
                            .addComponent(jComboBox1, javax.swing.GroupLayout.PREFERRED_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.PREFERRED_SIZE))
                        .addGap(29, 29, 29)
                        .addGroup(layout.createParallelGroup(javax.swing.GroupLayout.Alignment.BASELINE)
                            .addComponent(jLabel3)
                            .addComponent(jComboBox2, javax.swing.GroupLayout.PREFERRED_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.PREFERRED_SIZE))
                        .addPreferredGap(javax.swing.LayoutStyle.ComponentPlacement.RELATED, javax.swing.GroupLayout.DEFAULT_SIZE, Short.MAX_VALUE)
                        .addGroup(layout.createParallelGroup(javax.swing.GroupLayout.Alignment.BASELINE)
                            .addComponent(jButton2)
                            .addComponent(jButton3))
                        .addGap(32, 32, 32)))
                .addComponent(jPanel2, javax.swing.GroupLayout.PREFERRED_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.PREFERRED_SIZE))
        );

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

    private void jButton2ActionPerformed(java.awt.event.ActionEvent evt) {                                         
        // TODO add your handling code here:
        jPanel3.setVisible(true);
    }                                        

    private void jButton1ActionPerformed(java.awt.event.ActionEvent evt) {                                         
        // TODO add your handling code here:
        med frame = new med();  

        frame.setVisible(true);  
    }                                        

    /**
     * @param args the command line arguments
     */
    public static void main(String args[]) {         






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

    // Variables declaration - do not modify                     
    private javax.swing.JButton jButton1;
    private javax.swing.JButton jButton2;
    private javax.swing.JButton jButton3;
    private javax.swing.JComboBox<String> jComboBox1;
    private javax.swing.JComboBox<String> jComboBox2;
    private javax.swing.JLabel jLabel1;
    private javax.swing.JLabel jLabel3;
    private javax.swing.JLabel jLabel4;
    private javax.swing.JLabel jLabel5;
    private javax.swing.JLabel jLabel6;
    private javax.swing.JLabel jLabel7;
    private javax.swing.JLabel jLabel8;
    private javax.swing.JPanel jPanel1;
    private javax.swing.JPanel jPanel2;
    private javax.swing.JPanel jPanel3;
    // End of variables declaration                   


    private void IniciarBD(){

        try {
        Class.forName("org.apache.derby.jdbc.ClientDriver");
    } catch ( ClassNotFoundException e){
            System.out.println("Erro :" + e.getMessage() );
    }

    String url = "jdbc:derby://localhost:1527/bd" ;
    String user = "root" ;
    String pass = "senha" ;


        try {
            Connection con = DriverManager.getConnection( url, user, pass );

            Statement stmt = con.createStatement();

            System.out.println("Conexão Realizada com Sucesso");
        } catch (SQLException e) {
            System.out.println("Erro :" + e.getMessage() );
        }

}
}

StackTrace:

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
at NovoJFrame.PrencherComboBox(NovoJFrame.java:43)
at NovoJFrame.<init>(NovoJFrame.java:33)
at NovoJFrame$3.run(NovoJFrame.java:280)
at java.awt.event.InvocationEvent.dispatch(InvocationEvent.java:311)
at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:756)
at java.awt.EventQueue.access$500(EventQueue.java:97)
at java.awt.EventQueue$3.run(EventQueue.java:709)
at java.awt.EventQueue$3.run(EventQueue.java:703)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:76)
at java.awt.EventQueue.dispatchEvent(EventQueue.java:726)
at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:201)
at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:116)
at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:105)
at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:101)
at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:93)
at java.awt.EventDispatchThread.run(EventDispatchThread.java:82)

java swing jdbc nullpointerexception

asked by anonymous 29.07.2016 / 21:45

2 answers

Error installing Ionic + Cordova return query as zend string

score 1 · Answer 1

Your main problem is that in the PreencherComboBox method, you do not initialize the connection.

The simplest solution would be to do this:

    public void PrencherComboBox(){
        String sql = "Select * from ALUNO";
        try {
               con = DriverManager.getConnection(url, user, pass);
               stl = con.prepareStatement(sql);
               rs = stl.executeQuery();

               while(rs.next()){
                   jComboBox1.addItem(rs.getString("NOME"));
               }
           } catch(SQLException ex){
               JOptionPane.showMessageDialog(null,ex);
           }

    }

However, since we are here, we can analyze what other problems we have in your code. And there are several of them:

You have forgotten the private modifier in some attributes of the class. Package visibility is probably not what you want.
You open connections, Statement s, and ResultSet s, but do not close them properly. This is a bad practice that can lead to wasting memory consumption or maintaining open connections that are no longer useful. The most practical way to deal with this problem is to use the try syntax named try-with-resources in> .
You mix logic of visualization (swing) with data access logic (JDBC). In order for the code to be organized, it is better to separate them.
You are not following the Java language naming conventions correctly. Methods must have their name initialed with lowercase.

Let's then solve all these problems:

public class AcessoDados {
    private final String url = "jdbc:derby://localhost:1527/bd";
    private final String user = "root";
    private final String pass = "senha";

    static {
        try {
            Class.forName("org.apache.derby.jdbc.ClientDriver");
        } catch (ClassNotFoundException e) {
            System.out.println("Erro :" + e.getMessage());
            throw new ExceptionInInitializerError(e);
        }
    }

    public AcessoDados() {
        this.url = "jdbc:derby://localhost:1527/bd";
        this.user = "root";
        this.pass = "senha";
    }

    public AcessoDados(String url, String user, String pass) {
        this.url = url;
        this.user = user;
        this.pass = pass;
    }

    private Connection conectar() {
        return DriverManager.getConnection(url, user, pass);
    }

    public List<String> listarNomesDeAlunos() throws SQLException {
        List<String> lista = new ArrayList<>(10);

        String sql = "SELECT nome FROM ALUNO";
        try (
            Connection con = conectar();
            PreparedStatement stl = con.prepareStatement(sql);
            ResultSet rs = stl.executeQuery();
        ) {
           while (rs.next()) {
               lista.addItem(rs.getString("NOME"));
           }
        }
        return lista;
    }

    public void testarConexao() throws SQLException {
        try (
            Connection con = conectar();
            Statement stmt = con.createStatement();
        ) {
            System.out.println("Conexão Realizada com Sucesso");
        }
    }
}

public class NovoJFrame extends javax.swing.JFrame {

    private final AcessoDados acesso = new AcessoDados();

    public NovoJFrame() {
        initComponents();
        jPanel3.setVisible(false);

        try {
            acesso.testarConexao();
        } catch (SQLException e) {
            System.out.println("Erro :" + e.getMessage());
        }

        this.prencherComboBox();
    }

    public void prencherComboBox() {
        List<String> nomes;
        try {
            nomes = acesso.listarNomesDeAlunos();
       } catch (SQLException ex) {
           JOptionPane.showMessageDialog(null, ex);
       }

        for (String nome : nomes) {
            jComboBox1.addItem(nome);
        }
    }

    // ... Resto da classe...
}

There is yet another serious problem that you must solve. You are manipulating the database on the same swing thread. This may leave the application unresponsive. The reason is that the database may take a while to make the request, and during that time, your screen will be frozen. To resolve this, try to use the SwingWorker so as not to let the swing hang because it is doing operations in the database.

However, to propose a solution to this using SwingWorker , it would be necessary to change a lot in the structure of your application, much more than just the portion of what you posted in the question. Moreover, this subject is already another conversation far from the initial purpose of what is outlined in the question. If you are interested, I suggest you ask a new question about this topic.

score -3 · Answer 2

Connection con = null;

stl = con.prepareStatement(sql); // con = null

If IniciarBD() is initializing the variable con , then:

rs = stl.executeQuery();
jComboBox1.addItem(rs.getString("NOME"));

If query execution fails, rs = null .