error java.lang.NullPointerException [duplicate]

-1

I have this error and I can not find the problem.

  

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException     at windows.WebPage.getUserList (MainPage.java:53) at   windows.TelePrincipal.Show_Users_In_JTable (MainPage.java:69) at   windows.HomePrincipal. (MainPage.java:28) at   windows.Tap.main.lambda $ main $ 0 (Main.java:256) 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)

Main Class

/*
* 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.
*/
package janelas;

import classes.User;
import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.ResultSet;
import java.sql.SQLException;
import java.sql.Statement;
import java.util.ArrayList;
import javax.swing.table.DefaultTableModel;

/**
*
* @author danilo
*/
public final class TelaPrincipal extends javax.swing.JFrame {

    /**
    * Creates new form TelaPrincipal
    */
    public TelaPrincipal() {
        initComponents();
        Show_Users_In_JTable();
    }

    public Connection getConnection(){

        Connection con;
        try{
            Class.forName("com.mysql.jdbc.Driver");
            con = DriverManager.getConnection("jdbc:mysql://localhost:3306/sistema_clientes","danilo","2701232");
            return con;
        } catch (ClassNotFoundException | SQLException e){
            return null;
        }

    }

    public ArrayList<User> getUserList(){

        ArrayList<User> usersList = new ArrayList<>();
        Connection connection = getConnection();
        String query = "SELECT * FROM 'users'";
        Statement st;
        ResultSet rs;

        try{
            st = connection.createStatement();
            rs = st.executeQuery(query);
            User user;
            while(rs.next()){
                user = new User(rs.getInt("id"),rs.getString("first_name"),rs.getString("last_name"),rs.getInt("age"));
                usersList.add(user);
            }
        } catch(SQLException e){
        }
        return usersList;
    }

    // Display Data in JTable
    public void Show_Users_In_JTable(){

        ArrayList<User> list = getUserList();
        DefaultTableModel model = (DefaultTableModel)jTable_Display_Users.getModel();
        Object[] row = new Object[4];

        for(int i = 0; i < list.size();i++){
            row[0] = list.get(i).getId();
            row[1] = list.get(i).getFirstName();
            row[2] = list.get(i).getLastName();
            row[3] = list.get(i).getAge();

            model.addRow(row);
        }

    }

    /**
    * 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")
    private void initComponents() {

        jPanel1 = new javax.swing.JPanel();
        jScrollPane1 = new javax.swing.JScrollPane();
        jTable_Display_Users = new javax.swing.JTable();
        jLabel1 = new javax.swing.JLabel();
        jTextField_Id = new javax.swing.JTextField();
        jLabel2 = new javax.swing.JLabel();
        jTextField_FirstName = new javax.swing.JTextField();
        jLabel3 = new javax.swing.JLabel();
        jTextField_LastName = new javax.swing.JTextField();
        jLabel4 = new javax.swing.JLabel();
        jTextField_Age = new javax.swing.JTextField();
        jButton_Insert = new javax.swing.JButton();
        jButton_Update = new javax.swing.JButton();
        jButton_Delete = new javax.swing.JButton();

        setDefaultCloseOperation(javax.swing.WindowConstants.EXIT_ON_CLOSE);

        jTable_Display_Users.setModel(new javax.swing.table.DefaultTableModel(
            new Object [][] {

            },
            new String [] {
                "Id", "First Name", "Last Name", "Age"
            }
        ));
        jScrollPane1.setViewportView(jTable_Display_Users);

        javax.swing.GroupLayout jPanel1Layout = new javax.swing.GroupLayout(jPanel1);
        jPanel1.setLayout(jPanel1Layout);
        jPanel1Layout.setHorizontalGroup(
            jPanel1Layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
            .addGroup(javax.swing.GroupLayout.Alignment.TRAILING, jPanel1Layout.createSequentialGroup()
                .addContainerGap(153, Short.MAX_VALUE)
                .addComponent(jScrollPane1, javax.swing.GroupLayout.PREFERRED_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.PREFERRED_SIZE)
                .addGap(35, 35, 35))
        );
        jPanel1Layout.setVerticalGroup(
            jPanel1Layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
            .addGroup(jPanel1Layout.createSequentialGroup()
                .addContainerGap()
                .addComponent(jScrollPane1, javax.swing.GroupLayout.PREFERRED_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.PREFERRED_SIZE)
                .addContainerGap(52, Short.MAX_VALUE))
        );

        jLabel1.setFont(new java.awt.Font("Dialog", 0, 18)); // NOI18N
        jLabel1.setText("Id:");

        jTextField_Id.setFont(new java.awt.Font("Dialog", 0, 18)); // NOI18N

        jLabel2.setFont(new java.awt.Font("Dialog", 0, 18)); // NOI18N
        jLabel2.setText("First Name:");

        jTextField_FirstName.setFont(new java.awt.Font("Dialog", 0, 18)); // NOI18N

        jLabel3.setFont(new java.awt.Font("Dialog", 0, 18)); // NOI18N
        jLabel3.setText("Last Name:");

        jTextField_LastName.setFont(new java.awt.Font("Dialog", 0, 18)); // NOI18N

        jLabel4.setFont(new java.awt.Font("Dialog", 0, 18)); // NOI18N
        jLabel4.setText("Age:");

        jTextField_Age.setFont(new java.awt.Font("Dialog", 0, 18)); // NOI18N

        jButton_Insert.setText("Insert");

        jButton_Update.setText("Update");

        jButton_Delete.setText("Delete");

        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()
                .addGroup(layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
                    .addGroup(layout.createSequentialGroup()
                        .addGroup(layout.createParallelGroup(javax.swing.GroupLayout.Alignment.TRAILING)
                            .addComponent(jLabel1)
                            .addComponent(jLabel4)
                            .addComponent(jLabel3)
                            .addComponent(jLabel2))
                        .addPreferredGap(javax.swing.LayoutStyle.ComponentPlacement.RELATED)
                        .addGroup(layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
                            .addComponent(jTextField_Id, javax.swing.GroupLayout.PREFERRED_SIZE, 142, javax.swing.GroupLayout.PREFERRED_SIZE)
                            .addComponent(jTextField_FirstName, javax.swing.GroupLayout.PREFERRED_SIZE, 142, javax.swing.GroupLayout.PREFERRED_SIZE)
                            .addComponent(jTextField_LastName, javax.swing.GroupLayout.PREFERRED_SIZE, 142, javax.swing.GroupLayout.PREFERRED_SIZE)
                            .addComponent(jTextField_Age, javax.swing.GroupLayout.PREFERRED_SIZE, 142, javax.swing.GroupLayout.PREFERRED_SIZE))
                        .addGap(0, 1, Short.MAX_VALUE))
                    .addGroup(layout.createSequentialGroup()
                        .addGap(0, 0, Short.MAX_VALUE)
                        .addComponent(jButton_Insert)
                        .addGap(18, 18, 18)
                        .addComponent(jButton_Update)
                        .addGap(18, 18, 18)
                        .addComponent(jButton_Delete)))
                .addPreferredGap(javax.swing.LayoutStyle.ComponentPlacement.RELATED)
                .addComponent(jPanel1, javax.swing.GroupLayout.PREFERRED_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.PREFERRED_SIZE)
                .addContainerGap())
        );
        layout.setVerticalGroup(
            layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
            .addGroup(javax.swing.GroupLayout.Alignment.TRAILING, layout.createSequentialGroup()
                .addComponent(jPanel1, javax.swing.GroupLayout.PREFERRED_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.PREFERRED_SIZE)
                .addGap(0, 0, Short.MAX_VALUE))
            .addGroup(layout.createSequentialGroup()
                .addContainerGap()
                .addGroup(layout.createParallelGroup(javax.swing.GroupLayout.Alignment.BASELINE)
                    .addComponent(jLabel1)
                    .addComponent(jTextField_Id, javax.swing.GroupLayout.PREFERRED_SIZE, 31, javax.swing.GroupLayout.PREFERRED_SIZE))
                .addPreferredGap(javax.swing.LayoutStyle.ComponentPlacement.RELATED)
                .addGroup(layout.createParallelGroup(javax.swing.GroupLayout.Alignment.BASELINE)
                    .addComponent(jLabel2)
                    .addComponent(jTextField_FirstName, javax.swing.GroupLayout.PREFERRED_SIZE, 31, javax.swing.GroupLayout.PREFERRED_SIZE))
                .addPreferredGap(javax.swing.LayoutStyle.ComponentPlacement.UNRELATED)
                .addGroup(layout.createParallelGroup(javax.swing.GroupLayout.Alignment.BASELINE)
                    .addComponent(jLabel3)
                    .addComponent(jTextField_LastName, javax.swing.GroupLayout.PREFERRED_SIZE, 31, javax.swing.GroupLayout.PREFERRED_SIZE))
                .addPreferredGap(javax.swing.LayoutStyle.ComponentPlacement.UNRELATED)
                .addGroup(layout.createParallelGroup(javax.swing.GroupLayout.Alignment.BASELINE)
                    .addComponent(jLabel4)
                    .addComponent(jTextField_Age, javax.swing.GroupLayout.PREFERRED_SIZE, 31, javax.swing.GroupLayout.PREFERRED_SIZE))
                .addGap(102, 102, 102)
                .addGroup(layout.createParallelGroup(javax.swing.GroupLayout.Alignment.BASELINE)
                    .addComponent(jButton_Insert)
                    .addComponent(jButton_Update)
                    .addComponent(jButton_Delete))
                .addContainerGap(javax.swing.GroupLayout.DEFAULT_SIZE, Short.MAX_VALUE))
        );

        pack();
    }

    /**
    * @param args the command line arguments
    */
    public static void main(String args[]) {
        /* Set the Nimbus look and feel */
        /* 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(TelaPrincipal.class.getName()).log(java.util.logging.Level.SEVERE, null, ex);
        } catch (InstantiationException ex) {
            java.util.logging.Logger.getLogger(TelaPrincipal.class.getName()).log(java.util.logging.Level.SEVERE, null, ex);
        } catch (IllegalAccessException ex) {
            java.util.logging.Logger.getLogger(TelaPrincipal.class.getName()).log(java.util.logging.Level.SEVERE, null, ex);
        } catch (javax.swing.UnsupportedLookAndFeelException ex) {
            java.util.logging.Logger.getLogger(TelaPrincipal.class.getName()).log(java.util.logging.Level.SEVERE, null, ex);
        }

        /* Create and display the form */
        java.awt.EventQueue.invokeLater(() -> {
            new TelaPrincipal().setVisible(true);
        });
    }

    // Variables declaration - do not modify                     
    private javax.swing.JButton jButton_Delete;
    private javax.swing.JButton jButton_Insert;
    private javax.swing.JButton jButton_Update;
    private javax.swing.JLabel jLabel1;
    private javax.swing.JLabel jLabel2;
    private javax.swing.JLabel jLabel3;
    private javax.swing.JLabel jLabel4;
    private javax.swing.JPanel jPanel1;
    private javax.swing.JScrollPane jScrollPane1;
    private javax.swing.JTable jTable_Display_Users;
    private javax.swing.JTextField jTextField_Age;
    private javax.swing.JTextField jTextField_FirstName;
    private javax.swing.JTextField jTextField_Id;
    private javax.swing.JTextField jTextField_LastName;
    // End of variables declaration                   
}

User class

package classes;

public class User{

    //Atributos
    private int id,age;
    private String firstName, lastName;

    //Métodos Especiais
    public User(int id,String firstName, String lastName, int age) {
        this.id = id;
        this.age = age;
        this.firstName = firstName;
        this.lastName = lastName;
    }

    public int getId() {
        return id;
    }

    public void setId(int id) {
        this.id = id;
    }

    public int getAge() {
        return age;
    }

    public void setAge(int age) {
        this.age = age;
    }

    public String getFirstName() {
        return firstName;
    }

    public void setFirstName(String firstName) {
        this.firstName = firstName;
    }

    public String getLastName() {
        return lastName;
    }

    public void setLastName(String lastName) {
        this.lastName = lastName;
    }
}
    
asked by anonymous 17.09.2018 / 22:15

1 answer

0

When you try to create the Connection it is returning null in case of error and simply ignoring Exception.

This way, when you run, the code "seems" to work, but it is falling into the exception and returning null and when you run st = connection.getStatement() a NullPointerException is thrown because connection == null .

  

What is the NullPointerException and what are its main causes?

    
17.09.2018 / 22:26