編集：解決しました！みんなありがとう！：D
正しいクレデンシャルで入力しても、エラーshowMessageDialogが表示されます。rs.next（）は0より大きい値を返すと思いますが、ユーザーのクレデンシャルがusers_tableのクレデンシャルと一致するかどうかを確認する必要がありますが、クレデンシャルが一致していないかのように、常にエラーメッセージが表示されます。
import java.sql.*;
import javax.swing.*;
public class System extends javax.swing.JFrame {
// variables needed for db connection
Connection conn;
Statement stmt;
ResultSet rs;
String sql;
String user;
char[] pass;
//for table row count
int ctr = 0;
//temporary variables needed in retrieving and inserting records in our db
int user_id;
String username;
String password;
public System() {
initComponents();
setLocationRelativeTo(null);
dbConnect();
}
public void dbConnect() {
conn = null;
String url = "jdbc:mysql://localhost:3306/item_db";
String driver = "com.mysql.jdbc.Driver";
String userName = "root";
String password = "";
try {
Class.forName(driver).newInstance();
conn = DriverManager.getConnection(url, userName, password);
stmt = conn.createStatement(ResultSet.TYPE_SCROLL_INSENSITIVE, ResultSet.CONCUR_UPDATABLE);
sql = "Select * from users_table";
rs = stmt.executeQuery(sql);//rs - will hold the records from the databa sql = "Select * from item_table";se
} catch (Exception e) {
JOptionPane.showMessageDialog(this, e.getMessage());
}
}
private void login() {
try {
if (user != null && pass != null) {
sql = "Select * from users_table Where username='" + user + "' and password='" + pass + "'";
rs = stmt.executeQuery(sql);
rs.next();
if ((user.equals(username)) && (pass.equals(password))) {
JOptionPane.showMessageDialog(null, "You are now logon!");
}
else {
JOptionPane.showMessageDialog(null, "damn!", "alert", JOptionPane.ERROR_MESSAGE);
}
}
} catch (SQLException err) {
JOptionPane.showMessageDialog(this, err.getMessage());
}
}
@SuppressWarnings("unchecked")
// <editor-fold defaultstate="collapsed" desc="Generated Code">
private void initComponents() {
jLabel1 = new javax.swing.JLabel();
jLabel2 = new javax.swing.JLabel();
jTextField1 = new javax.swing.JTextField();
jButton1 = new javax.swing.JButton();
jButton2 = new javax.swing.JButton();
jPasswordField1 = new javax.swing.JPasswordField();
jLabel3 = new javax.swing.JLabel();
setDefaultCloseOperation(javax.swing.WindowConstants.EXIT_ON_CLOSE);
jLabel1.setFont(new java.awt.Font("Calibri", 0, 18)); // NOI18N
jLabel1.setText("Username:");
jLabel2.setFont(new java.awt.Font("Calibri", 0, 18)); // NOI18N
jLabel2.setText("Password:");
jButton1.setText("Login");
jButton1.addActionListener(new java.awt.event.ActionListener() {
public void actionPerformed(java.awt.event.ActionEvent evt) {
jButton1ActionPerformed(evt);
}
});
jButton2.setText("Cancel");
jLabel3.setIcon(new javax.swing.ImageIcon(getClass().getResource("/login.png"))); // NOI18N
javax.swing.GroupLayout layout = new javax.swing.GroupLayout(getContentPane());
getContentPane().setLayout(layout);
layout.setHorizontalGroup(
layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
.addGroup(layout.createSequentialGroup()
.addGroup(layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
.addGroup(layout.createSequentialGroup()
.addGap(46, 46, 46)
.addComponent(jLabel3))
.addGroup(layout.createSequentialGroup()
.addGap(82, 82, 82)
.addGroup(layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
.addGroup(layout.createSequentialGroup()
.addGroup(layout.createParallelGroup(javax.swing.GroupLayout.Alignment.TRAILING)
.addComponent(jLabel2)
.addComponent(jLabel1))
.addPreferredGap(javax.swing.LayoutStyle.ComponentPlacement.RELATED)
.addGroup(layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING, false)
.addComponent(jTextField1)
.addComponent(jPasswordField1, javax.swing.GroupLayout.PREFERRED_SIZE, 134, javax.swing.GroupLayout.PREFERRED_SIZE)))
.addGroup(layout.createSequentialGroup()
.addGap(51, 51, 51)
.addComponent(jButton1)
.addGap(28, 28, 28)
.addComponent(jButton2)))))
.addContainerGap(52, Short.MAX_VALUE))
);
layout.setVerticalGroup(
layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
.addGroup(javax.swing.GroupLayout.Alignment.TRAILING, layout.createSequentialGroup()
.addComponent(jLabel3, javax.swing.GroupLayout.DEFAULT_SIZE, 100, Short.MAX_VALUE)
.addPreferredGap(javax.swing.LayoutStyle.ComponentPlacement.RELATED)
.addGroup(layout.createParallelGroup(javax.swing.GroupLayout.Alignment.BASELINE)
.addComponent(jLabel1)
.addComponent(jTextField1, javax.swing.GroupLayout.PREFERRED_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.PREFERRED_SIZE))
.addPreferredGap(javax.swing.LayoutStyle.ComponentPlacement.RELATED)
.addGroup(layout.createParallelGroup(javax.swing.GroupLayout.Alignment.BASELINE)
.addComponent(jLabel2)
.addComponent(jPasswordField1, javax.swing.GroupLayout.PREFERRED_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.PREFERRED_SIZE))
.addGap(18, 18, 18)
.addGroup(layout.createParallelGroup(javax.swing.GroupLayout.Alignment.BASELINE)
.addComponent(jButton1)
.addComponent(jButton2))
.addGap(29, 29, 29))
);
pack();
}// </editor-fold>
private void jButton1ActionPerformed(java.awt.event.ActionEvent evt) {
// TODO add your handling code here:
//str = JOptionPane.showInputDialog(this, "Enter id number: ");
user = jTextField1.getText();
pass = jPasswordField1.getPassword();
login();
}
/**
* @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(System.class.getName()).log(java.util.logging.Level.SEVERE, null, ex);
} catch (InstantiationException ex) {
java.util.logging.Logger.getLogger(System.class.getName()).log(java.util.logging.Level.SEVERE, null, ex);
} catch (IllegalAccessException ex) {
java.util.logging.Logger.getLogger(System.class.getName()).log(java.util.logging.Level.SEVERE, null, ex);
} catch (javax.swing.UnsupportedLookAndFeelException ex) {
java.util.logging.Logger.getLogger(System.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 System().setVisible(true);
}
});
}
// Variables declaration - do not modify
private javax.swing.JButton jButton1;
private javax.swing.JButton jButton2;
private javax.swing.JLabel jLabel1;
private javax.swing.JLabel jLabel2;
private javax.swing.JLabel jLabel3;
private javax.swing.JPasswordField jPasswordField1;
private javax.swing.JTextField jTextField1;
// End of variables declaration
}