空指针异常错误使用getter和setter

Question

我正在处理的某些代码出现问题。我有一个登录系统和一个带有getter和setter的类来存储用户信息。如果登录成功，我使用setter来存储信息。登录后，会出现一个新的GUI，我正在尝试使用getter将信息放在新的GUI上。我一直得到一个空指针异常错误，我不明白为什么？

继承登录GUI代码

Database_Classes.User userclass;
User_Repository user;
Main_Frame main = new Main_Frame();
Connection connection;
ResultSet resultset;
JdbcRowSetImpl RowSet;
CallableStatement callablestatement;
String URL = "jdbc:mysql://localhost:3306/gamebacklog";
String DB_Username = "root";
String DB_Password = "root";

public Login_Frame()
{
initComponents();
user = new User_Repository();
userclass = new Database_Classes.User();
}

public final void Connect()
{

RowSet = new JdbcRowSetImpl();

try
{
    connection = DriverManager.getConnection(URL, DB_Username, DB_Password);
    RowSet.setUrl(URL);
    RowSet.setUsername(DB_Username);
    RowSet.setPassword(DB_Password);
} catch (Exception ex)
{
    JOptionPane.showMessageDialog(null, "There was a problem connecting to the database.");
    System.out.print(ex);
    System.exit(0);
}
}

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

    UsernameField = new javax.swing.JTextField();
    UsernameLabel = new javax.swing.JLabel();
    PasswordLabel = new javax.swing.JLabel();
    LoginButton = new javax.swing.JButton();
    RegisterLabel = new javax.swing.JLabel();
    WelcomeLabel = new javax.swing.JLabel();
    RegisterButton = new javax.swing.JButton();
    UnsuccessfulLabel = new javax.swing.JLabel();
    ExitButton = new javax.swing.JButton();
    PasswordField = new javax.swing.JPasswordField();

    setDefaultCloseOperation(javax.swing.WindowConstants.EXIT_ON_CLOSE);

    UsernameLabel.setFont(new java.awt.Font("Dialog", 0, 14)); // NOI18N
    UsernameLabel.setText("Username");

    PasswordLabel.setFont(new java.awt.Font("Dialog", 0, 14)); // NOI18N
    PasswordLabel.setText("Password");

    LoginButton.setText("Login");
    LoginButton.addActionListener(new java.awt.event.ActionListener()
    {
        public void actionPerformed(java.awt.event.ActionEvent evt)
        {
            LoginButtonActionPerformed(evt);
        }
    });

    RegisterLabel.setFont(new java.awt.Font("Dialog", 0, 14)); // NOI18N
    RegisterLabel.setText("Register Here");

    WelcomeLabel.setFont(new java.awt.Font("Dialog", 0, 18)); // NOI18N
    WelcomeLabel.setText(" Login form for the system");
    WelcomeLabel.setVerticalAlignment(javax.swing.SwingConstants.TOP);

    RegisterButton.setText("Register");
    RegisterButton.addActionListener(new java.awt.event.ActionListener()
    {
        public void actionPerformed(java.awt.event.ActionEvent evt)
        {
            RegisterButtonActionPerformed(evt);
        }
    });

    ExitButton.setFont(new java.awt.Font("Dialog", 0, 14)); // NOI18N
    ExitButton.setText("Exit");
    ExitButton.addActionListener(new java.awt.event.ActionListener()
    {
        public void actionPerformed(java.awt.event.ActionEvent evt)
        {
            ExitButtonActionPerformed(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()
            .addGroup(layout.createParallelGroup(javax.swing.GroupLayout.Alignment.TRAILING)
                .addGroup(layout.createSequentialGroup()
                    .addContainerGap()
                    .addComponent(ExitButton, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, Short.MAX_VALUE))
                .addGroup(javax.swing.GroupLayout.Alignment.LEADING, layout.createSequentialGroup()
                    .addContainerGap()
                    .addGroup(layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
                        .addComponent(UnsuccessfulLabel, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, Short.MAX_VALUE)
                        .addGroup(layout.createSequentialGroup()
                            .addComponent(UsernameLabel, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, Short.MAX_VALUE)
                            .addPreferredGap(javax.swing.LayoutStyle.ComponentPlacement.RELATED)
                            .addComponent(UsernameField, javax.swing.GroupLayout.PREFERRED_SIZE, 150, javax.swing.GroupLayout.PREFERRED_SIZE))
                        .addGroup(javax.swing.GroupLayout.Alignment.TRAILING, layout.createSequentialGroup()
                            .addComponent(RegisterLabel, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, Short.MAX_VALUE)
                            .addPreferredGap(javax.swing.LayoutStyle.ComponentPlacement.UNRELATED)
                            .addComponent(RegisterButton, javax.swing.GroupLayout.PREFERRED_SIZE, 95, javax.swing.GroupLayout.PREFERRED_SIZE))
                        .addGroup(javax.swing.GroupLayout.Alignment.TRAILING, layout.createSequentialGroup()
                            .addComponent(PasswordLabel, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, Short.MAX_VALUE)
                            .addPreferredGap(javax.swing.LayoutStyle.ComponentPlacement.RELATED)
                            .addGroup(layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING, false)
                                .addComponent(LoginButton, javax.swing.GroupLayout.DEFAULT_SIZE, 150, Short.MAX_VALUE)
                                .addComponent(PasswordField)))))
                .addGroup(javax.swing.GroupLayout.Alignment.LEADING, layout.createSequentialGroup()
                    .addGap(22, 22, 22)
                    .addComponent(WelcomeLabel)
                    .addGap(0, 21, Short.MAX_VALUE)))
            .addContainerGap())
    );
    layout.setVerticalGroup(
        layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
        .addGroup(javax.swing.GroupLayout.Alignment.TRAILING, layout.createSequentialGroup()
            .addContainerGap()
            .addComponent(WelcomeLabel)
            .addGap(25, 25, 25)
            .addGroup(layout.createParallelGroup(javax.swing.GroupLayout.Alignment.BASELINE)
                .addComponent(UsernameLabel)
                .addComponent(UsernameField, javax.swing.GroupLayout.PREFERRED_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.PREFERRED_SIZE))
            .addPreferredGap(javax.swing.LayoutStyle.ComponentPlacement.UNRELATED)
            .addGroup(layout.createParallelGroup(javax.swing.GroupLayout.Alignment.BASELINE)
                .addComponent(PasswordLabel)
                .addComponent(PasswordField, javax.swing.GroupLayout.PREFERRED_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.PREFERRED_SIZE))
            .addPreferredGap(javax.swing.LayoutStyle.ComponentPlacement.UNRELATED)
            .addComponent(LoginButton)
            .addPreferredGap(javax.swing.LayoutStyle.ComponentPlacement.UNRELATED)
            .addComponent(UnsuccessfulLabel, javax.swing.GroupLayout.PREFERRED_SIZE, 27, javax.swing.GroupLayout.PREFERRED_SIZE)
            .addPreferredGap(javax.swing.LayoutStyle.ComponentPlacement.RELATED)
            .addGroup(layout.createParallelGroup(javax.swing.GroupLayout.Alignment.BASELINE)
                .addComponent(RegisterButton)
                .addComponent(RegisterLabel, javax.swing.GroupLayout.PREFERRED_SIZE, 48, javax.swing.GroupLayout.PREFERRED_SIZE))
            .addPreferredGap(javax.swing.LayoutStyle.ComponentPlacement.RELATED)
            .addComponent(ExitButton)
            .addContainerGap(javax.swing.GroupLayout.DEFAULT_SIZE, Short.MAX_VALUE))
    );

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

private void LoginButtonActionPerformed(java.awt.event.ActionEvent evt)                                            
{                                                

try
{
    Connect();
    String username = UsernameField.getText();
    String password = PasswordField.getText();
    resultset = user.Login(connection, username, password);

    if (resultset.next())
    {
    result(resultset);

    } else
    {
    UnsuccessfulLabel.setText("The Login Attempt was unsuccessful");
    }
} catch (Exception ex)
{
    JOptionPane.showMessageDialog(null, "Encountered a problem with the Server. Please try again later");
    System.out.println("two \t" + ex.toString());
} finally
{
    try
    {
    user.Close();
    connection.close();
    main.setVisible(true);
    this.setVisible(false);
    } catch (Exception ex)
    {
    JOptionPane.showMessageDialog(null, "Encountered a problem with the Server. Please try again later");
    System.out.println("three \t" + ex.toString());
    }

}
}                                           

private void RegisterButtonActionPerformed(java.awt.event.ActionEvent evt)                                               
{                                                   
Register_Frame Register = new Register_Frame();
Register.setVisible(true);
this.setVisible(false);
}                                              

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

public static void main(String args[])
{
java.awt.EventQueue.invokeLater(new Runnable()
{
    public void run()
    {
    new Login_Frame().setVisible(true);
    }
});
}

// Variables declaration - do not modify                     
private javax.swing.JButton ExitButton;
private javax.swing.JButton LoginButton;
private javax.swing.JPasswordField PasswordField;
private javax.swing.JLabel PasswordLabel;
private javax.swing.JButton RegisterButton;
private javax.swing.JLabel RegisterLabel;
private javax.swing.JLabel UnsuccessfulLabel;
private javax.swing.JTextField UsernameField;
private javax.swing.JLabel UsernameLabel;
private javax.swing.JLabel WelcomeLabel;
// End of variables declaration                   

public void result(ResultSet resultSet)
{

try
{
    int ID = resultSet.getInt(1);
    userclass.SetID(ID);
    String firstname = resultSet.getString(2);
    userclass.SetFirstname(firstname);
    String lastname = resultSet.getString(3);
    userclass.SetLastname(lastname);
    String username = resultSet.getString(4);
    userclass.SetUsername(username);
    int accesslevel = resultSet.getInt(6);
    userclass.SetAccessLevel(accesslevel);

    System.out.println(userclass.GetID() + "  " + userclass.GetFirstname() + "  " + userclass.GetLastname() + "  " + userclass.GetUsername() + "  " + userclass.GetAccesslevel());

} catch (Exception ex)
{
    System.out.println(ex);
}
}

这是其他GUI

Database_Classes.User userclass;

public Main_Frame()
{
initComponents();
}

public void get()
{

jLabel2.setText("Firstname = " + userclass.GetFirstname());
jLabel1.setText("ID = " + userclass.GetID());
jLabel3.setText("Lastname = " + userclass.GetLastname());
jLabel4.setText("Username = " + userclass.GetUsername());
jLabel5.setText("accesslevel = " + userclass.GetAccesslevel());
}

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

    jLabel1 = new javax.swing.JLabel();
    jLabel2 = new javax.swing.JLabel();
    jLabel3 = new javax.swing.JLabel();
    jLabel4 = new javax.swing.JLabel();
    jLabel5 = new javax.swing.JLabel();
    LogoutButton = new javax.swing.JButton();
    SetButton = new javax.swing.JButton();

    setDefaultCloseOperation(javax.swing.WindowConstants.EXIT_ON_CLOSE);

    jLabel1.setText("jLabel1");

    jLabel2.setText("jLabel2");

    jLabel3.setText("jLabel3");

    jLabel4.setText("jLabel4");

    jLabel5.setText("jLabel5");

    LogoutButton.setText("Logout");
    LogoutButton.addActionListener(new java.awt.event.ActionListener()
    {
        public void actionPerformed(java.awt.event.ActionEvent evt)
        {
            LogoutButtonActionPerformed(evt);
        }
    });

    SetButton.setText("Results");
    SetButton.addActionListener(new java.awt.event.ActionListener()
    {
        public void actionPerformed(java.awt.event.ActionEvent evt)
        {
            SetButtonActionPerformed(evt);
        }
    });

    javax.swing.GroupLayout layout = new javax.swing.GroupLayout(getContentPane());
    getContentPane().setLayout(layout);
    layout.setHorizontalGroup(
        layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
        .addGroup(layout.createSequentialGroup()
            .addContainerGap()
            .addGroup(layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
                .addGroup(layout.createSequentialGroup()
                    .addGroup(layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING, false)
                        .addComponent(jLabel1, javax.swing.GroupLayout.DEFAULT_SIZE, 172, Short.MAX_VALUE)
                        .addComponent(jLabel2, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, Short.MAX_VALUE)
                        .addComponent(jLabel3, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, Short.MAX_VALUE)
                        .addComponent(jLabel4, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, Short.MAX_VALUE)
                        .addComponent(jLabel5, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, Short.MAX_VALUE))
                    .addGap(0, 0, Short.MAX_VALUE))
                .addGroup(layout.createSequentialGroup()
                    .addComponent(LogoutButton)
                    .addPreferredGap(javax.swing.LayoutStyle.ComponentPlacement.RELATED, 37, Short.MAX_VALUE)
                    .addComponent(SetButton, javax.swing.GroupLayout.PREFERRED_SIZE, 127, javax.swing.GroupLayout.PREFERRED_SIZE)))
            .addContainerGap())
    );
    layout.setVerticalGroup(
        layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
        .addGroup(layout.createSequentialGroup()
            .addContainerGap()
            .addComponent(jLabel1)
            .addGap(18, 18, 18)
            .addComponent(jLabel2)
            .addGap(18, 18, 18)
            .addComponent(jLabel3)
            .addGap(18, 18, 18)
            .addComponent(jLabel4)
            .addGap(18, 18, 18)
            .addComponent(jLabel5)
            .addGap(18, 18, 18)
            .addGroup(layout.createParallelGroup(javax.swing.GroupLayout.Alignment.BASELINE)
                .addComponent(LogoutButton)
                .addComponent(SetButton))
            .addContainerGap(javax.swing.GroupLayout.DEFAULT_SIZE, Short.MAX_VALUE))
    );

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

private void LogoutButtonActionPerformed(java.awt.event.ActionEvent evt)                                             
{                                                 
Login_Frame login = new Login_Frame();
login.setVisible(true);
this.setVisible(false);
}                                            

private void SetButtonActionPerformed(java.awt.event.ActionEvent evt)                                          
{                                              
    get();
}                                         


// Variables declaration - do not modify                     
private javax.swing.JButton LogoutButton;
private javax.swing.JButton SetButton;
private javax.swing.JLabel jLabel1;
private javax.swing.JLabel jLabel2;
private javax.swing.JLabel jLabel3;
private javax.swing.JLabel jLabel4;
private javax.swing.JLabel jLabel5;
// End of variables declaration                   

最后是用户类代码

private int ID;
private String firstname;
private String lastname;
private String username;
private String password;
private int accesslevel;

public int GetID()
{
return ID;
}

public void SetID(int ID)
{
this.ID = ID;
}

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;
}

public String GetUsername()
{
return username;
}

public void SetUsername(String username)
{
this.username = username;
}

public String GetPassword()
{
return password;
}

public void SetPassword(String password)
{
this.password = password;
}

public int GetAccesslevel()
{
return accesslevel;
}

public void SetAccessLevel(int accesslevel)
{
this.accesslevel = accesslevel;
}

这也是Null Pointer异常错误不断出现

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
at GameBacklog.Main_Frame.get(Main_Frame.java:16)
at GameBacklog.Main_Frame.SetButtonActionPerformed(Main_Frame.java:119)
at GameBacklog.Main_Frame.access$100(Main_Frame.java:3)
at GameBacklog.Main_Frame$2.actionPerformed(Main_Frame.java:62)
at javax.swing.AbstractButton.fireActionPerformed(AbstractButton.java:2022)
at javax.swing.AbstractButton$Handler.actionPerformed(AbstractButton.java:2348)
at javax.swing.DefaultButtonModel.fireActionPerformed(DefaultButtonModel.java:402)
at javax.swing.DefaultButtonModel.setPressed(DefaultButtonModel.java:259)
at javax.swing.plaf.basic.BasicButtonListener.mouseReleased(BasicButtonListener.java:252)
at java.awt.Component.processMouseEvent(Component.java:6535)
at javax.swing.JComponent.processMouseEvent(JComponent.java:3324)
at java.awt.Component.processEvent(Component.java:6300)
at java.awt.Container.processEvent(Container.java:2236)
at java.awt.Component.dispatchEventImpl(Component.java:4891)
at java.awt.Container.dispatchEventImpl(Container.java:2294)
at java.awt.Component.dispatchEvent(Component.java:4713)
at java.awt.LightweightDispatcher.retargetMouseEvent(Container.java:4888)
at java.awt.LightweightDispatcher.processMouseEvent(Container.java:4525)
at java.awt.LightweightDispatcher.dispatchEvent(Container.java:4466)
at java.awt.Container.dispatchEventImpl(Container.java:2280)
at java.awt.Window.dispatchEventImpl(Window.java:2750)
at java.awt.Component.dispatchEvent(Component.java:4713)
at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:758)
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.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:86)
at java.awt.EventQueue$4.run(EventQueue.java:731)
at java.awt.EventQueue$4.run(EventQueue.java:729)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:76)
at java.awt.EventQueue.dispatchEvent(EventQueue.java:728)
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)