所以我有一个奇怪的问题，我刚发布并修复了我的其他错误，但这似乎创造了这个，这没有任何意义。
基本上，用户按下一个按钮，此窗口打开，然后无论是按取消还是注册，此窗口都应该设置为false。但是当执行此代码时，它表示我的窗口为空。
package frontend;
import java.awt.EventQueue;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.sql.SQLException;
import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JPanel;
import javax.swing.JTextField;
import javax.swing.border.EmptyBorder;
public class Registration extends JFrame {
/**
*
*/
private static final long serialVersionUID = 1L;
private static boolean ranOnce = false;
private JPanel contentPane;
private Registration reg;
private JTextField userTF;
private JTextField passTF;
private JTextField emailTF;
private LoginProcess lp = new LoginProcess();
private JLabel error;
private JButton cancelBtn;
/**
* Create the frame.
*/
public Registration() {
if (!ranOnce) {
EventQueue.invokeLater(new Runnable() {
public void run() {
try {
reg = new Registration();
reg.setVisible(true);
ranOnce = true;
} catch (Exception e) {
e.printStackTrace();
}
}
});
}
setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
setBounds(100, 100, 245, 212);
setLocationRelativeTo(null);
contentPane = new JPanel();
contentPane.setBorder(new EmptyBorder(5, 5, 5, 5));
setContentPane(contentPane);
contentPane.setLayout(null);
JLabel lblUsername = new JLabel("Username:");
lblUsername.setBounds(10, 11, 75, 14);
contentPane.add(lblUsername);
JLabel lblPassword = new JLabel("Password:");
lblPassword.setBounds(10, 36, 75, 14);
contentPane.add(lblPassword);
JLabel lblEmail = new JLabel("Email:");
lblEmail.setBounds(10, 61, 75, 14);
contentPane.add(lblEmail);
userTF = new JTextField();
userTF.setBounds(95, 8, 130, 20);
contentPane.add(userTF);
userTF.setColumns(10);
passTF = new JTextField();
passTF.setColumns(10);
passTF.setBounds(95, 33, 130, 20);
contentPane.add(passTF);
emailTF = new JTextField();
emailTF.setColumns(10);
emailTF.setBounds(95, 58, 130, 20);
contentPane.add(emailTF);
error = new JLabel("");
error.setBounds(10, 154, 215, 14);
contentPane.add(error);
JButton regBtn = new JButton("Register");
regBtn.addActionListener(new ActionListener() {
public void actionPerformed(ActionEvent arg0) {
if (!userTF.getText().equals("") || !passTF.getText().equals("") || !emailTF.getText().equals("")) {
try {
if (lp.newUser(userTF.getText(), passTF.getText(), emailTF.getText())) {
// THIS IS LINE 117 reg.setVisible(false);
Main.mainFrame.setVisible(true);
} else {
if (lp.duplicateAccount) {
error.setText("Error: Username already in use.");
}
}
} catch (SQLException e) {
}
}
}
});
regBtn.setBounds(10, 120, 215, 23);
contentPane.add(regBtn);
cancelBtn = new JButton("Cancel");
cancelBtn.addActionListener(new ActionListener() {
public void actionPerformed(ActionEvent arg0) {
reg.setVisible(false);
Main.mainFrame.setVisible(true);
}
});
cancelBtn.setBounds(10, 86, 215, 23);
contentPane.add(cancelBtn);
}
}
这是异常错误：
Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
at frontend.Registration$3.actionPerformed(Registration.java:117)
at javax.swing.AbstractButton.fireActionPerformed(Unknown Source)
我真的不知道为什么它说我的窗口是空的，什么时候不是。它必须是我在初始化窗口时所做的事情。
答案 0 :(得分：2)
为什么不起作用？
您没有正确使用 Singleton 设计模式。您是否注意到有两个JFrame弹出？
if (!ranOnce)
{
EventQueue.invokeLater(new Runnable()//this call the run method in a separate thread
{
public void run()
{
try
{
// for the current instance of Registration, you are setting
// the reg variable by calling another instance of Registration => thats not the correct way to do a Singleton
reg = new Registration();
reg.setVisible(true);
ranOnce = true;
}
catch (Exception e)
{
e.printStackTrace();
}
}
});
}
所以，假设你在main方法中调用new Registration()，你将进入public Registration()构造函数（让我们称之为instance1）并通过再次调用来设置变量reg构造函数new Registration()（instance2）但到目前为止，ranOnce将为true，因此EventQueue.invokeLater将不会被调用=＆gt;不会在instance2 =＆gt;中设置reg点击第二帧中的NullPointerException（顶部的那个）（instance2）时Jbutton。单击第一帧（instance1）按钮将隐藏第二帧（instance2）。
如何解决？
使用正确的 Singleton ：
private static final long serialVersionUID = 1L;
private static boolean ranOnce = false;
private JPanel contentPane;
private static Registration reg;
private JTextField userTF;
private JTextField passTF;
private JTextField emailTF;
private LoginProcess lp = new LoginProcess();
private JLabel error;
private JButton cancelBtn;
public static synchronized Registration getRegistration()
{
if(reg == null)
reg = new Registration();
return reg;
}
/**
* Create the frame.
*/
private Registration()
{
setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
setBounds(100, 100, 245, 212);
setLocationRelativeTo(null);
contentPane = new JPanel();
contentPane.setBorder(new EmptyBorder(5, 5, 5, 5));
setContentPane(contentPane);
contentPane.setLayout(null);
JLabel lblUsername = new JLabel("Username:");
lblUsername.setBounds(10, 11, 75, 14);
contentPane.add(lblUsername);
JLabel lblPassword = new JLabel("Password:");
lblPassword.setBounds(10, 36, 75, 14);
contentPane.add(lblPassword);
JLabel lblEmail = new JLabel("Email:");
lblEmail.setBounds(10, 61, 75, 14);
contentPane.add(lblEmail);
userTF = new JTextField();
userTF.setBounds(95, 8, 130, 20);
contentPane.add(userTF);
userTF.setColumns(10);
passTF = new JTextField();
passTF.setColumns(10);
passTF.setBounds(95, 33, 130, 20);
contentPane.add(passTF);
emailTF = new JTextField();
emailTF.setColumns(10);
emailTF.setBounds(95, 58, 130, 20);
contentPane.add(emailTF);
error = new JLabel("");
error.setBounds(10, 154, 215, 14);
contentPane.add(error);
JButton regBtn = new JButton("Register");
regBtn.addActionListener(new ActionListener() {
public void actionPerformed(ActionEvent arg0) {
if (!userTF.getText().equals("") || !passTF.getText().equals("") || !emailTF.getText().equals("")) {
try {
if (lp.newUser(userTF.getText(), passTF.getText(), emailTF.getText())) {
setVisible(false);
Main.mainFrame.setVisible(true);
} else {
if (lp.duplicateAccount) {
error.setText("Error: Username already in use.");
}
}
} catch (SQLException e) {
}
}
}
});
regBtn.setBounds(10, 120, 215, 23);
contentPane.add(regBtn);
cancelBtn = new JButton("Cancel");
cancelBtn.addActionListener(new ActionListener()
{
public void actionPerformed(ActionEvent arg0)
{
setVisible(false);
}
});
cancelBtn.setBounds(10, 86, 215, 23);
contentPane.add(cancelBtn);
}
public static void main(String[] args)
{
Registration registration = Registration.getRegistration();
registration.setVisible(true);
}
答案 1 :(得分：0)
尝试使用setVisible(fasle)代替reg.setVisible(false)