为什么我的Swing GUI不能从一个GUI切换到另一个GUI？

Question

我正在用临时用户名系统构建一个简单的聊天服务器。当屏幕弹出时，它会以简单的屏幕开始询问您的用户名。你可以放入任何你想要的东西，这纯粹是暂时的（我也在试验）。该代码告诉程序继续，将用户名保存到临时使用的代码中，然后输入主聊天服务器（目前为止，这只是一个GUI）。但这不是什么情况。相反，Java非常粗鲁地告诉我，我犯了一个错误，并且我是个坏人。我不确定我做错了什么或者发生了什么，因为一切似乎都很好，我认为我的按钮听众是正确的。我的错误是什么，我该如何解决？这里是我的代码：

package coltGUI; 

import java.awt.BorderLayout; 
import java.awt.Color; 
import java.awt.Font; 
import java.awt.GridBagConstraints; 
import java.awt.GridBagLayout; 
import java.awt.event.ActionEvent; 
import java.awt.event.ActionListener; 

import javax.swing.JButton; 
import javax.swing.JFrame; 
import javax.swing.JLabel; 
import javax.swing.JPanel; 
import javax.swing.JScrollPane; 
import javax.swing.JTextArea; 
import javax.swing.JTextField; 
import javax.swing.UIManager; 

public class MainGUI { 

    String username; 

    public static void main(String[] args) { 
     try { 
      UIManager.setLookAndFeel(UIManager.getSystemLookAndFeelClassName()); 
     } catch (Exception e) { 
      e.printStackTrace(); 
     } 

     MainGUI gui = new MainGUI(); 
     gui.preDisplay(); 
    } 

    JFrame frame; 
    JButton sendMessage; 
    JTextField messageBox; 
    JTextArea chatBox; 
    JTextArea usernameChooser; 
    JFrame preFrame; 

    private void preDisplay() { 
     JFrame preFrame = new JFrame("Choose your username!(Colt chat v 0.1"); 
     JTextField usernameChooser = new JTextField(); 
     JLabel chooseUsernameLabel = new JLabel("Pick a username:"); 
     JButton enterServer = new JButton("Enter Chat Server"); 
     JPanel prePanel = new JPanel(new GridBagLayout()); 
     GridBagConstraints preRight = new GridBagConstraints(); 
     preRight.anchor = GridBagConstraints.EAST; 
     GridBagConstraints preLeft = new GridBagConstraints(); 
     preLeft.anchor = GridBagConstraints.WEST; 
     preRight.weightx = 2.0; 
     preRight.fill = GridBagConstraints.HORIZONTAL; 
     preRight.gridwidth = GridBagConstraints.REMAINDER; 
     preFrame.add(BorderLayout.CENTER, prePanel); 
     prePanel.add(chooseUsernameLabel, preLeft); 
     prePanel.add(usernameChooser, preRight); 
     preFrame.add(BorderLayout.SOUTH, enterServer); 
     preFrame.setVisible(true); 
     preFrame.setSize(300, 300); 

     enterServer.addActionListener(new enterServerButtonListener()); 
    } 

    public void display() { 

     frame = new JFrame("Colt Chat (Alpha 0.1)"); 
     JPanel southPanel = new JPanel(); 

     frame.add(BorderLayout.SOUTH, southPanel); 
     southPanel.setBackground(Color.BLUE); 
     southPanel.setLayout(new GridBagLayout()); 

     messageBox = new JTextField(30); 
     sendMessage = new JButton("Send Message"); 
     chatBox = new JTextArea(); 
     chatBox.setEditable(false); 
     frame.add(new JScrollPane(chatBox), BorderLayout.CENTER); 

     chatBox.setLineWrap(true); 

     GridBagConstraints left = new GridBagConstraints(); 
     left.anchor = GridBagConstraints.WEST; 
     GridBagConstraints right = new GridBagConstraints(); 
     right.anchor = GridBagConstraints.EAST; 
     right.weightx = 2.0; 

     southPanel.add(messageBox, left); 
     southPanel.add(sendMessage, right); 

     chatBox.setFont(new Font("Serif", Font.PLAIN, 15)); 
     sendMessage.addActionListener(new sendMessageButtonListener()); 
     frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); 
     frame.setSize(470, 300); 
    } 

    class sendMessageButtonListener implements ActionListener { 
     public void actionPerformed(ActionEvent event) { 
      if (messageBox.getText().length() < 1) { 
       // do nothing 
      } else if (messageBox.getText().equals(".clear")) { 
       chatBox.setText("Cleared all messages\n"); 
       messageBox.setText(""); 
      } else { 
       chatBox.append("<" + username + ">:" + messageBox.getText() + "\n"); 
       messageBox.setText(""); 
      } 
     } 
    } 

    class enterServerButtonListener implements ActionListener { 
     public void actionPerformed(ActionEvent event) { 
      username = usernameChooser.getText(); 
      preFrame.setVisible(false); 
      frame.setVisible(true); 

     } 

    } 
} 

而这就是那个错误。当我尝试按它发生了“输入服务器”上输入用户名GUI按钮：

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException 
    at coltGUI.MainGUI$enterServerButtonListener.actionPerformed(MainGUI.java:113) 
    at javax.swing.AbstractButton.fireActionPerformed(Unknown Source) 
    at javax.swing.AbstractButton$Handler.actionPerformed(Unknown Source) 
    at javax.swing.DefaultButtonModel.fireActionPerformed(Unknown Source) 
    at javax.swing.DefaultButtonModel.setPressed(Unknown Source) 
    at javax.swing.plaf.basic.BasicButtonListener.mouseReleased(Unknown Source) 
    at java.awt.Component.processMouseEvent(Unknown Source) 
    at javax.swing.JComponent.processMouseEvent(Unknown Source) 
    at java.awt.Component.processEvent(Unknown Source) 
    at java.awt.Container.processEvent(Unknown Source) 
    at java.awt.Component.dispatchEventImpl(Unknown Source) 
    at java.awt.Container.dispatchEventImpl(Unknown Source) 
    at java.awt.Component.dispatchEvent(Unknown Source) 
    at java.awt.LightweightDispatcher.retargetMouseEvent(Unknown Source) 
    at java.awt.LightweightDispatcher.processMouseEvent(Unknown Source) 
    at java.awt.LightweightDispatcher.dispatchEvent(Unknown Source) 
    at java.awt.Container.dispatchEventImpl(Unknown Source) 
    at java.awt.Window.dispatchEventImpl(Unknown Source) 
    at java.awt.Component.dispatchEvent(Unknown Source) 
    at java.awt.EventQueue.dispatchEventImpl(Unknown Source) 
    at java.awt.EventQueue.access$000(Unknown Source) 
    at java.awt.EventQueue$3.run(Unknown Source) 
    at java.awt.EventQueue$3.run(Unknown Source) 
    at java.security.AccessController.doPrivileged(Native Method) 
    at java.security.ProtectionDomain$1.doIntersectionPrivilege(Unknown Source) 
    at java.security.ProtectionDomain$1.doIntersectionPrivilege(Unknown Source) 
    at java.awt.EventQueue$4.run(Unknown Source) 
    at java.awt.EventQueue$4.run(Unknown Source) 
    at java.security.AccessController.doPrivileged(Native Method) 
    at java.security.ProtectionDomain$1.doIntersectionPrivilege(Unknown Source) 
    at java.awt.EventQueue.dispatchEvent(Unknown Source) 
    at java.awt.EventDispatchThread.pumpOneEventForFilters(Unknown Source) 
    at java.awt.EventDispatchThread.pumpEventsForFilter(Unknown Source) 
    at java.awt.EventDispatchThread.pumpEventsForHierarchy(Unknown Source) 
    at java.awt.EventDispatchThread.pumpEvents(Unknown Source) 
    at java.awt.EventDispatchThread.pumpEvents(Unknown Source) 
    at java.awt.EventDispatchThread.run(Unknown Source) 

Answer 1

因为你的听众是依靠尚未初始化

JFrame frame; 
JButton sendMessage; 
JTextField messageBox; 
JTextArea chatBox; 
JTextArea usernameChooser; 
JFrame preFrame; 

private void preDisplay() { 
    // All the above fields have been redeclared here and are local to the constrcutor 
    JFrame preFrame = new JFrame("Choose your username!(Colt chat v 0.1"); 
    JTextField usernameChooser = new JTextField(); 
    JLabel chooseUsernameLabel = new JLabel("Pick a username:"); 
    JButton enterServer = new JButton("Enter Chat Server"); 
    JPanel prePanel = new JPanel(new GridBagLayout()); 
    GridBagConstraints preRight = new GridBagConstraints(); 
    preRight.anchor = GridBagConstraints.EAST; 
    GridBagConstraints preLeft = new GridBagConstraints(); 

应该看起来更像领域...

JFrame frame; 
JButton sendMessage; 
JTextField messageBox; 
JTextArea chatBox; 
JTextArea usernameChooser; 
JFrame preFrame; 

private void preDisplay() { 
    // All the above fields have been redeclared here and are local to the constrcutor 
    preFrame = new JFrame("Choose your username!(Colt chat v 0.1"); 
    usernameChooser = new JTextField(); 
    JLabel chooseUsernameLabel = new JLabel("Pick a username:"); 
    JButton enterServer = new JButton("Enter Chat Server"); 
    JPanel prePanel = new JPanel(new GridBagLayout());