空指針異常問題

Question

我首先使用名爲chatWindow的JTextArea製作了即時通訊工具。之後我將其更改爲JTextPane。添加了我的樣式屬性之後，我在eclipse的控制檯中得到了一個空指針異常。這裏是我的構造函數和方法的類代碼：

import java.io.*; 
    import java.net.*; 
    import java.awt.*; 
    import java.awt.event.*; 

    import javax.swing.*; 
    import javax.swing.text.AttributeSet; 
    import javax.swing.text.SimpleAttributeSet; 
    import javax.swing.text.StyleConstants; 
    import javax.swing.text.StyledDocument; 

    public class Server extends JFrame{ 

     private JTextField userText; 

     private JTextPane chatWindow; 

     StyledDocument doc = chatWindow.getStyledDocument(); 

     //Defining attributes to varibles (keyword) 
     SimpleAttributeSet keyWord = new SimpleAttributeSet(); 



     private ObjectOutputStream output; 
     private ObjectInputStream input; 
     private ServerSocket server; 
     private Socket connection; 

     //constructor 
     public Server(){ 
      super("Mikey Mac Instant Messenger"); 
      userText = new JTextField(); 
      userText.setEditable(false); 
      userText.addActionListener(
       new ActionListener(){ 
        public void actionPerformed(ActionEvent event){ 
         sendMessage(event.getActionCommand()); 
         userText.setText(""); 
        } 
       } 
      ); 
      add(userText, BorderLayout.NORTH); 
      chatWindow = new JTextPane(); 
      chatWindow.setBackground(Color.getHSBColor(207, 24, 87)); 
      add(new JScrollPane(chatWindow)); 
      setSize(850,600); 
      setVisible(true); 
     } 

     //set up and run the server 
     public void startRunning(){ 
      try{ 
       server = new ServerSocket(6789, 100); 
       while(true){ 
        try{ 
         waitForConnection(); 
         setupStreams(); 
         whileChatting(); 
        }catch(EOFException eofException){ 
         showMessage("\n SYSTEM - Server ended the connection!"); 
        }finally{ 
         closeWindow(); 
        } 
       } 
      }catch(IOException ioException){ 
       ioException.printStackTrace(); 
      } 
     } 
     //wait for connection, then display connection information 
     private void waitForConnection() throws IOException{ 
      chatWindow.setEditable(false); 
      showMessage("SYSTEM - Waiting for someone to connect... \n"); 
      connection = server.accept(); 
      showMessage("SYSTEM - Now connected to " + connection.getInetAddress().getHostName()); 
     } 

     //get stream to send and recieve data 
     private void setupStreams() throws IOException{ 
      output = new ObjectOutputStream(connection.getOutputStream()); 
      output.flush(); 
      input = new ObjectInputStream(connection.getInputStream()); 
      showMessage("\n SYSTEM - Streams are now setup! \n"); 
     } 

     //during the chat conversation 
     private void whileChatting() throws IOException{ 
      String message = " SYSTEM - You are now connected!"; 
      sendMessage(message); 
      ableToType(true); 
      do{ 
       //have a conversation 
       try{ 
        message = (String) input.readObject(); 
        showMessage("\n" + message); 
       }catch(ClassNotFoundException classNotFoundException){ 
        showMessage("\n SYSTEM - I have no clue what the user just said!"); 
       } 
      }while(!message.equals("CLIENT - END")); 
     } 

     //close streams and sockets after you are done chatting 
     private void closeWindow(){ 
      showMessage("\n SYSTEM - Closing Connections... \n"); 
      ableToType(false); 
      try{ 
       output.close(); 
       input.close(); 
       connection.close(); 
      }catch(IOException ioException){ 
       ioException.printStackTrace(); 
      } 
     } 

     //send message to client 
     private void sendMessage(String message){ 
      try{ 

       output.writeObject("SERVER - " + message); 
       output.flush(); 
       showMessage("\nSERVER - " + message); 
      }catch(IOException ioException){ 
       //chatWindow.append("\n System Error: Dude I can't send this..."); 

       try 
       { 
        StyleConstants.setForeground(keyWord, Color.getHSBColor(351, 95, 95)); 
        StyleConstants.setBackground(keyWord, Color.YELLOW); 
        StyleConstants.setBold(keyWord, true); 

        doc.insertString(0, "System Error: Dude, I can't send this...", keyWord); 
       } 
       catch(Exception e) { System.out.println(e); } 
      } 
     } 

     //updates chatWindow 
     private void showMessage(final String string){ 
      SwingUtilities.invokeLater(
       new Runnable(){ 
        public void run(){ 
         //chatWindow.append(string); 
         //THE BOTTOM METHOD IS USED FOR APPENDING A STRING JTEXTPANE STYLE HAHA 
         try 
         { 
          //doc.insertString(0, "Start of text\n", null); 
          //doc.insertString(doc.getLength(), "", string); 
          //doc.insertString(int offset, String str, ArributeSet a); 

          //SETTING THE STYLE FOR THE STRING (down below) 

          StyleConstants.setForeground(keyWord, Color.getHSBColor(251, 89, 87)); 
          //StyleConstants.setBackground(keyWord, Color.YELLOW); 
          StyleConstants.setBold(keyWord, false); 

          doc.insertString(0, string, keyWord); 
         } 
         catch(Exception e) { System.out.println(e); } 
        } 
       } 
      ); 
     } 

     //let the user type stuff into their box 
     private void ableToType(final boolean tof){ 
      SwingUtilities.invokeLater(
        new Runnable(){ 
         public void run(){ 
          userText.setEditable(tof); 
         } 
        } 
       ); 
     } 

    } 

這裏是我的主要方法類代碼：

import javax.swing.JFrame; 

    public class ServerTest { 
     public static void main(String[] args) { 

      Server messaging = new Server(); 
      messaging.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); 
      messaging.startRunning(); 
     } 

    } 

我得到我的空指針異常在2點：

- The first one is here (In constructor): 
    **`StyledDocument doc = chatWindow.getStyledDocument();`** 

- The second one is here (In main): 
    **`Server messaging = new Server();`** 

以下是消息中我的控制檯甾體抗炎藥：

Exception in thread "main" java.lang.NullPointerException 
    at Server.<init>(Server.java:18) 
    at ServerTest.main(ServerTest.java:6) 

---

有誰知道我該如何解決這個空指針異常錯誤？謝謝。

Answer 1

StyledDocument doc = chatWindow.getStyledDocument(); 

是因爲你從未初始化chatWindow。將上面的初始化代碼移到你的構造函數中，它應該沒問題。您需要保留成員變量聲明：

StyledDocument doc; 

以便您稍後可以參考。引用評論：

chatWindow get在構造函數中初始化。但是在構造函數之外的初始化 發生得更早，並且chatWindow 在那時是空的。