Получение исключения: - java.lang.NullPointerException?

Question

Получение следующее исключение: -

java.lang.NullPointerException 
    at com.local.testing.ChatClient$sendButtonActionListener.actionPerformed(ChatClient.java:53) 
    at javax.swing.AbstractButton.fireActionPerformed(Unknown Source) 
    at javax.swing.AbstractButton$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.dispatchEvent(Unknown Source) 
    at java.awt.EventDispatchThread.pumpOneEventForHierarchy(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) 

Я использую следующие два класса: -

package com.local.testing; 

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

public class ChatClient { 
    JTextField outgoing=null; 
    Socket sock=null; 
    PrintWriter writer=null; 
    public void buildGui(){ 
     JFrame frame = new JFrame("ChatClient"); 
     JPanel mainPanel = new JPanel(); 
     outgoing = new JTextField(20); 
     JButton send = new JButton("send"); 
     send.addActionListener(new sendButtonActionListener()); 
     mainPanel.add(outgoing); 
     mainPanel.add(send); 
     frame.getContentPane().add(BorderLayout.CENTER,mainPanel); 
     frame.setVisible(true); 
     frame.setSize(400, 400); 
     frame.pack(); 

     setUpNetworking(); 
    } 

    private void setUpNetworking(){ 
     try{ 
     sock = new Socket("127.0.0.1",4242); 
     writer = new PrintWriter(sock.getOutputStream()); 
     System.out.println("Connection Established"); 
    } 
     catch(UnknownHostException uhe){ 
      uhe.printStackTrace(); 
     } 
     catch(IOException ioe){ 
      ioe.getMessage(); 
     } 
     finally{ 
      if(writer !=null){ 
       writer.close(); 
      } 
     } 
} 
    public class sendButtonActionListener implements ActionListener{ 
     public void actionPerformed(ActionEvent arg0) { 
      // TODO Auto-generated method stub 
      //System.out.println(outgoing.getText()); 
      try{ 

      writer.println(outgoing.getText()); 
      writer.flush(); 
      } 
      catch(Exception e){ 
       e.printStackTrace(); 
      } 
      outgoing.setText(""); 
      outgoing.requestFocus(); 
     } 
    } 

    public static void main(String []x){ 
     ChatClient client = new ChatClient(); 
     client.buildGui(); 
    } 
} 

---

package com.local.testing; 

import java.io.*; 
import java.net.*; 

public class ChatServer { 

    public void connectToClient(){ 
     try { 
      ServerSocket serverSocket = new ServerSocket(4242); 
      while(true) 
      { 
       Socket sock = serverSocket.accept(); 
       InputStreamReader stream = new InputStreamReader(sock.getInputStream()); 
       BufferedReader reader = new BufferedReader(stream); 
       String chat=reader.readLine(); 
       System.out.println(chat); 
      } 
     } 
     catch (IOException e) { 
      // TODO Auto-generated catch block 
      e.printStackTrace(); 
     } 
    } 

    public static void main(String [] x){ 
     ChatServer server = new ChatServer(); 
     server.connectToClient(); 
    } 
} 

Что может быть проблема?

Answer 1

Существует что-то не так с вашим методом setUpNetworking. Если запустить следующую, вы увидите, что он печатает test/test2/null . Таким образом, вы пойманной в IOException и, следовательно, writer не инициализируется, который будет бросками NPE:

private void setUpNetworking(){ 
     System.out.println("test"); 
     try{ 
     sock = new Socket("127.0.0.1",4242); 
     writer = new PrintWriter(sock.getOutputStream()); 
     } 
     catch(UnknownHostException uhe){ 
      System.out.println("test1"); 
      uhe.printStackTrace(); 
     } 
     catch(IOException ioe){ 
      System.out.println("test2"); 
      ioe.getMessage(); 
     } 
     finally{ 
      if(writer !=null){ 
       writer.close(); 
      } 
     } 
     System.out.println(writer); 
} 

При печати ГНА ck trace:

java.net.ConnectException: Connection refused: connect 
at java.net.DualStackPlainSocketImpl.connect0(Native Method) 
at java.net.DualStackPlainSocketImpl.socketConnect(Unknown Source) 
at java.net.AbstractPlainSocketImpl.doConnect(Unknown Source) 
at java.net.AbstractPlainSocketImpl.connectToAddress(Unknown Source) 
at java.net.AbstractPlainSocketImpl.connect(Unknown Source) 
at java.net.PlainSocketImpl.connect(Unknown Source) 
at java.net.SocksSocketImpl.connect(Unknown Source) 
at java.net.Socket.connect(Unknown Source) 
at java.net.Socket.connect(Unknown Source) 
at java.net.Socket.<init>(Unknown Source) 
at java.net.Socket.<init>(Unknown Source) 
at com.local.testing.ChatClient.setUpNetworking(ChatClient.java:31) 
at com.local.testing.ChatClient.buildGui(ChatClient.java:14) 
at com.local.testing.ChatClient.main(ChatClient.java:62) 

Таким образом, вы должны исправить эту ошибку.

Answer 2

Кажется, что ваш обработчик событий был вызван до вашей инициализации в buildGui(). Поэтому в основном убедитесь, что эта функция называется earlied, например. в конструкторе.

Answer 3

writer.println(outgoing.getText()); 

автор не обнуляется и не устанавливался на объект перед выше вызовом. (JTextField outgoing=null;