为什么当我尝试在 PrintWriter 上写入时会抛出异常？

Question

我有一个简单的 ChatClient，它将数据发送到服务器。为了让聊天客户端发送数据，我创建一个 Socket，为 socket.getOutputStream() 创建一个 PrintWriter，然后执行 writer.println("")。由于某种原因，我得到了一个巨大的异常。我查看了与我类似的异常，很多人都说更新java。但我的 Mac 上有最新版本。是因为我有Leopard，还是我的程序粗制滥造？请看一下。

private void setUpNetworking() {

        try {
            **sock = new Socket("127.0.0.1", 65534);**
            InputStreamReader streamReader = new InputStreamReader(sock.getInputStream());
            reader = new BufferedReader(streamReader);

            **writer = new PrintWriter(sock.getOutputStream());**
            System.out.println("made connection... everything here works");

        }
        catch (IOException ex) {
            ex.printStackTrace();
            System.out.println("setup fail, but i didn't get this");

        }
    }

    public class SendButtonListener implements ActionListener{
        public void actionPerformed (ActionEvent ev){
            try {
                **writer.println(entertext.getText());**
                **writer.flush();**
                System.out.println("PROBLEM IS RIGHT HERE!!!");

            }
            catch (Exception ex) {
                ex.printStackTrace();
                System.out.println("i get a exception. HELP");

            }

这是我的例外：

java.lang.NullPointerException
    at SimpleChatClient$SendButtonListener.actionPerformed(SimpleChatClient.java:52)
    at javax.swing.AbstractButton.fireActionPerformed(AbstractButton.java:1882)
    at javax.swing.AbstractButton$Handler.actionPerformed(AbstractButton.java:2202)
    at javax.swing.DefaultButtonModel.fireActionPerformed(DefaultButtonModel.java:420)
    at javax.swing.DefaultButtonModel.setPressed(DefaultButtonModel.java:258)
    at javax.swing.plaf.basic.BasicButtonListener.mouseReleased(BasicButtonListener.java:246)
    at java.awt.Component.processMouseEvent(Component.java:5617)
    at javax.swing.JComponent.processMouseEvent(JComponent.java:3129)
    at java.awt.Component.processEvent(Component.java:5382)
    at java.awt.Container.processEvent(Container.java:2010)
    at java.awt.Component.dispatchEventImpl(Component.java:4083)
    at java.awt.Container.dispatchEventImpl(Container.java:2068)
    at java.awt.Component.dispatchEvent(Component.java:3918)
    at java.awt.LightweightDispatcher.retargetMouseEvent(Container.java:4256)
    at java.awt.LightweightDispatcher.processMouseEvent(Container.java:3936)
    at java.awt.LightweightDispatcher.dispatchEvent(Container.java:3866)
    at java.awt.Container.dispatchEventImpl(Container.java:2054)
    at java.awt.Window.dispatchEventImpl(Window.java:1801)
    at java.awt.Component.dispatchEvent(Component.java:3918)
    at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:501)
    at java.awt.EventQueue.access$000(EventQueue.java:80)
    at java.awt.EventQueue$1.run(EventQueue.java:462)
    at java.awt.EventQueue$1.run(EventQueue.java:461)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.security.AccessControlContext$1.doIntersectionPrivilege(AccessControlContext.java:84)
    at java.security.AccessControlContext$1.doIntersectionPrivilege(AccessControlContext.java:95)
    at java.awt.EventQueue$2.run(EventQueue.java:476)
    at java.awt.EventQueue$2.run(EventQueue.java:475)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.security.AccessControlContext$1.doIntersectionPrivilege(AccessControlContext.java:84)
    at java.awt.EventQueue.dispatchEvent(EventQueue.java:473)
    at java.awt.EventDispatchThread.pumpOneEventForHierarchy(EventDispatchThread.java:269)
    at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:190)
    at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:184)
    at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:176)
    **at java.awt.EventDispatchThread.run(EventDispatchThread.java:110)**

我不知道这意味着什么。请帮忙。

Answer 1

从堆栈跟踪和注释来看，听起来像是当您调用

writer.println(entertext.getText());

SendButtonListener 的 actionPerformed 方法时，或者 writer enterText 为空。

哪个是 null，以及它是如何以这种方式获得的，从代码中尚不清楚，但您应该能够通过调试器或日志记录来找出它。

Answer 2

以下是如何调试它 - 要么：

• 放入一个单独的 println 来打印每个可能为空的东西，就在它出现异常之前，
• 或者在调试器中运行它（例如 Eclipse 提供了一个）并且当你得到该行时看看变量的值是什么

Answer 3

Chekc.SimpleChatClient的第52行，有ir.null的东西