为什么我尝试在PrintWriter上写异常时抛出异常？

| 我有一个简单的ChatClient，可以将数据发送到服务器。为了使Chat Client发送数据，我制作了一个Socket，将PrintWriter制作到socket.getOutputStream（），然后执行writer.println（\“ \”）。由于某种原因，我得到了一个很大的例外。我查看了与我类似的例外，很多人说要更新Java。但是我的Mac上有最新版本。是因为我有Leopard，还是我的伪劣编程？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)**

我不知道这是什么意思。请帮忙。