尝试更改JFrame的标题时发生NullPointerException

如何解决尝试更改JFrame的标题时发生NullPointerException

| 我正在创建一个简单的文本编辑器，当我打开文件时，我想更改为JFrame的标题栏，以包括用户刚刚打开的文件的目录和文件名。我在JFrame API中使用setTitle（..）方法，但是由于某种原因，我收到了空指针异常。

public void actionPerformed(ActionEvent ae) 
  {
    //find out what event occured
     String command = ae.getActionCommand();

     String s = \" command: \" + command;
     JOptionPane.showMessageDialog(null,s);

     if(command.equals(\"New..\")) //the New file option was selected
     {
        output.setEditable(true);
        //createNewFile();
        isNew = true;
     }
     else if(command.equals(\"Open..\"))//the Open file option was selected
     {
        output.setEditable(true);
        openFile();
        //both className and currentFile are defined
        frame.setTitle(className + \": \" + currentFile.toString()); //Exception here
     }
     else if(command.equals(\"Save..\"))//the Save file option was selected
     {
        saveFile();
     }
     else if(command.equals(\"Save As..\"))//the Save file option was selected
     {
        saveFileAs();
     }
     else if(command.equals(\"Exit..\"))//the Exit program option was selected
     {
        exitProgram();
     }
  }     

这也是堆栈跟踪：

Exception in thread \"AWT-EventQueue-0\" java.lang.NullPointerException
at jtexteditor.JTextEditor.actionPerformed(JTextEditor.java:307)
at javax.swing.AbstractButton.fireActionPerformed(AbstractButton.java:1995)
at javax.swing.AbstractButton$Handler.actionPerformed(AbstractButton.java:2318)
at javax.swing.DefaultButtonModel.fireActionPerformed(DefaultButtonModel.java:387)
at javax.swing.DefaultButtonModel.setPressed(DefaultButtonModel.java:242)
at javax.swing.AbstractButton.doClick(AbstractButton.java:357)
at javax.swing.plaf.basic.BasicMenuItemUI.doClick(BasicMenuItemUI.java:809)
at javax.swing.plaf.basic.BasicMenuItemUI$Handler.mouseReleased(BasicMenuItemUI.java:850)
at java.awt.Component.processMouseEvent(Component.java:6288)
at javax.swing.JComponent.processMouseEvent(JComponent.java:3267)
at java.awt.Component.processEvent(Component.java:6053)
at java.awt.Container.processEvent(Container.java:2041)
at java.awt.Component.dispatchEventImpl(Component.java:4651)
at java.awt.Container.dispatchEventImpl(Container.java:2099)
at java.awt.Component.dispatchEvent(Component.java:4481)
at java.awt.LightweightDispatcher.retargetMouseEvent(Container.java:4577)
at java.awt.LightweightDispatcher.processMouseEvent(Container.java:4238)
at java.awt.LightweightDispatcher.dispatchEvent(Container.java:4168)
at java.awt.Container.dispatchEventImpl(Container.java:2085)
at java.awt.Window.dispatchEventImpl(Window.java:2478)
at java.awt.Component.dispatchEvent(Component.java:4481)
at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:643)
at java.awt.EventQueue.access$000(EventQueue.java:84)
at java.awt.EventQueue$1.run(EventQueue.java:602)
at java.awt.EventQueue$1.run(EventQueue.java:600)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.AccessControlContext$1.doIntersectionPrivilege(AccessControlContext.java:87)
at java.security.AccessControlContext$1.doIntersectionPrivilege(AccessControlContext.java:98)
at java.awt.EventQueue$2.run(EventQueue.java:616)
at java.awt.EventQueue$2.run(EventQueue.java:614)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.AccessControlContext$1.doIntersectionPrivilege(AccessControlContext.java:87)
at java.awt.EventQueue.dispatchEvent(EventQueue.java:613)
at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:269)
at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:184)
at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:174)
at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:169)
at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:161)
at java.awt.EventDispatchThread.run(EventDispatchThread.java:122)

---- jGRASP：操作完成。 任何帮助都会很棒。 猎人。     

解决方法

        您对

frame

的引用为空。核实。