Java 为什么会出现空指针错误。？

选择菜单项后，我试图显示一个自定义对话框，但是当我设置onClickListener时，如果没有onClickListner，onClickListener工作正常，将显示一个空指针错误。我检查了按钮id的错误，一切都是正确的

你知道为什么吗

  public void  unlinkDeviceClick(MenuItem item){

   final Dialog alertDiag = new Dialog(Home.this); 
   alertDiag.setContentView(R.layout.unlinkdialog);
   alertDiag.setTitle("Unlink Device"); 

   Button yes = (Button)findViewById(R.id.signout);
   Button cancel = (Button)findViewById(R.id.cancelsignout);


   final EditText usernameField = (EditText)findViewById(R.id.usernameField);
   final EditText passwordField = (EditText)findViewById(R.id.passwordField);   
   final TextView descr = (TextView)findViewById(R.id.unlinkresults);    


   yes.setOnClickListener(new OnClickListener(){

       public void onClick(View v) {
        // TODO Auto-generated method stub
        String userName = usernameField.getText().toString();
        String password = passwordField.getText().toString();

        if(checkLoginFields(userName,password)){        
            if(checkCredentials(userName,password)){
                //loadUnlink(); 
                descr.setText("Sucessfully unlinked");
            }
            else{
                showWrongInfo(new View(Home.this));
            }
        }
        else{

        }

       }

   });


   cancel.setOnClickListener(new OnClickListener(){

        public void onClick(View v) {
        // TODO Auto-generated method stub
            alertDiag.dismiss();
        }

   });


   alertDiag.show();
}

//////////////////////堆栈跟踪//////////////////////////////////////

    04-19 10:30:32.198: E/AndroidRuntime(20386): FATAL EXCEPTION: main
04-19 10:30:32.198: E/AndroidRuntime(20386): java.lang.RuntimeException: java.lang.reflect.InvocationTargetException
04-19 10:30:32.198: E/AndroidRuntime(20386):    at android.view.MenuInflater$InflatedOnMenuItemClickListener.onMenuItemClick(MenuInflater.java:203)
04-19 10:30:32.198: E/AndroidRuntime(20386):    at com.android.internal.view.menu.MenuItemImpl.invoke(MenuItemImpl.java:154)
04-19 10:30:32.198: E/AndroidRuntime(20386):    at com.android.internal.view.menu.MenuBuilder.performItemAction(MenuBuilder.java:964)
04-19 10:30:32.198: E/AndroidRuntime(20386):    at com.android.internal.view.menu.MenuBuilder$ActionMenuAdapter$1.onClick(MenuBuilder.java:1565)
04-19 10:30:32.198: E/AndroidRuntime(20386):    at android.view.View.performClick(View.java:3135)
04-19 10:30:32.198: E/AndroidRuntime(20386):    at android.view.View.onKeyUp(View.java:5055)
04-19 10:30:32.198: E/AndroidRuntime(20386):    at android.view.KeyEvent.dispatch(KeyEvent.java:2623)
04-19 10:30:32.198: E/AndroidRuntime(20386):    at android.view.View.dispatchKeyEvent(View.java:4603)
04-19 10:30:32.198: E/AndroidRuntime(20386):    at android.view.ViewGroup.dispatchKeyEvent(ViewGroup.java:1157)
04-19 10:30:32.198: E/AndroidRuntime(20386):    at android.view.ViewGroup.dispatchKeyEvent(ViewGroup.java:1159)
04-19 10:30:32.198: E/AndroidRuntime(20386):    at android.widget.ListView.dispatchKeyEvent(ListView.java:2061)
04-19 10:30:32.198: E/AndroidRuntime(20386):    at android.view.ViewGroup.dispatchKeyEvent(ViewGroup.java:1159)
04-19 10:30:32.198: E/AndroidRuntime(20386):    at android.view.ViewGroup.dispatchKeyEvent(ViewGroup.java:1159)
04-19 10:30:32.198: E/AndroidRuntime(20386):    at android.view.ViewGroup.dispatchKeyEvent(ViewGroup.java:1159)
04-19 10:30:32.198: E/AndroidRuntime(20386):    at android.view.ViewGroup.dispatchKeyEvent(ViewGroup.java:1159)
04-19 10:30:32.198: E/AndroidRuntime(20386):    at android.view.ViewGroup.dispatchKeyEvent(ViewGroup.java:1159)
04-19 10:30:32.198: E/AndroidRuntime(20386):    at com.android.internal.policy.impl.tv.TvWindow$DecorView.superDispatchKeyEvent(TvWindow.java:1161)
04-19 10:30:32.198: E/AndroidRuntime(20386):    at com.android.internal.policy.impl.tv.TvWindow.superDispatchKeyEvent(TvWindow.java:803)
04-19 10:30:32.198: E/AndroidRuntime(20386):    at android.app.Dialog.dispatchKeyEvent(Dialog.java:706)
04-19 10:30:32.198: E/AndroidRuntime(20386):    at com.android.internal.policy.impl.tv.TvWindow$DecorView.dispatchKeyEvent(TvWindow.java:1117)
04-19 10:30:32.198: E/AndroidRuntime(20386):    at android.view.ViewRoot.deliverKeyEventPostIme(ViewRoot.java:2902)
04-19 10:30:32.198: E/AndroidRuntime(20386):    at android.view.ViewRoot.deliverKeyEvent(ViewRoot.java:2860)
04-19 10:30:32.198: E/AndroidRuntime(20386):    at android.view.ViewRoot.handleMessage(ViewRoot.java:2059)
04-19 10:30:32.198: E/AndroidRuntime(20386):    at android.os.Handler.dispatchMessage(Handler.java:99)
04-19 10:30:32.198: E/AndroidRuntime(20386):    at android.os.Looper.loop(Looper.java:132)
04-19 10:30:32.198: E/AndroidRuntime(20386):    at android.app.ActivityThread.main(ActivityThread.java:4083)
04-19 10:30:32.198: E/AndroidRuntime(20386):    at java.lang.reflect.Method.invokeNative(Native Method)
04-19 10:30:32.198: E/AndroidRuntime(20386):    at java.lang.reflect.Method.invoke(Method.java:491)
04-19 10:30:32.198: E/AndroidRuntime(20386):    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:841)
04-19 10:30:32.198: E/AndroidRuntime(20386):    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:599)
04-19 10:30:32.198: E/AndroidRuntime(20386):    at dalvik.system.NativeStart.main(Native Method)
04-19 10:30:32.198: E/AndroidRuntime(20386): Caused by: java.lang.reflect.InvocationTargetException
04-19 10:30:32.198: E/AndroidRuntime(20386):    at java.lang.reflect.Method.invokeNative(Native Method)
04-19 10:30:32.198: E/AndroidRuntime(20386):    at java.lang.reflect.Method.invoke(Method.java:491)
04-19 10:30:32.198: E/AndroidRuntime(20386):    at android.view.MenuInflater$InflatedOnMenuItemClickListener.onMenuItemClick(MenuInflater.java:199)
04-19 10:30:32.198: E/AndroidRuntime(20386):    ... 30 more
04-19 10:30:32.198: E/AndroidRuntime(20386): Caused by: java.lang.NullPointerException
04-19 10:30:32.198: E/AndroidRuntime(20386):    at com.ameba.api.activityClasses.Home.unlinkDeviceClick(Home.java:604)
04-19 10:30:32.198: E/AndroidRuntime(20386):    ... 33 more

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没有CallStack，很难说。不过有一个想法，也许findViewById提供的小部件是空的

最好的方法是检查调用堆栈，以确定是“是”按钮还是“取消”按钮出错，或者最好使用断点并逐步查找代码中缺少的内容

如果对话框中有“是”和“取消”按钮，则需要在设置的对话框布局中搜索它们：

Button yes = (Button) alertDialog.findViewById(R.id.signout);
Button cancel = (Button) alertDialog.findViewById(R.id.cancelsignout);

---

我猜这是因为findViewByIdR.id.signout返回null？我检查了signout按钮的id，它确实是正确的。为什么为空？signout.SetOnClickListener新建OnClickListener{我不确定你的意思，我创建了自己的自定义对话框xml，其中有两个按钮“是”和“取消”。你的意思是：dialog.findviewbydr.id.signout？@Fabii你的按钮只存在于你在对话框R.layout.unlinkdialog中设置的布局中。如果你找到了findViewById，你将在当前活动布局中搜索它们，你将在其中我找不到它们并且有一个NullPointerException。您必须在对话框对象上使用findViewById，在对话框对象上设置包含按钮的布局。我认为@Luksprog猜对了：findViewById方法查找其所有者的小部件层次结构。从活动调用它意味着您希望在活动v的层次结构中有一个小部件查看。但是“是/取消”按钮位于对话框层次结构中，因此正确答案应该是alertDiag.findViewById。。。