java - NullPointerException:LinearLayout 为 Null

Question

请看下面的代码

private class OpenFileEvent implements OnClickListener
    {

        @Override
        public void onClick(View arg0) {

            LinearLayout openFileDialogView = (LinearLayout)findViewById(R.id.open_file_dialog);

            // TODO Auto-generated method stub
            final Dialog openFileDialog = new Dialog(VoiceNotes.this);
            openFileDialog.setTitle("Open File");
            openFileDialog.setContentView(R.layout.open_dialog);


            //First, list all the available Files
            File folder = new File(Environment.getExternalStorageDirectory()+"/Voice/Notes/");
            File file = new File(folder.getAbsolutePath());

            File[] fileNameList = file.listFiles();


            if(fileNameList != null && fileNameList.length>0)
            {
                for(int i=0;i<fileNameList.length;i++)
                {
                    //Get the sub views first
                    LayoutInflater inflater = (LayoutInflater)getSystemService(Context.LAYOUT_INFLATER_SERVICE);

                    View openThisFileView = inflater.inflate(R.layout.open_dialog_file, null);      
                    Button openThisFileButton = (Button)openThisFileView.findViewById(R.id.open_this_file_button);
                    Button appendThisFileButton = (Button)openThisFileView.findViewById(R.id.append_note_this_file);
                    TextView openThisFileNameTxt = (TextView)openThisFileView.findViewById(R.id.open_this_file_name);

                    //Set the Text
                    openThisFileNameTxt.setText(fileNameList[i].getName());

                    //Set the Listeners


                    //Add the View
                    openFileDialogView.addView(openThisFileView);

                }
            }
            //Show the Dialog
            openFileDialog.show();

        }

    }

一旦运行此代码，我就会收到以下错误消息，即 NullPointerException

11-18 16:24:20.832: E/AndroidRuntime(1019): FATAL EXCEPTION: main
11-18 16:24:20.832: E/AndroidRuntime(1019): java.lang.NullPointerException
11-18 16:24:20.832: E/AndroidRuntime(1019):     at com.x.xxx.VoiceNotes$OpenFileEvent.onClick(VoiceNotes.java:227)
11-18 16:24:20.832: E/AndroidRuntime(1019):     at android.view.View.performClick(View.java:4204)
11-18 16:24:20.832: E/AndroidRuntime(1019):     at android.view.View$PerformClick.run(View.java:17355)
11-18 16:24:20.832: E/AndroidRuntime(1019):     at android.os.Handler.handleCallback(Handler.java:725)
11-18 16:24:20.832: E/AndroidRuntime(1019):     at android.os.Handler.dispatchMessage(Handler.java:92)
11-18 16:24:20.832: E/AndroidRuntime(1019):     at android.os.Looper.loop(Looper.java:137)
11-18 16:24:20.832: E/AndroidRuntime(1019):     at android.app.ActivityThread.main(ActivityThread.java:5041)
11-18 16:24:20.832: E/AndroidRuntime(1019):     at java.lang.reflect.Method.invokeNative(Native Method)
11-18 16:24:20.832: E/AndroidRuntime(1019):     at java.lang.reflect.Method.invoke(Method.java:511)
11-18 16:24:20.832: E/AndroidRuntime(1019):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:793)
11-18 16:24:20.832: E/AndroidRuntime(1019):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:560)
11-18 16:24:20.832: E/AndroidRuntime(1019):     at dalvik.system.NativeStart.main(Native Method)

错误指向此处

openFileDialogView.addView(openThisFileView);

由于它表示 LinearLayout 为 null，因此这里是该布局所属的 XML 文件。

open_dialog.xml

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:id="@+id/open_file_dialog"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical" 
    android:background="#ffffff">

</LinearLayout>

这与充当 Dialog 的内容 View 的布局相同。

为什么我认为 LinearLayout 为 null，而不是类内膨胀的布局？因为下面的代码还生成了 NullPointerException

openFileDialogView.addView(new Button(Notes.this));

为什么我会收到此错误？

Answer 1

LinearLayout openFileDialogView = (LinearLayout)findViewById(R.id.open_file_dialog);

此处 findViewById() 尝试在 Activity 的 View 层次结构中查找指定 View (由 setContentView() 设置)，但您仅使用该 id 来扩展布局稍后。findViewById() 返回 null 并尝试调用 null 上的方法会导致 NPE。