NullPointerException：无法列出目录中的文件

Question

请看下面的代码

private class OpenFileEvent implements OnClickListener
{
    LinearLayout openFileDialogView = (LinearLayout)findViewById(R.id.open_file_dialog);

    @Override
    public void onClick(View arg0) {
        // TODO Auto-generated method stub
        final Dialog openFileDialog = new Dialog(Notes.this);
        openFileDialog.setTitle("Open File");
        openFileDialog.setContentView(R.layout.open_dialog);


        //First, list all the available Files
        File folder = new File(Environment.getExternalStorageDirectory()+"/Main Notes/Notes");
        File[] fileNameList = folder.listFiles();

        if(fileNameList.length>0 && fileNameList!=null)
        {
            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，就会出现以下错误

11-17 17:31:18.681: E/AndroidRuntime(4868): FATAL EXCEPTION: main
11-17 17:31:18.681: E/AndroidRuntime(4868): java.lang.NullPointerException
11-17 17:31:18.681: E/AndroidRuntime(4868):     at com.a.Notter.Notes$OpenFileEvent.onClick(Notes.java:203)
11-17 17:31:18.681: E/AndroidRuntime(4868):     at android.view.View.performClick(View.java:4204)
11-17 17:31:18.681: E/AndroidRuntime(4868):     at android.view.View$PerformClick.run(View.java:17355)
11-17 17:31:18.681: E/AndroidRuntime(4868):     at android.os.Handler.handleCallback(Handler.java:725)
11-17 17:31:18.681: E/AndroidRuntime(4868):     at android.os.Handler.dispatchMessage(Handler.java:92)
11-17 17:31:18.681: E/AndroidRuntime(4868):     at android.os.Looper.loop(Looper.java:137)
11-17 17:31:18.681: E/AndroidRuntime(4868):     at android.app.ActivityThread.main(ActivityThread.java:5041)
11-17 17:31:18.681: E/AndroidRuntime(4868):     at java.lang.reflect.Method.invokeNative(Native Method)
11-17 17:31:18.681: E/AndroidRuntime(4868):     at java.lang.reflect.Method.invoke(Method.java:511)
11-17 17:31:18.681: E/AndroidRuntime(4868):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:793)
11-17 17:31:18.681: E/AndroidRuntime(4868):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:560)
11-17 17:31:18.681: E/AndroidRuntime(4868):     at dalvik.system.NativeStart.main(Native Method)

在这里，我也处理了NULL ，因此会发生此错误

if(fileNameList.length>0 && fileNameList!=null)

这是怎么回事

Answer 1

评估的第一个条件将是fileNameList的长度。 否则，如果fileNameList为null ，则将抛出NPE因为您首先尝试访问其length属性。

您应该逆转您的情况：

if(fineNameList != null && fileNameList.length>0)

为什么呢

这些运算符表现出“短路”行为，这意味着仅在需要时才评估第二个操作数。

因此，如果fileNameList为null ，则将不评估第二个操作数，因此避免抛出NPE 。

Answer 2

你应该用

if(fileNameList!=null && fileNameList.length>0)

否则，由于fileNameList.length而导致NullPointerException ，其中fileNameList为NULL 。