java - AsyncTask 复制文件

Question

我正在尝试使用 AsyncTask 将一些文件从一个文件夹复制到另一个文件夹，然后列出它们。但是，当我执行 CopyandList 时，我的应用程序只是强制关闭。任何意见，将不胜感激。

这是我的 AsyncTask 代码

private class CopyandList extends AsyncTask<Void, Void, Void> {

    private ProgressDialog dialog;

    @Override
    protected void onPreExecute() {
        dialog.setMessage("Copying Files ...");
        dialog.show();
    }

    @Override
    protected Void doInBackground(Void... arg0) {
        File file;
        file = new File(Snapdes);
        File list[] = file.listFiles();

        for( int i=0; i< list.length; i++)
        {
            String x=(directory + list[i].getName());
            String y=(Snapdes);
            if(list[i].equals(y)){
            }else{
                RootTools.copyFile(x, y, false, true);
            }

        }
        return null;
    }

    protected void onPostExecute(Void result) {
        if (dialog.isShowing()) {
            dialog.dismiss();

            File file;
            file = new File(Snapdes);
            File list[] = file.listFiles();
            ListView ListView = (ListView) findViewById(R.id.SnapList);
            List<String> SnapList;
            SnapList = new ArrayList<String>();

            for( int i=0; i< list.length; i++){

                SnapList.add( list[i].getName() );
                System.out.print(list[i]);

            }

            ArrayAdapter<String> adapter =
                new ArrayAdapter<String> (MainActivity.this,
                        android.R.layout.simple_list_item_1,
                        android.R.id.text1, SnapList);
            ListView.setAdapter(adapter);
        }
    }
}

然后我在主 Activity 中执行它

new CopyandList().execute();

抱歉，这是我的 logcat。

05-23 21:26:56.145: D/AndroidRuntime(24915): Shutting down VM
05-23 21:26:56.145: W/dalvikvm(24915): threadid=1: thread exiting with uncaught exception (group=0x41770930)
05-23 21:26:56.165: E/AndroidRuntime(24915): FATAL EXCEPTION: main
05-23 21:26:56.165: E/AndroidRuntime(24915): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.itsallwhite.sneakysnapper/com.itsallwhite.sneakysnapper.MainActivity}: java.lang.NullPointerException
05-23 21:26:56.165: E/AndroidRuntime(24915):    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2307)
05-23 21:26:56.165: E/AndroidRuntime(24915):    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2357)
05-23 21:26:56.165: E/AndroidRuntime(24915):    at android.app.ActivityThread.access$600(ActivityThread.java:153)
05-23 21:26:56.165: E/AndroidRuntime(24915):    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1247)
05-23 21:26:56.165: E/AndroidRuntime(24915):    at android.os.Handler.dispatchMessage(Handler.java:99)
05-23 21:26:56.165: E/AndroidRuntime(24915):    at android.os.Looper.loop(Looper.java:137)
05-23 21:26:56.165: E/AndroidRuntime(24915):    at android.app.ActivityThread.main(ActivityThread.java:5226)
05-23 21:26:56.165: E/AndroidRuntime(24915):    at java.lang.reflect.Method.invokeNative(Native Method)
05-23 21:26:56.165: E/AndroidRuntime(24915):    at java.lang.reflect.Method.invoke(Method.java:511)
05-23 21:26:56.165: E/AndroidRuntime(24915):    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:795)
05-23 21:26:56.165: E/AndroidRuntime(24915):    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:562)
05-23 21:26:56.165: E/AndroidRuntime(24915):    at com.android.internal.os.ZygoteInit.main(Native Method)
05-23 21:26:56.165: E/AndroidRuntime(24915):    at dalvik.system.NativeStart.main(Native Method)
05-23 21:26:56.165: E/AndroidRuntime(24915): Caused by: java.lang.NullPointerException
05-23 21:26:56.165: E/AndroidRuntime(24915):    at com.itsallwhite.sneakysnapper.MainActivity$CopyandList.onPreExecute(MainActivity.java:119)
05-23 21:26:56.165: E/AndroidRuntime(24915):    at android.os.AsyncTask.executeOnExecutor(AsyncTask.java:586)
05-23 21:26:56.165: E/AndroidRuntime(24915):    at android.os.AsyncTask.execute(AsyncTask.java:534)
05-23 21:26:56.165: E/AndroidRuntime(24915):    at com.itsallwhite.sneakysnapper.MainActivity.onCreate(MainActivity.java:66)
05-23 21:26:56.165: E/AndroidRuntime(24915):    at android.app.Activity.performCreate(Activity.java:5104)
05-23 21:26:56.165: E/AndroidRuntime(24915):    at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1080)
05-23 21:26:56.165: E/AndroidRuntime(24915):    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2261)
05-23 21:26:56.165: E/AndroidRuntime(24915):    ... 12 more

Answer 1

当您调用 dialog.setMessage("Copying Files ..."); 时，您会得到一个 NullPointerException。您需要先初始化对话框。