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com.example.qkobe.asynctask;android.graphics.Bitmap;android.graphics.BitmapFactory;android.os.AsyncTask;android.os.Bundle;android.support.v7.app.AppCompatActivity;android.view.View;android.widget.ImageView;android.widget.ProgressBar;java.io.BufferedInputStream;java.io.IOException;java.io.InputStream;java.net.URL;java.net.URLConnection;ImageTest AppCompatActivity { ImageView ; ProgressBar ; String = ; onCreate(Bundle savedInstanceState) { .onCreate(savedInstanceState); = (ImageView)findViewById(R.id.); = (ProgressBar)findViewById(R.id.); MyAsyncTask().execute(); } MyAsyncTask AsyncTask{ onPreExecute() { .onPreExecute(); .setVisibility(View.); } onPostExecute(Bitmap bitmap) { .onPostExecute(bitmap); .setImageBitmap(bitmap); .setVisibility(View.); } Bitmap doInBackground(String... params) { String url = params[]; Bitmap bitmap = ; URLConnection connection; InputStream is; { connection = URL(url).openConnection(); is = connection.getInputStream(); BufferedInputStream bis = BufferedInputStream(is); bitmap = BitmapFactory.(bis); is.close(); bis.close(); }(IOException e){ e.printStackTrace(); } bitmap; } }}报错为java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.ProgressBar.setVisibility(int)' on a null object reference

错误行代码为
mProgressBar.setVisibility(View.VISIBLE);

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点击Image Test按钮闪退

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