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<String,Void,Bitmap>{
????????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;
????????}
????}
}

報錯為java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.ProgressBar.setVisibility(int)' on a null object reference

錯誤行代碼為

mProgressBar.setVisibility(View.VISIBLE);