android中的java.lang.nullpointer错误

我正在制作一个应用程序，它将显示名人的图像和 4 个名字，其中一个是正确的。

我添加了一个“更改”按钮来更改图像中的名人，但是在按下该按钮时，我的应用程序崩溃，显示空 java.lang.null 指针异常。

我在 oncreate 方法中有加载图像和四个名称的代码，该方法在应用程序首次启动时加载图像和四个名称，但是在调用函数时按下按钮，虽然我有相同的代码，但会生成错误在里面。

public class MainActivity extends AppCompatActivity {
    //declaring all the widgets to be used
    Bitmap celebimg;
    Random rand;
    ImageView img;
    Button btn0, btn1, btn2, btn3;
    int optionnum, imgnum;
    ArrayList<String> celebnames = new ArrayList<String>();
    ArrayList<String> celebimages = new ArrayList<String>();
    ArrayList<String> buttonoptions = new ArrayList<String>(); 

    //Invoked when one out of the four options is clicked
    public void row(View view) {
        if (view.getTag().toString().equals(Integer.toString(optionnum))){
            Toast.makeText(this, "correct Answer", Toast.LENGTH_SHORT).show();
        } else {
            Toast.makeText(this, "wrong Answer, It was" + 
            celebnames.get(imgnum), Toast.LENGTH_SHORT).show();
        }
    }

    //Class for downloading of image
    public class Imagedownload extends AsyncTask<String, Void, Bitmap> {     
    @Override
    protected Bitmap doInBackground(String... strings) {

        try {
            URL url = new URL(strings[0]);
            HttpURLConnection connection = (HttpURLConnection)url.openConnection();
            connection.connect();
            InputStream in = connection.getInputStream();
            Bitmap myimage = BitmapFactory.decodeStream(in);
            return myimage;
        } catch (MalformedURLException e) {
            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        }

        return null;
    }
}

/**
  * Class for downloading the website html code
  */
public class Download extends AsyncTask<String, Void, String > {
    @Override
    protected String doInBackground(String... strings) {
        String result = "";
        URL url;
        HttpURLConnection connection = null;

        try {
            url = new URL(strings[0]);
            connection = (HttpURLConnection)url.openConnection();
            InputStream input = connection.getInputStream();
            InputStreamReader reader = new InputStreamReader(input);
            int data = reader.read();
            while(data != -1) {
                char current = (char)data;
                result += current;
                data = reader.read();
            }
            return result;
        } catch (MalformedURLException e) {
            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        }

        return null;
    }
}

单击更改按钮时执行的代码

public void change(View view) {
    rand = new Random();
    imgnum = rand.nextInt(celebimages.size());
    Imagedownload image = new Imagedownload();

    try {
        celebimg = image.execute(celebimages.get(imgnum)).get();
        if (celebimg == null)
            Log.i("its", "null douchebag");
        img.setImageBitmap(celebimg);
    } catch (InterruptedException e) {
        if (celebimg == null)
            Log.i("its", "null douchebag");
        e.printStackTrace();
    } catch (ExecutionException e) {
        if (celebimg == null)
            Log.i("its", "null douchebag");
        e.printStackTrace();
    }

    optionnum = rand.nextInt(4);

    for(int i = 0;i < 4;i++){
        if(i == optionnum)
            buttonoptions.add(celebnames.get(imgnum));
        else {
            int random = rand.nextInt(celebnames.size());
            while (celebnames.get(imgnum) == celebnames.get(random)){
                random = rand.nextInt(celebnames.size());
            }
            buttonoptions.add(celebnames.get(random));
        }
    }
    btn0.setText(buttonoptions.get(0));
    btn1.setText(buttonoptions.get(1));
    btn2.setText(buttonoptions.get(2));
    btn3.setText(buttonoptions.get(3));
}

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    ImageView img = (ImageView)findViewById(R.id.celeb);

    btn0 = (Button)findViewById(R.id.btn0);
    btn1 = (Button)findViewById(R.id.btn1);
    btn2 = (Button)findViewById(R.id.btn2);
    btn3 = (Button)findViewById(R.id.btn3);
    String data = "";
    Download load = new Download();
    try {
        data = load.execute("http://www.posh24.se/kandisar").get();
        String[] splitdata = data.split("<div class=\"title\">Lista:</div>");

        // seperating out the required img src from the html code
        Pattern p = Pattern.compile("src=\"(.*?)\"");
        Matcher M = p.matcher(splitdata[1]);

        while (M.find()){
            //adding all the img src values to celebimages arraylist
            celebimages.add(M.group(1));
        }
        Pattern pi = Pattern.compile("alt=\"(.*?)\"");
        Matcher Mi = pi.matcher(splitdata[1]);

        while (Mi.find()) {
           // adding all the alt values to celebnames arraylist
            celebnames.add(Mi.group(1));
        }

    } catch (InterruptedException e) {
        e.printStackTrace();
    } catch (ExecutionException e) {
        e.printStackTrace();
    }
    Random rand = new Random();
    imgnum = rand.nextInt(celebimages.size());
    Imagedownload image = new Imagedownload();
    Bitmap celebimg;
    try {
        //downloading the image from stored img src values from celebimages 
        //arraylist
        celebimg = image.execute(celebimages.get(imgnum)).get();
        img.setImageBitmap(celebimg);
    } catch (InterruptedException e) {
        e.printStackTrace();
    } catch (ExecutionException e) {
        e.printStackTrace();
    }
    //Setting the correct value in one button and random values in other 
    //three
    optionnum = rand.nextInt(4);

    for(int i = 0;i < 4;i++){
        if(i == optionnum)
            buttonoptions.add(celebnames.get(imgnum));
        else {
            int random = rand.nextInt(85);
            while (celebnames.get(imgnum) == celebnames.get(random)){
                random = rand.nextInt(celebnames.size());
            }
            buttonoptions.add(celebnames.get(random));
        }
    }
    btn0.setText(buttonoptions.get(0));
    btn1.setText(buttonoptions.get(1));
    btn2.setText(buttonoptions.get(2));
    btn3.setText(buttonoptions.get(3));
}

错误：

引起：java.lang.NullPointerException：尝试在空对象引用上调用虚方法“void android.widget.ImageView.setImageBitmap(android.graphics.Bitmap)”