將代碼更改爲android註釋

Question

我試圖將我的代碼更改爲android註釋。我的應用程序開始，但是當我點擊按鈕，它崩潰，出現錯誤：

FATAL EXCEPTION: main 
Process: com.example.albertpula.apka3, PID: 2523 
java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.ProgressBar.setVisibility(int)' on a null object reference 
    at com.example.albertpula.apka3.MainActivity$ReadPic.onPreExecute(MainActivity.java:83) 
    at android.os.AsyncTask.executeOnExecutor(AsyncTask.java:604) 
    at android.os.AsyncTask.execute(AsyncTask.java:551) 
    at com.example.albertpula.apka3.MainActivity.click(MainActivity.java:54) 
    at com.example.albertpula.apka3.MainActivity_$1.onClick(MainActivity_.java:72) 
    at android.view.View.performClick(View.java:5198) 
    at android.view.View$PerformClick.run(View.java:21147) 
    at android.os.Handler.handleCallback(Handler.java:739) 
    at android.os.Handler.dispatchMessage(Handler.java:95) 
    at android.os.Looper.loop(Looper.java:148) 
    at android.app.ActivityThread.main(ActivityThread.java:5417) 
    at java.lang.reflect.Method.invoke(Native Method) 
    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726) 
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616) 

這裏是代碼：

@EActivity(R.layout.activity_main) 
public class MainActivity extends Activity { 

String adres ="http://www.heavens-above.com/orbitdisplay.aspx?icon=iss&width=300&height=300&satid=25544"; 
Bitmap bmp; 
ProgressBar progressBar; 
Button button1; 
ImageView image; 
TextView text; 

@Override 
public void onCreate(Bundle savedInstanceState) 
{ 
    image = (ImageView)findViewById(R.id.imageView); 
    button1 = (Button)findViewById(R.id.button1); 
    progressBar = (ProgressBar)findViewById(R.id.progressBar1); 
    text = (TextView)findViewById(R.id.textView1); 

    super.onCreate(savedInstanceState); 
    setContentView(R.layout.activity_main); 
} 

@Click(R.id.button1) 
void click(View view){ 
    ReadPic rp = new ReadPic(); 
    rp.execute(); 
} 

class ReadPic extends AsyncTask<Void, Void, Void> 
{ 
    boolean sukces = true; 

    @Override 
    protected Void doInBackground(Void... arg0){ 
     URL u; 
     InputStream is; 
     try{ 

      u = new URL(adres); 
      is = u.openStream(); 
      Bitmap temp = BitmapFactory.decodeStream(is); 
      bmp=temp.copy(Bitmap.Config.ARGB_8888, true); 
     } 
     catch(Exception e){ 
      sukces = false; 

     } 
     return null; 
    } 

    protected void onPreExecute(){ 
     progressBar.setVisibility(ProgressBar.VISIBLE); 
     button1.setEnabled(false); 
     super.onPreExecute(); 
    } 

    protected void onPostExecute(Void result){ 
     if(sukces){ 
      Canvas c = new Canvas(bmp); 
      Paint p = new Paint(); 
      int width = bmp.getWidth(), height = bmp.getHeight(); 
      p.setColor(Color.WHITE); 
      p.setStyle(Paint.Style.STROKE); 
      c.drawRect(0,0,width-1,height-1,p); 

      image.setImageBitmap(bmp); 
      text.setText("Downloading finished, img " +width+ "x" + height); 
     } 
     else{ 
      text.setText("Error when downloading"); 
     } 
     button1.setEnabled(true); 
     progressBar.setVisibility(ProgressBar.INVISIBLE); 

     super.onPostExecute(result); 
    } 
} 
} 

Answer 1

1. 使用ViewBinding綁定您的XML看法是這樣的：

   @ViewById(R.id.button1)Button button1; 
   
   @ViewById(R.id.imageView)ImageView image; 
   
   @ViewById(R.id.textView1)TextView text; 
   

從您的onCreate刪除（）的代碼。

不要添加：

super.onCreate(savedInstanceState); 

setContentView(R.layout.activity_main); 

對於錯誤：顯示java.lang.NullPointerException：嘗試調用虛擬方法「無效android.widget.ProgressBar。 setVisibility（INT）」上的空對象引用在

選項：1個檢查它是否爲空。

protected void onPreExecute(){ 
    if(progressBar != null) 
     progressBar.setVisibility(ProgressBar.VISIBLE); 

    button1.setEnabled(false); 
    super.onPreExecute(); 
} 

AND，

button1.setEnabled(true); 

if(progressBar != null) 
progressBar.setVisibility(ProgressBar.INVISIBLE); 

super.onPostExecute(result); 

選項：2 - 初始化ProgressDialog

class ReadPic extends AsyncTask<Void, Void, Void> { 
    ProgressBar progressBar; 
    boolean sukces = true; 

    public ReadPic(){ 
     progressBar = new ProgressBar();//Set the Theme, context & call constructor as per your choice. 
    } 

    //YOUR CODE GOES BELOW// 
    }//ReadPic class closes here.... 

編輯1：Gorthez，實例化進度條是這樣的：

progressBar = (ProgressBar) findViewById(R.id.progress_bar);

還添加進度欄在你的XML：

<LinearLayout 
     android:orientation="horizontal" 
     ... > 
     <ProgressBar 
      android:id="@+id/progress_bar" 
      android:layout_width="wrap_content" 
      android:layout_height="wrap_content" 
      style="@android:style/Widget.ProgressBar.Small" 
      android:layout_marginRight="5dp" /> 
     <TextView 
      android:layout_width="wrap_content" 
      android:layout_height="wrap_content" 
      android:text="@string/loading" /> </LinearLayout> 

EDIT1參考https://developer.android.com/reference/android/widget/ProgressBar.html

Answer 2

@Override 

public void onCreate(Bundle savedInstanceState) 
{ 

    **//put setContentView and super call here 

    super.onCreate(savedInstanceState); 
    setContentView(R.layout.activity_main);** 

    image = (ImageView)findViewById(R.id.imageView); 
    button1 = (Button)findViewById(R.id.button1); 
    progressBar = (ProgressBar)findViewById(R.id.progressBar1); 
    text = (TextView)findViewById(R.id.textView1); 

    //setContentView(R.layout.activity_main); 
} 

在這裏，你試圖膨脹的佈局之前訪問視圖元素。 首先通過調用setContentView（R.layout.activity_main）來擴充佈局; 然後嘗試訪問元素。