将代码更改为 android 注释

Question

我正在尝试将我的代码更改为 android 注释。我的应用程序启动了，但是当我点击按钮时它崩溃并出现错误：

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$ReadPic.onPreExecute(MainActivity.java:83)
    at android.os.AsyncTask.executeOnExecutor(AsyncTask.java:604)
    at android.os.AsyncTask.execute(AsyncTask.java:551)
    at com.example.albertpula.apka3.MainActivity.click(MainActivity.java:54)
    at com.example.albertpula.apka3.MainActivity_.onClick(MainActivity_.java:72)
    at android.view.View.performClick(View.java:5198)
    at android.view.View$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)

代码如下：

@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

@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);
}

在这里，您试图在扩充布局之前访问视图元素。首先通过调用 setContentView(R.layout.activity_main); 来膨胀布局然后尝试访问元素。

Answer 2

使用 ViewBinding 绑定您的 xml 视图，如下所示：

@ViewById(R.id.button1)Button button1;

@ViewById(R.id.imageView)ImageView image;

@ViewById(R.id.textView1)TextView text;

从您的 onCreate() 中删除代码。

不要添加：

super.onCreate(savedInstanceState);

setContentView(R.layout.activity_main);

对于错误：java.lang.NullPointerException：尝试在

选项:1 检查是否为空

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

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

和，

button1.setEnabled(true); 

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

super.onPostExecute(result);

选项 :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....

编辑 1：Gorthez，像这样实例化您的进度条：

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

同时在您的xml中添加进度条：

<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 参考 https://developer.android.com/reference/android/widget/ProgressBar.html

将代码更改为 android 注释

Change code to android annotations

java

android

android-annotations