的NullPointerException當處理程序，用於更新進度

Question

public class MainActivity extends Activity { 

    Button button = (Button)findViewById(R.id.btn); 
    ProgressBar bar = (ProgressBar)findViewById(R.id.pbar); 

    @Override 
    protected void onCreate(Bundle savedInstanceState) { 
     super.onCreate(savedInstanceState); 
     setContentView(R.layout.activity_main); 


     button.setOnClickListener(new OnClickListener() { 

      @Override 
      public void onClick(View v) { 
       // TODO Auto-generated method stub 
       updateHandler.post(updateThread); 
      } 
     }); 

    } 

    @Override 
    public boolean onCreateOptionsMenu(Menu menu) { 
     // Inflate the menu; this adds items to the action bar if it is present. 
     getMenuInflater().inflate(R.menu.main, menu); 
     return true; 
    } 

    @Override 
    public boolean onOptionsItemSelected(MenuItem item) { 
     // Handle action bar item clicks here. The action bar will 
     // automatically handle clicks on the Home/Up button, so long 
     // as you specify a parent activity in AndroidManifest.xml. 
     int id = item.getItemId(); 
     if (id == R.id.action_settings) { 
      return true; 
     } 
     return super.onOptionsItemSelected(item); 
    } 


    Handler updateHandler = new Handler(){ 

     public void handleMessage(Message msg) { 
      bar.setProgress(msg.arg1); 
      updateHandler.post(updateThread); 
     }; 
    }; 

    Runnable updateThread = new Runnable() { 

     int i = 0; 
     @Override 
     public void run() { 
      // TODO Auto-generated method stub 
      System.out.println("------------"); 
      i = i+10; 
      Message msg = updateHandler.obtainMessage(); 
      msg.arg1 = i; 

      try { 
       Thread.sleep(1000); 
      } catch (InterruptedException e) { 
       // TODO Auto-generated catch block 
       e.printStackTrace(); 
      } 

      updateHandler.sendMessage(msg); 
      if (i==100) { 
       updateHandler.removeCallbacks(updateThread); 
      } 
     } 
    }; 


} 

logcat的：

了java.lang.RuntimeException：無法實例活動ComponentInfo {com.zxy.handlerpbtest/com.zxy.handlerpbtest.MainActivity}：JAVA。 lang.NullPointerException

Answer 1

可以重寫行的這段代碼：

ProgressBar bar = (ProgressBar)findViewById(R.id.pbar); 
Button button = (Button)findViewById(R.id.btn); 

由於：

public class MainActivity extends Activity { 

    Button button ; 
    ProgressBar bar ; 

    @Override 
    protected void onCreate(Bundle savedInstanceState) { 
     super.onCreate(savedInstanceState); 
     setContentView(R.layout.activity_main); 
     bar = (ProgressBar)findViewById(R.id.pbar); 
     button = (Button)findViewById(R.id.btn); 
..... 
}} 

那麼您將無法獲得空指針異常。

Answer 2

此行

ProgressBar bar = (ProgressBar)findViewById(R.id.pbar); 
Button button = (Button)findViewById(R.id.btn); 

應該在你onCreate方法

@Override 
protected void onCreate(Bundle savedInstanceState) { 
    super.onCreate(savedInstanceState); 
    setContentView(R.layout.activity_main); 

    ProgressBar bar = (ProgressBar)findViewById(R.id.pbar); 
    Button button = (Button)findViewById(R.id.btn); 
} 

Answer 3

下面是完整的解決方案：

public class MainActivity extends Activity { 

    Button button; 
    ProgressBar bar; 

    @Override 
    protected void onCreate(Bundle savedInstanceState) { 
     super.onCreate(savedInstanceState); 
     setContentView(R.layout.activity_main2); 

     button = (Button) findViewById(R.id.btn); 
     bar = (ProgressBar) findViewById(R.id.pbar); 

     button.setOnClickListener(new View.OnClickListener() { 

      @Override 
      public void onClick(View v) { 
       // TODO Auto-generated method stub 
       updateHandler.post(updateThread); 
      } 
     }); 

    } 


    Handler updateHandler = new Handler() { 

     public void handleMessage(Message msg) { 
      bar.setProgress(msg.arg1); 
      updateHandler.post(updateThread); 
     } 
    }; 

    Runnable updateThread = new Runnable() { 

     int i = 0; 

     @Override 
     public void run() { 
      // TODO Auto-generated method stub 
      System.out.println("------------"); 
      i = i + 10; 
      Message msg = updateHandler.obtainMessage(); 
      msg.arg1 = i; 

      try { 
       Thread.sleep(1000); 
      } catch (InterruptedException e) { 
       // TODO Auto-generated catch block 
       e.printStackTrace(); 
      } 

      updateHandler.sendMessage(msg); 
      if (i == 100) { 
       updateHandler.removeCallbacks(updateThread); 
      } 
     } 
    }; 

}