为什么MainActivity无法解决或不是字段？(Why MainActivity cannot be resolved or is not a field?)

 我有一个小型项目，我需要使用Android设备的电池。 起初，我希望能够打印电池，所以我使用了本教程 。 
 
 然后，我在Eclipse中创建了一个名为“Testing”的新Android项目，并在MainActivity.java中添加了以下代码： 
 
package com.example.testing;

import android.app.Activity;
import android.content.BroadcastReceiver;
import android.content.Context;
import android.content.Intent;
import android.content.IntentFilter;
import android.os.BatteryManager;
import android.os.Bundle;
import android.util.Log;
import android.view.View;
import android.view.View.OnClickListener;
import android.widget.TextView;
import android.widget.Toast;
import android.R.*;



public class MainActivity extends Activity {  
     TextView mTextView;  
     @Override  
     protected void onCreate(Bundle savedInstanceState) {  
          super.onCreate(savedInstanceState);  
          setContentView(R.layout.MainActivity); //This is the MainActivity that contains the error
          mTextView = (TextView) findViewById(R.id.batterTest);  
          findViewById(R.id.getBattery).setOnClickListener(new OnClickListener() {  
               @Override  
               public void onClick(View v) {  
                    mTextView.setText(String.valueOf(batteryLevel));  
               }  
          });  
          new Thread(new ThreadM()).start();  
     }  
     @Override  
     protected void onDestroy() {  
          super.onDestroy();  
          unregisterReceiver(mArrow);  
     }  
     BatteryReceiver mArrow;  
     private class ThreadM implements Runnable {  
          @Override  
          public void run() {  
               mArrow = new BatteryReceiver();  
               IntentFilter mIntentFilter = new IntentFilter();  
               mIntentFilter.addAction(Intent.ACTION_BATTERY_LOW);  
               mIntentFilter.addAction(Intent.ACTION_BATTERY_CHANGED);  
               mIntentFilter.addAction(Intent.ACTION_BATTERY_OKAY);  
               Intent batteryIntent = registerReceiver(mArrow, mIntentFilter);  
               batteryLevel = getBatteryLevel(batteryIntent);  
               Log.e("Battery Level", String.valueOf(batteryLevel));  
          }  
     }  
     float batteryLevel;  
     private class BatteryReceiver extends BroadcastReceiver {  
          @Override  
          public void onReceive(Context arg0, Intent arg1) {  
               if (arg1.getAction().equalsIgnoreCase(Intent.ACTION_BATTERY_LOW)  || arg1.getAction().equalsIgnoreCase(Intent.ACTION_BATTERY_CHANGED) || arg1.getAction().equalsIgnoreCase(Intent.ACTION_BATTERY_OKAY)) {  
                    int level = arg1.getIntExtra("level", 0);  
                    Toast.makeText(MainActivity.this,"Current Battery " + level + " %", Toast.LENGTH_LONG).show();  
                    mTextView.setText(String.valueOf("Battery Level Change Detect through Receiver = " + level));  
               }  
          }
     }  
     public float getBatteryLevel(Intent batteryIntent) {  
          int level = batteryIntent.getIntExtra(BatteryManager.EXTRA_LEVEL, -1);  
          int scale = batteryIntent.getIntExtra(BatteryManager.EXTRA_SCALE, -1);  
          if (level == -1 || scale == -1) {  
               return 50.0f;  
          }  
          return ((float) level / (float) scale) * 100.0f;  
     }  
}
 
 实际上，这应该只是在屏幕上打印电池电量的情况下执行简单的活动。 但这并不是因为一个错误。 “MainActivity无法解析或不是字段”（请参阅​​注释行）。 你有什么想法，为什么？ 我想让这段代码有效，所以我可以继续。 
 
 谢谢。 

I have a small project where I need to play with the Battery of an Android device. At first, I wanted to be able to print the Battery so I used this tutorial.
 
Then, I have created a new Android project called "Testing" in Eclipse, and in MainActivity.java I put this code :
 
package com.example.testing;

import android.app.Activity;
import android.content.BroadcastReceiver;
import android.content.Context;
import android.content.Intent;
import android.content.IntentFilter;
import android.os.BatteryManager;
import android.os.Bundle;
import android.util.Log;
import android.view.View;
import android.view.View.OnClickListener;
import android.widget.TextView;
import android.widget.Toast;
import android.R.*;



public class MainActivity extends Activity {  
     TextView mTextView;  
     @Override  
     protected void onCreate(Bundle savedInstanceState) {  
          super.onCreate(savedInstanceState);  
          setContentView(R.layout.MainActivity); //This is the MainActivity that contains the error
          mTextView = (TextView) findViewById(R.id.batterTest);  
          findViewById(R.id.getBattery).setOnClickListener(new OnClickListener() {  
               @Override  
               public void onClick(View v) {  
                    mTextView.setText(String.valueOf(batteryLevel));  
               }  
          });  
          new Thread(new ThreadM()).start();  
     }  
     @Override  
     protected void onDestroy() {  
          super.onDestroy();  
          unregisterReceiver(mArrow);  
     }  
     BatteryReceiver mArrow;  
     private class ThreadM implements Runnable {  
          @Override  
          public void run() {  
               mArrow = new BatteryReceiver();  
               IntentFilter mIntentFilter = new IntentFilter();  
               mIntentFilter.addAction(Intent.ACTION_BATTERY_LOW);  
               mIntentFilter.addAction(Intent.ACTION_BATTERY_CHANGED);  
               mIntentFilter.addAction(Intent.ACTION_BATTERY_OKAY);  
               Intent batteryIntent = registerReceiver(mArrow, mIntentFilter);  
               batteryLevel = getBatteryLevel(batteryIntent);  
               Log.e("Battery Level", String.valueOf(batteryLevel));  
          }  
     }  
     float batteryLevel;  
     private class BatteryReceiver extends BroadcastReceiver {  
          @Override  
          public void onReceive(Context arg0, Intent arg1) {  
               if (arg1.getAction().equalsIgnoreCase(Intent.ACTION_BATTERY_LOW)  || arg1.getAction().equalsIgnoreCase(Intent.ACTION_BATTERY_CHANGED) || arg1.getAction().equalsIgnoreCase(Intent.ACTION_BATTERY_OKAY)) {  
                    int level = arg1.getIntExtra("level", 0);  
                    Toast.makeText(MainActivity.this,"Current Battery " + level + " %", Toast.LENGTH_LONG).show();  
                    mTextView.setText(String.valueOf("Battery Level Change Detect through Receiver = " + level));  
               }  
          }
     }  
     public float getBatteryLevel(Intent batteryIntent) {  
          int level = batteryIntent.getIntExtra(BatteryManager.EXTRA_LEVEL, -1);  
          int scale = batteryIntent.getIntExtra(BatteryManager.EXTRA_SCALE, -1);  
          if (level == -1 || scale == -1) {  
               return 50.0f;  
          }  
          return ((float) level / (float) scale) * 100.0f;  
     }  
}
 
Actually this should just run a simple activity with the Battery level printed on the screen. But it does not because of one error. "MainActivity cannot be resolved or is not a field" (see the commented line). Do you have any idea why ? I would like to have this code working so I can go on then.
 
Thank you.

原文：https://stackoverflow.com/questions/21852485