Proximity Sensor Activity in main activity-Duplicate class

I want to call ProximitySensor activity in main activity, I've tried this:

Intent intentSensor = new Intent(context, ProximitySensor.class);
startActivity(intentSensor);

But, I have an errror:

duplicate class: ProximitySensor and cannot access ProximitySensor

Here is my ProximitySensor.java:

    import org.qtproject.qt5.android.bindings.QtActivity;
    import android.hardware.Sensor;
    import android.hardware.SensorEvent;
    import android.hardware.SensorEventListener;
    import android.hardware.SensorManager;
    import android.os.Bundle;
    import android.app.Activity;
    import android.content.Context;
    import android.widget.Toast;
    import android.os.PowerManager;
    import android.view.WindowManager;
    import android.view.WindowManager.LayoutParams;

    public class ProximitySensor extends Activity implements SensorEventListener{
     //SensorManager lets you access the device's sensors
     //declare Variables
     private SensorManager sensorManager;

     /** Called when the activity is first created. */
     @Override
     public void onCreate(Bundle savedInstanceState) {
      super.onCreate(savedInstanceState);

      //create instance of sensor manager and get system service to interact with Sensor
      sensorManager= (SensorManager)getSystemService(Context.SENSOR_SERVICE);
      WindowManager.LayoutParams params = getWindow().getAttributes();
      Sensor proximitySensor= sensorManager.getDefaultSensor(Sensor.TYPE_PROXIMITY);
         .
         .
         .
     // called when sensor value have changed
     @Override
     public void onSensorChanged(SensorEvent event)
     {
         WindowManager.LayoutParams params = this.getWindow().getAttributes();
      if(event.sensor.getType()==Sensor.TYPE_PROXIMITY)
      {

          if(event.values[0]==0)
          {
            params.flags |= LayoutParams.FLAG_KEEP_SCREEN_ON;
            params.screenBrightness = 0;
            getWindow().setAttributes(params);
          }
          else
          {
            params.flags |= LayoutParams.FLAG_KEEP_SCREEN_ON;
            params.screenBrightness = -1f;
            getWindow().setAttributes(params);
          }
      }
     }
    }

And my main actinvity:

    .
    .
public class CustomMainActivity extends QtActivity
{    
    .
    .
    .

    @Override
    public void onCreate(Bundle savedInstanceState)
    { 
        super.onCreate(savedInstanceState);  
        Context context = getApplicationContext();
        Intent intentSensor = new Intent(context, ProximitySensor.class);
        startActivity(intentSensor);

First Question: How to fix it?
Second Question: Could I use just onSensorChanged in main activity?