android.app.Application cannot be cast to android.app.Activity

public class ApokusActivity extends Activity {
	
	@Override
	protected void onCreate(Bundle savedInstanceState) {
		super.onCreate(savedInstanceState);
		setContentView(new SampleView(this));
	}

	private static class SampleView extends View {

		// CONSTRUCTOR
		public SampleView(Context context) {
			super(context);
			setFocusable(true);

		}
		@Override
		protected void onDraw(Canvas canvas) {
			Paint paint = new Paint();
			
			canvas.drawColor(Color.GREEN);
            
           Bitmap b = Bitmap.createBitmap(200, 200, Bitmap.Config.ALPHA_8);
           Canvas c = new Canvas(b);
           c.drawRect(0, 0, 200, 200, paint);
           
           paint.setXfermode(new PorterDuffXfermode(PorterDuff.Mode.SRC));
           paint.setTextSize(40);
           paint.setTextScaleX(1.f);
           paint.setAlpha(0);
           paint.setAntiAlias(true);
           c.drawText("Your text", 30, 40, paint);
           paint.setColor(Color.RED);
           
           canvas.drawBitmap(b, 10,10, paint);
		}

	}
}

"id cannot be resolved or is not a field” error
If you copy project to workspace or some code from other project you can get this error.

Try remove or add import your R file:

import com.example.app.R

Check xml files.
Clean project by menu Project-Clean

Maybe you have to create new project end copy old source files to new project folders.

1.) check your xml files in res/layout folder if contain errors or warnings, repair this
2.) try comment import R in Activity file
// import com.mysite.packagename.R;
3.) every clean and rebuild project

Screen Android example source code for developers.
Get orientation of screen.

 public int getScreenOrientation() {
  if (getResources().getConfiguration().orientation == Configuration.ORIENTATION_PORTRAIT)                {
       return 1; // Portrait Mode
   
  }else if (getResources().getConfiguration().orientation == Configuration.ORIENTATION_LANDSCAPE) {
       return 2;   // Landscape mode
  }
  return 0;
 }

You can get list by class Build
For example:

if(Build.MANUFACTURER.equals("unknown")) {
		// Emulator
	}

List of Build class getting from emulator:
"BOARD=unknown
BRAND=generic
CPU_ABI=armeabi
DEVICE=generic
DISPLAY=sdk-eng 2.1-update1 ECLAIR 35983 test-keys
FINGERPRINT=generic/sdk/generic/:2.1-update1/ECLAIR/35983:eng/test-keys
HOST=android-test-13.mtv.corp.google.com
ID=ECLAIR
MANUFACTURER=unknown
MODEL=sdk
PRODUCT=sdk
TAGS=test-keys
TIME=1273161972000
TYPE=eng
USER=android-build
"