public class MeinActivity extends Activity {
    private EditText txtVon;
    private EditText txtBis;
    private Button btnStart;
    private TextView txtView;
    
    //Keys
    public static final String NUM_VON = "NUM_VON";
    public static final String NUM_BIS = "NUM_BIS";
    
    private Intent intentService;
    
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);
        
        
        txtVon = (EditText) findViewById(R.id.txtVon);
        
        txtBis = (EditText) findViewById(R.id.txtBis);
        
        txtView = (TextView) findViewById(R.id.txtAusgabe);
        
        btnStart = (Button) findViewById(R.id.btnStart);
        btnStart.setOnClickListener(new BtnStartClickListener());
        System.out.println("registering...");
        intentService  = new Intent(MeinActivity.this,PrimBerechner.class);
        registerReceiver(new primBroadcastReceiver(),new IntentFilter(PrimBerechner.PRIM_ACTION));
    }
    
    public class primBroadcastReceiver extends BroadcastReceiver {
		@Override
		public void onReceive(Context context, Intent intent) {
			if(intent.getAction().equals(PrimBerechner.PRIM_ACTION) ) {
				Bundle b = intent.getExtras();
				txtView.append(b.getInt("PRIM")+"\n");
			}
		}    	
    }    
    
    public class BtnStartClickListener implements OnClickListener {
		@Override
		public void onClick(View v) {
			if(v.equals(btnStart)) {
				if(((Button)v).getText().toString().equals("Start")) {
					
					//Service starten
					intentService.putExtra(MeinActivity.NUM_VON, Integer.parseInt(txtVon.getText().toString()));
					intentService.putExtra(MeinActivity.NUM_BIS, Integer.parseInt(txtBis.getText().toString()));
					
					startService(intentService);
					
					btnStart.setText("Stop");
				}
				else {
					txtView.append("Berechnung abgebrochen");
					Intent i = new Intent(MeinActivity.this,PrimBerechner.class);
					stopService(i);
					btnStart.setText("Start");
				}
			}
		}
    	
    }
}

public class PrimBerechner extends Service {
	private int von;
	private int bis;
	
	private String zahlen = "";
	
	//Intent key 
	public static final String PRIM_ACTION = "PRIM_ACTION";
	
	@Override
	public IBinder onBind(Intent arg0) {
		return null;
	}
	
	public void onStart(Intent intent, int ID) {
		super.onStart(intent, ID);
		
		Bundle b = intent.getExtras();
		this.von = b.getInt(MeinActivity.NUM_VON);
		this.bis = b.getInt(MeinActivity.NUM_BIS);
		
		Thread t = new Thread(new Rechner());
		t.start();
	}	

	public class Rechner implements Runnable {
		@Override
		public void run() {
			if (von % 2 == 0)
				von++;
					
			int teiler;
			int y;
			
			Intent i = new Intent(PRIM_ACTION);
					
			while (von <= bis)
			{
				teiler = 2;
				y = 0;
						
				while (teiler < von)
				{
					if ( von % teiler == 0 )
						y = 1;
					teiler++; 
				}
				
				if ( y == 0 ){
					//Broadcast senden
					i.putExtra("PRIM", von);
					sendBroadcast(i);
					
					zahlen += von + "\n";
				}			
				von += 2;
			}
			
			NotificationManager notiManager = (NotificationManager) getSystemService(Context.NOTIFICATION_SERVICE);
			Notification noti = new Notification(R.drawable.sieben,"Primzahlberechnung abgeschlossen",System.currentTimeMillis());
			
			noti.ledARGB = Color.BLUE;
			noti.ledOffMS = 1000;
			noti.ledOnMS = 2000;
			noti.flags = Notification.FLAG_SHOW_LIGHTS;
			
			Intent intent = new Intent(PrimBerechner.this,MeinActivity.class);
			intent.putExtra("ZAHLEN", zahlen);
			
			PendingIntent penIntent = PendingIntent.getActivity(getApplicationContext(), 0, intent, 0);
			
			noti.setLatestEventInfo(getApplicationContext(), "Primzahlberechner", "Ihre Primzahlberechnung ist abgeschlossen", penIntent);
			notiManager.notify(1, noti);
		}
		
	}
}

<activity android:name=".MainActivity"
             android:label="@string/app_name"
             android:launchMode="singleInstance"
             >
</activity>