Print Messages to a Log File(Android)

To write log messages from your application:

1. Import android.util.Log.
2. Use Log.v(), Log.d(), Log.i(), Log.w(), or Log.e() to log messages.
   E.g., Log.e(this.toString(), "error: " + err.toString())
3. Launch DDMS from a terminal by executing ddms in your Android SDK /tools path.
4. Run your application in the Android emulator.
5. From the DDMS application, select the emulator (e.g., "emulator-5554") and click Device > Run logcat... to view all the log data.

MORE http://developer.android.com/resources/faq/commontasks.html#newandroidprojectnoeclipse