What type of Looper should a Handler use to send data from a background thread to the main thread of the application?

To send data from a background thread to the main application thread in Android, the Handler must use the Looper of the main (UI) thread. Usually, this is Looper.getMainLooper().

Example of creating a Handler in the main thread:

Handler mainHandler = new Handler(Looper.getMainLooper());

This ensures that messages and Runnables sent through this Handler will be processed in the main thread, which is necessary for updating the UI.