In which method should you handle the actions when a button in the app bar is pressed?

The method where you should handle the actions when a button in the app bar is pressed is onOptionsItemSelected(). This method is specifically designed to respond to user interactions with the options menu, including button presses from the app bar.

When a user taps an item in the options menu, the Android framework calls onOptionsItemSelected(), providing you with the MenuItem that represents the selected option. Inside this method, you can determine which menu item was selected by using its ID and perform the associated actions, such as starting a new activity, opening a dialog, or updating the UI.

The other methods listed serve different purposes in the context of menu management. onCreateOptionsMenu() is used to inflate the options menu when it is first created, while openOptionsMenu() is a method that can be called to open the options menu programmatically. onPrepareOptionsMenu() allows you to customize the options menu each time it is about to be displayed, but it does not handle the actions triggered by button presses. Thus, onOptionsItemSelected() is the appropriate method for processing those actions.