Android :: Setting Return Key On Android Keyboard

Does anyone of you friendly coders know how to change the text on the return key of the android keyboard. I am catching the return event of an EditText element to start an action.So I would like to show the user, that he can start the action with that key.Some applications do that. E.g. they replace the key with a search symbol. I am also wondering why apps always have an additional button aside the text field. Clearly, this takes away screen space. But it may be necessary for some purpose I am not aware off. The one problem I could imagine is task switching. After switching back, the user would have to call the keyboard before he could start the action. The other is a hardware keyboard, but anyone will assume that the return key will start the action.