TextView
Class (widget) for displaying text on the screen
SimpleDateFormat
Class that can specify the date format The format used this time is "kk: mm: ss" → hour: minute: second
Timer
A class that schedules tasks in the background
TimerTask
A class with the function of a task that schedules timer iterations to run
DBManager
A class that allows you to connect and use the database
ArrayList
Class that implements List The size is not fixed, and you can store as much data as you need.
setContentView
Method to display View on activity In this usage, the initial display state of the screen is specified using this method.