android: How can I send an activity to background & call her from there

I am in process of creating some test application "Time Tracker" app like stopwatch so I start app go to Activity where I start time counter and now I need some functionality to send my activity to background or minimize or hide and after some period to call this activity again and stop her counter to get passed time period .

How can I send Activity to back and call her t开发者_如何学编程o front but not to stop timer counter ??

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Using a Service would be the best way I guess. You can than call finish() on your Activity and the Service can start your application when a specific time was reached.