need help with game

I need help with a game i make for school.
My idea for the game is a reaction game;
first 2 buttons or some sort of circle whatever, When one button dissapear, you have to click on the other button as fast as possible.
The problem is, i have absolutely no idea how i must make the timer who counts the time between the button that dissapear, and when you click the button
Sorry for my bad english. :P

Quote:

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timer who counts the time between the button that dissapear, and when you click the button

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You could use a method in the System class that returns the current time. Capture the time of the first event and then the time of the second event and take the difference.

well can you give a example of that?
I am a beginner with java, and i think the teacher doesn't get it himself either ;p

long startTime = <GET THE TIME>;
... do something that takes time
long duration = <GET THE TIME> - startTime;