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What would passing input into its own constructor do? What would passing it into TimerPanel's constructor before it's initialized do? Depending on how things are defined, that might even result in a...

A setter is just a method that takes a parameter, then sets something in the class based on that parameter. It would probably work exactly like your constructor.

Recommended reading: Passing...

Can't you just use a setter method in one of them?

But many times, this is a sign of a bad design. Are you sure you're instantiating things where they really belong?