New to Java

If I understand it correctly, the guidling principle to 'thread safety' is to separate the classes for user interfaces and the logic behind the interfaces.

So, what I've been doing is to set up pairs of classes; one for the user interface (JFrame, JPanel, JTextBox, layout etc) and another 'checker' class (with regexes, logic etc) to interpret the inputs into the UI and to pass them back to the 'engine' code classes. For instance, the user might input the string '2000' into a JTextBox and the checker class will interpret this as a date with a value of 01/01/2000 00:00:00.000 and pass it on. The checker class will also check that a finish date occurs after a start date, and so on.

Question 1: does it matter which way round I call the classes? Should the UI class call the checker class, should it be the other way round or does it make no difference (other than following up my own conventions)?

Question 2: can you briefly explain what 'thread safety' means anyway?

Thanks in advance