Open the Thingies folder in Sublime, or open Thingies.pde in Processing.
Run the program. You should get an error message that says The field Thingy.myX is not visible because the client code in setup() is trying to access private member variables.
Fix the program by
finishing the four setter and getter functions in Thingy.java
rewriting the client code in setup() to use the four setter and getter functions
The finished program should print Distance between bob and mary is 56.568542