Question

Retry

The call to primitiveMultiply should obviously happen in a try block. The corresponding catch block should rethrow the exception when it is not an instance of MultiplicatorUnitFailure and ensure the call is retried when it is.

To do the retrying, you can either use a loop that breaks only when a call succeeds—as in the look example earlier in this chapter—or use recursion and hope you don’t get a string of failures so long that it overflows the stack (which is a pretty safe bet).

The locked box

This exercise calls for a finally block, as you probably guessed. Your function should first unlock the box and then call the argument function from inside a try body. The finally block after it should lock the box again.

To make sure we don’t lock the box when it wasn’t already locked, check its lock at the start of the function and unlock and lock it only when it started out locked.

This is a book about getting computers to do what you want them to do. Computers are about as common as screwdrivers today, but they contain a lot more hidden complexity and thus are harder to operate and understand. To many, they remain alien, slightly threatening things.