Custom Exceptions

Java ships with dozens of built-in exception types, but real applications run into failures that none of them name precisely. An order lookup returns nothing. A cart checks out with no items. A payment provider declines a card. These are domain failures, and giving each one its own exception type lets callers tell them apart, lets logs read clearly, and lets the type signature document what can go wrong. This lesson covers how to declare a custom exception, what base class to extend, the four constructors every custom exception should provide, how to attach domain data as fields, and when a standard exception already fits and a custom one is overkill.