Sealed Classes & Interfaces (Java 17): The Modern Java Lens

Sealed types let a parent class or interface name the exact set of subtypes that are allowed to extend or implement it. The syntax basics, including the permits clause, the location rule, and the three modifiers a permitted subtype must pick, were covered earlier. This lesson is the modern-Java view: how sealed pairs with records to model closed domains, how it makes pattern-matching switches exhaustive, how to model state machines with it, and how it shows up in real API design.