Java 21 Pattern Matching Tutorial

Java 21 finalizes all essential pattern matching features, enabling a clean alternative to the visitor pattern. The tutorial walks through extended instanceof with type patterns, switch enhancements using arrow syntax, sealed interfaces for exhaustiveness checking, and record patterns for destructuring. It shows how combining sealed types with pattern-matching switches gives compile-time safety when new subtypes are added. The tutorial also covers guarded patterns with 'when' clauses, the four principles of data-oriented programming (immutable records, sealed alternatives, boundary validation, no illegal states), and Java 21 preview unnamed patterns using underscore to declutter switches while preserving exhaustiveness checks.