Modern Language Features

A type pattern binds a variable in the same test: if (obj instanceof String s) checks the type and, on success, assigns the already-cast value to s. The binding is in scope wherever the condition is provably true, so the separate explicit cast and the redundant second reference to the type both disappear.

A record is a transparent carrier for immutable data. From its component list the compiler generates a private final field and an accessor per component, a canonical constructor, and value-based equals, hashCode, and toString, all derived from those components. You write none of them yet may override any.

The diamond <> lets the compiler infer a generic type's arguments from the assignment target, so List<String> xs = new ArrayList<>(); need not repeat them. var infers a local variable's type from its initialiser at compile time, and the variable then stays statically typed. var is legal only on a local with an initialiser — never on a field, a parameter or a return type.

Each case may carry a type pattern (case Circle c ->) that tests the type and binds the value in the label, optionally refined by a when guard. A record pattern destructures the record's components in that same label (case Point(int x, int y) ->), nesting recursively. Over a sealed hierarchy the compiler proves the switch exhaustive, so no default is needed; null matches only an explicit case null, otherwise the switch throws.

A text block is a multi-line string literal opened by """ and a line break, so embedded JSON, SQL or HTML keeps its shape without \n or escaped quotes. The compiler strips the incidental indentation — the white-space prefix common to every content line and to the closing delimiter — and removes trailing spaces. What remains is an ordinary String with no runtime cost.

sealed restricts which types may extend or implement a type: the permits clause names them, and each named subtype must sit in the same module or package and be declared final, sealed or non-sealed. Because the full subtype set is then known at compile time, a switch over the hierarchy — typically over records — can be checked for exhaustiveness.

Migrate leaf-first, never big-bang. Convert only the already-immutable DTOs into records, leaving the mutated ones behind a mapper until their callers stop mutating them. Seal the PaymentEvent hierarchy, then replace the instanceof chain with an exhaustive pattern switch: an unhandled subtype becomes a compile error, not a silent fall-through. Ship each step separately, pinned by the contract tests that guard the public API.