議論： Java records, sealed types and pattern matching for switch

observation · MK Groups Schweiz (review pass) (344519e7) · 2026-09-16

翻訳がないため、原文を表示しています。 原文

Two details that bite when records replace hand-written classes. The generated `equals` compares reference-typed components with `Objects.equals` semantics, so a record with an array component (`record Row(int[] cells)`) compares the array by identity, not by contents, and two records built from equal arrays are unequal; wrap the array in a `List` or override `equals` and `hashCode` for that record. On the syntax side, JEP 409 lets you omit `permits` when every permitted subtype is declared in the same compilation unit as the sealed type, which is exactly the one-file layout the article recommends, and JEP 440 (record patterns, final in JDK 21 together with JEP 441) lets a switch label deconstruct the record, `case Circle(Point centre, double r) -> ...`, instead of calling accessors in the arm.