Record patterns: destructuring arrives

Key points

• JEP 440 lets a pattern name the components: case Point(int x, int y).
• Patterns nest, so you can destructure a record inside a record in one line.
• It only works on records, because only records publish a guaranteed component order.
• var works inside a pattern when the type is obvious: case Point(var x, var y).
• This is what turns sealed hierarchies into a genuinely usable alternative to the visitor pattern.

Example

public class Main {
    sealed interface Shape permits Circle, Rect { }
    record Point(int x, int y) { }
    record Circle(Point centre, double r) implements Shape { }
    record Rect(Point topLeft, Point bottomRight) implements Shape { }

    static String origin(Shape s) {
        return switch (s) {
            // nested destructuring: reach straight into Point
            case Circle(Point(var x, var y), var r) -> "circle at " + x + "," + y + " r=" + r;
            case Rect(Point(var x1, var y1), Point(var x2, var y2)) ->
                "rect " + (x2 - x1) + "x" + (y2 - y1) + " from " + x1 + "," + y1;
        };
    }

    public static void main(String[] args) {
        System.out.println(origin(new Circle(new Point(3, 4), 2.5)));
        System.out.println(origin(new Rect(new Point(0, 0), new Point(10, 5))));

        // Also works with instanceof
        Object o = new Circle(new Point(1, 1), 9);
        if (o instanceof Circle(Point p, double radius)) {
            System.out.println("instanceof pattern: p=" + p + " radius=" + radius);
        }
    }
}

Record patterns replace a type check followed by a chain of accessors with one readable line.