Record Patterns

Example

An expression evaluator — the classic use case for sealed interfaces + record patterns.

public class Main {
    static int eval(Expr e) {
        return switch (e) {
            case Num(int value) -> value;
            case Add(Expr left, Expr right) -> eval(left) + eval(right);
            case Mul(Num(int a), Num(int b)) -> a * b;              // nested pattern: fast path
            case Mul(Expr left, Expr right) -> eval(left) * eval(right);
        };
    }

    static String show(Expr e) {
        return switch (e) {
            case Num(var v) -> String.valueOf(v);
            case Add(var l, var r) -> "(" + show(l) + " + " + show(r) + ")";
            case Mul(var l, var r) -> show(l) + " * " + show(r);
        };
    }

    public static void main(String[] args) {
        Expr e = new Add(new Num(2), new Mul(new Num(3), new Num(4)));
        System.out.println(show(e) + " = " + eval(e));

        Object point = new Point(3, 4);
        if (point instanceof Point(int x, int y) && x > 0) {
            System.out.println("x=" + x + ", y=" + y + ", distance^2=" + (x * x + y * y));
        }
    }
}

sealed interface Expr permits Num, Add, Mul {}
record Num(int value) implements Expr {}
record Add(Expr left, Expr right) implements Expr {}
record Mul(Expr left, Expr right) implements Expr {}
record Point(int x, int y) {}

Output

(2 + 3 * 4) = 14
x=3, y=4, distance^2=25