Java Pattern Matching, Sealed Classes and Records

Modern Java (17, 21 and 25) has features which make code shorter and safer. The three most important work together: sealed types, records and pattern matching.

Sealed classes and interfaces

A sealed type says exactly which classes are allowed to extend or implement it. So the compiler knows all the possible cases.

sealed interface Shape permits Circle, Rectangle, Triangle { }

record Circle(double radius) implements Shape { }
record Rectangle(double width, double height) implements Shape { }
record Triangle(double base, double height) implements Shape { }

public class Main {
    static double area(Shape shape) {
        return switch (shape) {
            case Circle c -> Math.PI * c.radius() * c.radius();
            case Rectangle r -> r.width() * r.height();
            case Triangle t -> 0.5 * t.base() * t.height();
        };                                             // no default needed
    }

    public static void main(String[] args) {
        System.out.printf("%.2f%n", area(new Circle(2)));
        System.out.printf("%.2f%n", area(new Rectangle(3, 4)));
        System.out.printf("%.2f%n", area(new Triangle(6, 5)));
    }
}

Because Shape is sealed, the switch covers all the cases and needs no default. If you add a new shape and forget the switch, the compiler gives an error.

Pattern matching for switch

A switch can test the type of an object and use a guard (when):

public class Pattern {
    static String describe(Object o) {
        return switch (o) {
            case null -> "nothing";
            case Integer i when i > 100 -> "big number " + i;
            case Integer i -> "number " + i;
            case String s when s.isEmpty() -> "empty text";
            case String s -> "text of length " + s.length();
            case int[] a -> "int array of " + a.length;
            default -> "something else: " + o;
        };
    }

    public static void main(String[] args) {
        System.out.println(describe(5));
        System.out.println(describe(500));
        System.out.println(describe(""));
        System.out.println(describe("hello"));
        System.out.println(describe(new int[3]));
        System.out.println(describe(null));
        System.out.println(describe(2.5));
    }
}

Record patterns

A pattern can take a record apart and give you its parts at once:

record Point(int x, int y) { }
record Line(Point from, Point to) { }

public class Rec {
    static String show(Object o) {
        if (o instanceof Point(int x, int y)) {
            return "Point at " + x + "," + y;
        }
        if (o instanceof Line(Point(var x1, var y1), Point(var x2, var y2))) {
            return "Line from " + x1 + "," + y1 + " to " + x2 + "," + y2;
        }
        return "unknown";
    }

    public static void main(String[] args) {
        System.out.println(show(new Point(1, 2)));
        System.out.println(show(new Line(new Point(0, 0), new Point(5, 5))));
        System.out.println(show("text"));
    }
}

A small expression calculator

This is a classic use: a sealed interface, records and a recursive switch.

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 { }

public class Calc {
    static int eval(Expr e) {
        return switch (e) {
            case Num n -> n.value();
            case Add(Expr l, Expr r) -> eval(l) + eval(r);
            case Mul(Expr l, Expr r) -> eval(l) * eval(r);
        };
    }

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

What each feature gives

Feature	Since	Benefit
Records	Java 16	Short data classes
Pattern matching for instanceof	Java 16	No manual cast
Sealed classes	Java 17	A closed list of subtypes
Pattern matching for switch	Java 21	Type tests with guards, no long if chains
Record patterns	Java 21	Take a record apart in one step

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