switch แบบครบถ้วนบนชนิดแบบปิดผนึก

สวิตช์ที่ครอบคลุมทุกกรณี

สวิตช์แบบรูปแบบชนิด

public class Main {
    sealed interface Shape permits Circle, Square {}
    record Circle(double radius) implements Shape {}
    record Square(double side) implements Shape {}

    static String describe(Shape s) {
        return switch (s) {
            case Circle c -> "circle r=" + c.radius();
            case Square sq -> "square s=" + sq.side();
        };
    }

    public static void main(String[] args) {
        System.out.println(describe(new Circle(2)));
        System.out.println(describe(new Square(3)));
    }
}

ไม่จำเป็นต้องมีกรณีเริ่มต้น

ความครบถ้วนขณะคอมไพล์

การคำนวณ Result

public class Main {
    sealed interface Shape permits Circle, Square {}
    record Circle(double radius) implements Shape {}
    record Square(double side) implements Shape {}

    static double area(Shape s) {
        return switch (s) {
            case Circle c -> Math.PI * c.radius() * c.radius();
            case Square sq -> sq.side() * sq.side();
        };
    }

    public static void main(String[] args) {
        System.out.printf("%.2f%n", area(new Circle(1)));
        System.out.printf("%.2f%n", area(new Square(2)));
    }
}

คำสั่งสวิตช์กับนิพจน์สวิตช์

การจัดการค่า null

public class Main {
    sealed interface Shape permits Circle, Square {}
    record Circle(double radius) implements Shape {}
    record Square(double side) implements Shape {}

    static String describe(Shape s) {
        return switch (s) {
            case null -> "no shape";
            case Circle c -> "circle";
            case Square sq -> "square";
        };
    }

    public static void main(String[] args) {
        System.out.println(describe(null));
        System.out.println(describe(new Circle(1)));
    }
}

เงื่อนไขกำกับไม่ทำลายความครอบคลุมทุกกรณี

public class Main {
    sealed interface Shape permits Circle, Square {}
    record Circle(double radius) implements Shape {}
    record Square(double side) implements Shape {}

    static String size(Shape s) {
        return switch (s) {
            case Circle c when c.radius() > 10 -> "big circle";
            case Circle c -> "small circle";
            case Square sq -> "square";
        };
    }

    public static void main(String[] args) {
        System.out.println(size(new Circle(20)));
        System.out.println(size(new Circle(2)));
    }
}

การใช้สวิตช์กับคลาสแบบปิด

public class Main {
    sealed static abstract class Token permits Word, Number {}
    static final class Word extends Token { final String text; Word(String t){text=t;} }
    static final class Number extends Token { final int value; Number(int v){value=v;} }

    static String show(Token t) {
        return switch (t) {
            case Word w -> "word:" + w.text;
            case Number n -> "num:" + n.value;
        };
    }

    public static void main(String[] args) {
        System.out.println(show(new Word("hi")));
        System.out.println(show(new Number(7)));
    }
}

เหตุใดจึงทรงพลัง

ตัวอย่างที่สมบูรณ์

public class Main {
    sealed interface Result permits Ok, Err {}
    record Ok(int value) implements Result {}
    record Err(String message) implements Result {}

    static String render(Result r) {
        return switch (r) {
            case Ok ok -> "value = " + ok.value();
            case Err err -> "error: " + err.message();
        };
    }

    public static void main(String[] args) {
        System.out.println(render(new Ok(10)));
        System.out.println(render(new Err("bad input")));
    }
}

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