permits 절

하위 타입 허용 절

기본 문법

public class Main {
    sealed interface Vehicle permits Car, Truck, Motorcycle {}
    record Car() implements Vehicle {}
    record Truck() implements Vehicle {}
    record Motorcycle() implements Vehicle {}

    public static void main(String[] args) {
        Vehicle v = new Truck();
        System.out.println(v.getClass().getSimpleName());
    }
}

계약 구현하기

허용 목록 생략

public class Main {
    sealed interface Status {}
    record Active() implements Status {}
    record Inactive() implements Status {}

    public static void main(String[] args) {
        Status s = new Active();
        System.out.println(s.getClass().getSimpleName());
    }
}

허용 목록이 필요한 경우

순서는 중요하지 않습니다

허용 목록을 사용하는 클래스 계층 구조

public class Main {
    sealed static abstract class Node permits Leaf, Branch {
        abstract int size();
    }
    static final class Leaf extends Node {
        int size() { return 1; }
    }
    static final class Branch extends Node {
        final Node a, b;
        Branch(Node a, Node b) { this.a = a; this.b = b; }
        int size() { return a.size() + b.size(); }
    }

    public static void main(String[] args) {
        Node tree = new Branch(new Leaf(), new Branch(new Leaf(), new Leaf()));
        System.out.println("Leaves: " + tree.size());
    }
}

하위 타입 누락

허용된 타입은 공개 정보입니다

final과 봉인된 하위 타입 혼합하기

public class Main {
    sealed interface Shape permits Circle, Polygon {}
    record Circle(double r) implements Shape {}
    sealed interface Polygon extends Shape permits Triangle, Rectangle {}
    record Triangle() implements Polygon {}
    record Rectangle() implements Polygon {}

    public static void main(String[] args) {
        Shape s = new Triangle();
        System.out.println(s instanceof Polygon);
    }
}

권장 사항

빠른 확인

복습