1Sealed.java

public sealed interface Item permits Box, Container { }

Usage.java

public class WeightCalculator {
    public int calculate(Item item) {
        return switch (item) {
            case Box box -> calculate(box);
            case Container container -> calculate(container);
            // the default case is removed
        };
    }
    public int calculate(Box box) {
        return box.size();
    }
    public int calculate(Container container) {
        int sum = 2;
        for (Item item : container.items()) {
            sum += calculate(item);
        }
        return sum;
    }
}