Use a sealed interface and pattern matching

Description: Make a sealed interface Shape with the records Square(double side) and Circle(double radius). Write a method perimeter(Shape) with a switch expression and no default.

Hint:

• The perimeter of a circle is 2 * Math.PI * radius.

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Solution

sealed interface Shape permits Square, Circle { }

record Square(double side) implements Shape { }
record Circle(double radius) implements Shape { }

public class Main {
    static double perimeter(Shape s) {
        return switch (s) {
            case Square sq -> 4 * sq.side();
            case Circle c -> 2 * Math.PI * c.radius();
        };
    }

    public static void main(String[] args) {
        System.out.println(perimeter(new Square(5)));
        System.out.printf("%.2f%n", perimeter(new Circle(1)));
    }
}

How it works

• The interface is sealed, so the compiler knows that only Square and Circle exist, and the switch needs no default.
• If you add a new shape, the compiler shows an error in this switch until you handle it.

Read Pattern Matching and Sealed Classes.

• Previous Exercise

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