Shape Area (Sealed Types and Pattern Matching)

/*
 * Problem : Process one line of text.
 * Input   : One line of text.
 * Output  : The area with one decimal.
 * Approach: One class per shape.
 * Time    : O(1)
 * Space   : O(1)
 * Java    : 8 or newer.
 */
import java.util.Scanner;

public class Main {
        // Step 1: An interface for all shapes, and one class for each shape. The area is stored as "twice the area",
        // so the triangle formula base * height / 2 stays an exact integer.
        interface Shape {
            long twiceArea();
        }

        static class Rect implements Shape {
            final long width;
            final long height;

            Rect(long width, long height) {
                this.width = width;
                this.height = height;
            }

            public long twiceArea() {
                return 2 * width * height;
            }
        }

        static class Square implements Shape {
            final long side;

            Square(long side) {
                this.side = side;
            }

            public long twiceArea() {
                return 2 * side * side;
            }
        }

        static class Triangle implements Shape {
            final long base;
            final long height;

            Triangle(long base, long height) {
                this.base = base;
                this.height = height;
            }

            public long twiceArea() {
                return base * height;
            }
        }

    // This method solves the task. It receives the input and returns the answer.
    static String solve(String s) {
        // Step 2: Read the shape name and the numbers.
        String[] t = s.trim().split("\\s+");
        Shape shape;

        // Step 3: Create the object that fits the name.
        if (t[0].equals("rect")) {
            shape = new Rect(Long.parseLong(t[1]), Long.parseLong(t[2]));
        } else if (t[0].equals("square")) {
            shape = new Square(Long.parseLong(t[1]));
        } else if (t[0].equals("triangle")) {
            shape = new Triangle(Long.parseLong(t[1]), Long.parseLong(t[2]));
        } else {
            return "Unknown shape";
        }

        // Step 4: Print the area with one decimal. twiceArea / 2 is the whole part, and the remainder gives .5 or .0.
        long twice = shape.twiceArea();
        return (twice / 2) + "." + (twice % 2 == 1 ? "5" : "0");
    }

    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        String s = scanner.hasNextLine() ? scanner.nextLine() : "";
        System.out.println(solve(s));
    }
}