Shape Areas with Polymorphism

/*
 * Problem : Process commands line by line.
 * Input   : Several lines.
 * Output  : One area per line.
 * Approach: Abstract parent, child classes.
 * Time    : O(n)
 * Space   : O(n)
 * Java    : 8 or newer.
 */
import java.util.Scanner;

public class Main {
        // Step 1: An abstract class is a common parent. It says every shape must give its area.
        static abstract class Shape {
            // Step 2: We return twice the area, so the answer stays an exact whole number.
            abstract long doubledArea();
        }

        static class Rect extends Shape {
            private final long width;
            private final long height;

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

            @Override
            long doubledArea() {
                return 2 * width * height;
            }
        }

        static class Square extends Shape {
            private final long side;

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

            @Override
            long doubledArea() {
                return 2 * side * side;
            }
        }

        static class Triangle extends Shape {
            private final long base;
            private final long height;

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

            @Override
            long doubledArea() {
                return base * height;
            }
        }

    // This method solves the task. It receives the input and returns the answer.
    static String[] solve(String[] lines) {
        String[] output = new String[lines.length];
        for (int i = 0; i < lines.length; i++) {
            String[] parts = lines[i].split(" ");

            // Step 3: Make the right child object. We store it in a variable of the parent type.
            Shape shape;
            if (parts[0].equals("rect")) {
                shape = new Rect(Long.parseLong(parts[1]), Long.parseLong(parts[2]));
            } else if (parts[0].equals("square")) {
                shape = new Square(Long.parseLong(parts[1]));
            } else {
                shape = new Triangle(Long.parseLong(parts[1]), Long.parseLong(parts[2]));
            }

            // Step 4: The same call works for every child. This is polymorphism.
            long doubled = shape.doubledArea();
            output[i] = (doubled / 2) + (doubled % 2 == 0 ? ".0" : ".5");
        }
        return output;
    }

    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        java.util.List<String> all = new java.util.ArrayList<>();
        while (scanner.hasNextLine()) {
            String line = scanner.nextLine().trim();
            if (!line.isEmpty()) {
                all.add(line);
            }
        }
        String[] lines = all.toArray(new String[0]);
        System.out.println(String.join("\n", solve(lines)));
    }
}