Vector2D Class

/*
 * Problem : Run the commands.
 * Input   : Command lines.
 * Output  : One line per answer.
 * Approach: Vec2 class
 * Time    : O(n)
 * Space   : O(n)
 * Java    : 8 or newer.
 */
import java.util.Scanner;

public class Main {
        // Step 1: An immutable vector. Every operation returns a new vector and keeps the old ones as they were.
        static class Vec2 {
            final long x;
            final long y;

            Vec2(long x, long y) {
                this.x = x;
                this.y = y;
            }

            Vec2 plus(Vec2 other) {
                return new Vec2(x + other.x, y + other.y);
            }

            Vec2 times(long k) {
                return new Vec2(x * k, y * k);
            }

            // Step 2: The dot product and the squared length are methods of the vector itself.
            long dot(Vec2 other) {
                return x * other.x + y * other.y;
            }

            @Override
            public String toString() {
                return "(" + x + ", " + y + ")";
            }
        }

    // This method solves the task. It receives the input and returns the answer.
    static String[] solve(String[] lines) {
        java.util.Map<String, Vec2> vectors = new java.util.HashMap<String, Vec2>();
        java.util.List<String> out = new java.util.ArrayList<String>();
        for (String line : lines) {
            String[] part = line.split(" ");
            if (part[0].equals("vec")) {
                vectors.put(part[1], new Vec2(Long.parseLong(part[2]), Long.parseLong(part[3])));
            } else if (part[0].equals("add")) {
                vectors.put(part[3], vectors.get(part[1]).plus(vectors.get(part[2])));
            } else if (part[0].equals("scale")) {
                vectors.put(part[3], vectors.get(part[1]).times(Long.parseLong(part[2])));
            } else if (part[0].equals("dot")) {
                out.add(String.valueOf(vectors.get(part[1]).dot(vectors.get(part[2]))));
            } else if (part[0].equals("len2")) {
                out.add(String.valueOf(vectors.get(part[1]).dot(vectors.get(part[1]))));
            } else {
                out.add(vectors.get(part[1]).toString());
            }
        }
        return out.toArray(new String[0]);
    }

    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)));
    }
}