Function Pipeline (Composition)

/*
 * Problem : Function pipeline.
 * Input   : Start value, then operation lines.
 * Output  : The final value.
 * Approach: Compose functions.
 * Time    : O(n)
 * Space   : O(n)
 * Java    : 8 or newer.
 */
import java.util.Scanner;

public class Main {
    // This method solves the task. It receives the input and returns the answer.
    static long solve(String[] lines) {
        // Step 1: Start with a function that returns its input (the identity function).
        java.util.function.LongUnaryOperator pipeline = value -> value;

        // Step 2: For each operation line, make a small function and join it at the end with andThen.
        for (int i = 1; i < lines.length; i++) {
            String[] parts = lines[i].split(" ");
            java.util.function.LongUnaryOperator step;
            if (parts[0].equals("add")) {
                long amount = Long.parseLong(parts[1]);
                step = value -> value + amount;
            } else if (parts[0].equals("mul")) {
                long amount = Long.parseLong(parts[1]);
                step = value -> value * amount;
            } else {
                step = value -> value * value;
            }
            pipeline = pipeline.andThen(step);
        }

        // Step 3: Run the one combined function on the starting number.
        return pipeline.applyAsLong(Long.parseLong(lines[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(solve(lines));
    }
}