Builder Pattern: Pizza Order

/*
 * Problem : Process lines.
 * Input   : Several lines.
 * Output  : One description per order.
 * Approach: A builder with checks.
 * Time    : O(total options)
 * Space   : O(n)
 * Java    : 8 or newer.
 */
import java.util.Scanner;

public class Main {
        // Step 1: The Pizza is built by a Builder. The Builder collects the options step by step and checks them in build().
        static final class Pizza {
            private final String size;
            private final boolean cheese;
            private final java.util.List<String> toppings;

            private Pizza(Builder builder) {
                this.size = builder.size;
                this.cheese = builder.cheese;
                this.toppings = builder.toppings;
            }

            static class Builder {
                private String size = null;
                private boolean cheese = false;
                private final java.util.List<String> toppings = new java.util.ArrayList<>();

                // Step 2: Each setter returns the builder itself, so calls can be chained.
                Builder size(String value) {
                    this.size = value;
                    return this;
                }

                Builder extraCheese() {
                    this.cheese = true;
                    return this;
                }

                Builder topping(String value) {
                    this.toppings.add(value);
                    return this;
                }

                // Step 3: build() checks the rule "size is required" and makes the final object.
                Pizza build() {
                    if (size == null) {
                        throw new IllegalStateException("size required");
                    }
                    return new Pizza(this);
                }
            }

            @Override
            public String toString() {
                java.util.List<String> parts = new java.util.ArrayList<>();
                if (cheese) {
                    parts.add("extra cheese");
                }
                java.util.List<String> sorted = new java.util.ArrayList<>(toppings);
                java.util.Collections.sort(sorted);
                parts.addAll(sorted);
                return size + " pizza" + (parts.isEmpty() ? "" : " with " + String.join(", ", parts));
            }
        }

    // 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++) {
            // Step 4: Read the options of the line and give each one to the builder.
            Pizza.Builder builder = new Pizza.Builder();
            for (String option : lines[i].split(" ")) {
                if (option.startsWith("size:")) {
                    builder.size(option.substring(5));
                } else if (option.equals("cheese")) {
                    builder.extraCheese();
                } else if (option.startsWith("top:")) {
                    builder.topping(option.substring(4));
                }
            }

            // Step 5: build() throws an exception if the size is missing. We print an error message then.
            try {
                output[i] = builder.build().toString();
            } catch (IllegalStateException error) {
                output[i] = "error: " + error.getMessage();
            }
        }
        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)));
    }
}