Java - Record

public record User(String name, int age, String phoneNumber) { }

User user = new User("Alice", 30, "050 12345678");
System.out.println(user.name()); // "Alice"
System.out.println(user);        // User[name=Alice, age=30, phoneNumber=050 12345678]

public record User(String name, int age, String phoneNumber) {

    // 1. Static constant fields shared across all record instances
    public static final int DEFAULT_AGE = 18;
    public static final String DEFAULT_PHONE = "050 00000000";

    // Compact Constructor for standard validation
    public User {
        if (name == null || name.isBlank()) {
            throw new IllegalArgumentException("Name cannot be null or blank!");
        }
    }

    // 2. Static Factory Method: Creates a User with a default phone number
    public static User of(String name, int age) {
        // Calls the standard constructor with a fallback phone number
        return new User(name, age, DEFAULT_PHONE);
    }

    // 3. Static Factory Method: Creates an adult user with a default age
    public static User createAdult(String name, String phoneNumber) {
        // Uses the static constant DEFAULT_AGE
        return new User(name, DEFAULT_AGE, phoneNumber);
    }
}

public class Main {
    public static void main(String[] args) {

        // Using static factory method 'of' (provides default phone automatically)
        User user1 = User.of("Alice", 25);
        System.out.println(user1);
        // Output: User[name=Alice, age=25, phoneNumber=050 00000000]

        // Using static factory method 'createAdult' (provides default age automatically)
        User user2 = User.createAdult("Bob", "054 12345678");
        System.out.println(user2);
        // Output: User[name=Bob, age=18, phoneNumber=054 12345678]

        // Accessing the static constant fields directly from the Record name
        System.out.println("Default user age is: " + User.DEFAULT_AGE);
    }
}

public record User(String name, int age, String phoneNumber) {

    // Compact Constructor - no parameter parentheses required!
    public User {
        // 1. Check if the name is null or blank
        if (name == null || name.isBlank()) {
            throw new IllegalArgumentException("Name cannot be null or blank!");
        }

        // 2. Check if the age is valid
        if (age < 0) {
            throw new IllegalArgumentException("Age cannot be negative: " + age);
        }

        // 3. Validate the phone number:
        //   Must not be null
        //   Must be strictly greater than 12 characters long
        //   Must have a hyphen (' ') exactly after the first 3 characters (index 3)
        //   All other characters (before and after the hyphen) must be digits only
        if (phoneNumber == null || phoneNumber.length() != 12) {
            throw new IllegalArgumentException("Phone number must be exactly 12 characters long!");
        }

        if (phoneNumber.charAt(3) != ' ') {
            throw new IllegalArgumentException("Phone number must contain a hyphen (' ') after the first 3 characters!");
        }

        // Validate that everything else consists strictly of digits
        for (int i = 0; i < phoneNumber.length(); i++) {
            if (i == 3) continue; // Skip the hyphen at index 3

            if (!Character.isDigit(phoneNumber.charAt(i))) {
                throw new IllegalArgumentException("Phone number can only contain digits (except for the hyphen at index 3)!");
            }
        }

        // No need to write this.name = name; > Java assigns the fields automatically at the end of the block!
    }
}