Java Use a Record for a Small Data Carrier

Understand the idea

A record declaration names its components. The compiler supplies a canonical constructor, accessors, equals, hashCode and toString, subject to the language rules. A compact constructor can validate or normalise the incoming component values. A record is final and cannot extend an arbitrary application class. If a component is a mutable list, make a defensive copy when the record’s contract should not expose future changes.

Run it locally

Use a local JDK 21 or newer. Save the complete example as Main.java in a separate lab folder, then compile and run it with the commands below. Java examples do not run in Bhylu’s browser playground. The -ea option enables assertion checks used by some lessons.

JDK and terminal setup

javac --release 21 -encoding UTF-8 -d out Main.java
java -ea -cp out Main

A worked example

import java.util.List;

public class Main {
    record Booking(String session, List<String> names) {
        Booking {
            if (session == null || session.isBlank()) throw new IllegalArgumentException("session required");
            names = List.copyOf(names);
        }
    }

    public static void main(String[] args) {
        Booking booking = new Booking("Woodwork", List.of("Asha", "Noor"));
        System.out.println(booking.session());
        System.out.println(booking.names().size());
    }
}

Expected output

Woodwork
2

The compact constructor checks the session and copies the names into an unmodifiable list. The generated names accessor then returns that protected list reference. List.copyOf also rejects null elements, which is a deliberate contract here rather than an incidental implementation detail. Records have been a final language feature since Java 16 and are available in this course baseline.