Records in Java

The lesson "How to Create an Immutable Class in Java" built an immutable Money class by hand: private final fields, a constructor, accessors, equals, hashCode and toString. That's about forty lines for "an amount and a currency". A record (standard since Java 16) says the same thing in one line: record Money(long paise, String currency) {}. The compiler writes everything else, correctly, every time. Records are now the everyday way to model plain data in Java.

One Line, Many Members

import java.util.HashSet;
import java.util.Set;
 
public class Records {
    record Money(long paise, String currency) {
        static final Money ZERO_INR = new Money(0, "INR");
 
        Money plus(Money other) {
            return new Money(paise + other.paise, currency);
        }
 
        String display() {
            return currency + " " + paise / 100 + "."
                + String.format("%02d", paise % 100);
        }
    }
 
    public static void main(String[] args) {
        Money ticket = new Money(25000, "INR");
        Money same = new Money(25000, "INR");
        System.out.println(ticket);
        System.out.println(ticket.paise() + " " + ticket.currency());
        System.out.println(ticket.equals(same) + " " + (ticket == same));
 
        Set<Money> seen = new HashSet<>();
        seen.add(ticket);
        System.out.println(seen.contains(same));
 
        System.out.println(ticket.plus(new Money(12050, "INR")).display());
        System.out.println(Money.ZERO_INR);
    }
}

Money[paise=25000, currency=INR]
25000 INR
true false
true
INR 370.50
Money[paise=0, currency=INR]

How it works

1. record Money(long paise, String currency) declares two components. From them, the compiler generates private final fields, a constructor, accessors, equals, hashCode and toString.
2. Accessors are named after the components, without get: ticket.paise(), not getPaise().
3. toString prints Money[paise=25000, currency=INR].
4. equals compares every component, so two records with the same data are equal (though they're still different objects, so == is false). hashCode matches it, which makes records safe in HashSet and as HashMap keys.
5. You can add methods (plus, display) and static fields and methods (ZERO_INR).

What the Compiler Generates

javap -p on record Point(int x, int y) {} shows it all:

public final class Point extends java.lang.Record {
  private final int x;
  private final int y;
  public Point(int, int);
  public final java.lang.String toString();
  public final int hashCode();
  public final boolean equals(java.lang.Object);
  public int x();
  public int y();
}

The Rules

You can also write a component's accessor yourself (for example, to return a copy), as long as it keeps the same name and return type.

When to Use a Record

Common Mistakes

Calling getX(). Record accessors are x().

Expecting a mutable List component to be protected. Copy it with List.copyOf in a compact constructor.

Using records for objects with identity and changing state. An account whose balance changes is a class, not a record.

Interview Corner

Practice

1. Easy: Write record Student(String name, int marks) and print a student, their name and whether two students with the same data are equal.
2. Harder: Write record DateRange(LocalDate start, LocalDate end) with a method long days() that returns the number of days between them, and a static factory ofWeek(LocalDate start).

Key Takeaways