Java record

last modified March 8, 2024

In this article we show how to work with record type in Java. Record is a type that is primarily used to hold immutable data.

A record is a type that is designed to hold immutable data. It is very useful for data analysis. The record type simplifies code and improves its readability. It removes unnecessary boilerplate.

record User(String name, String occupation) {
}

This is a definition of a simple record. The compiler automatically creates two final fields and two public read accessors. A public constructor is derived from the record parameters.

The compiler auto-generates hashCode, equals, and toString methods.

Java record cannot extend any class, cannot declare instance fields, and cannot be abstract. A record can implement interfaces. We can also define our own constructors and methods.

Other languages also have similar types. C# and F# have records, Scala case classes, and Kotlin data classes.

Simple example

The following is a simple example with a Java record.

void main() {

    var u = new User("John Doe", "gardener");
    System.out.println(u);

    System.out.println(u.name());
    System.out.println(u.occupation());
}

record User(String name, String occupation) {
}

The program defines a User record with two string parameters.

record User(String name, String occupation) {
}

The definition of a Java record is concise.

var u = new User("John Doe", "gardener");

A new instance of a record is created with the new keyword.

System.out.println(u);

We pass the record instance to the System.out.println, invoking record's toString method.

System.out.println(u.name());
System.out.println(u.occupation());

We call the two auto-generated getters.

$ java Main.java
User[name=John Doe, occupation=gardener]
John Doe
gardener

Comparing records

Java record auto-generates the equals method. It provides a value-based equality.

void main() {

    var u = new User("John Doe", "gardener");
    var u2 = new User("Roger Roe", "driver");
    var u3 = new User("John Doe", "gardener");

    if (u.equals(u2)) {
        System.out.println("users are equal");
    } else {
        System.out.println("users are not equal");
    }

    if (u.equals(u3)) {
        System.out.println("users are equal");
    } else {
        System.out.println("users are not equal");
    }
}

record User(String name, String occupation) {
}

The program compares three User record types.

$ java Main.java
users are not equal
users are equal

Sorting records

The next example sorts records.

import java.util.Comparator;
import java.util.List;

void main() {

    var users = List.of(
            new User("John", "Doe", 1230),
            new User("Lucy", "Novak", 670),
            new User("Ben", "Walter", 2050),
            new User("Robin", "Brown", 2300),
            new User("Amy", "Doe", 1250),
            new User("Joe", "Draker", 1190),
            new User("Janet", "Doe", 980),
            new User("Albert", "Novak", 1930));

    var sorted = users.stream().sorted(Comparator.comparingInt(User::salary)).toList();
    System.out.println(sorted);
}

record User(String fname, String lname, int salary) {
}

We have a User record with three arguments; we sort the users by their salaries.

var sorted = users.stream().sorted(Comparator.comparingInt(User::salary)).toList();

We use Java stream to sort the data.

Customize record

We can customize record types by overriding methods or defining custom ones.

import java.util.List;

void main() {

    var users = List.of(
            new User("John", "Doe", 1230),
            new User("Lucy", "Novak", 670),
            new User("Ben", "Walter", 2050),
            new User("Robin", "Brown", 2300),
            new User("Amy", "Doe", 1250),
            new User("Joe", "Draker", 1190),
            new User("Janet", "Doe", 980),
            new User("Albert", "Novak", 1930));

    var sorted = users.stream().sorted().toList();
    System.out.println(sorted);
}

record User(String fname, String lname, int salary) implements Comparable<User> {
    @Override
    public int compareTo(User u) {
        return this.lname.compareTo(u.lname);
    }
}

The program defines a custom comparator. The default comparison of the User type is by last name.

record User(String fname, String lname, int salary) implements Comparable<User> {
    @Override
    public int compareTo(User u) {
        return this.lname.compareTo(u.lname);
    }
}

The record implements the Comparable interface and overrides the compareTo method where we compare the users' last names.

Filtering records

The next example filters a list of records.

import java.util.List;
import java.util.stream.Collectors;

void main() {

    var users = List.of(
            User.of("John", "Doe", 1230),
            User.of("Lucy", "Novak", 670),
            User.of("Ben", "Walter", 2050),
            User.of("Robin", "Brown", 2300),
            User.of("Amy", "Doe", 1250),
            User.of("Joe", "Draker", 1190),
            User.of("Janet", "Doe", 980),
            User.of("Albert", "Novak", 1930));

    var filtered = users.stream().filter(e -> e.salary() > 2000)
            .collect(Collectors.toList());
    System.out.println(filtered);
}

record User(String fname, String lname, int salary) {
    public static User of(String fname, String lname, int salary) {
        return new User(fname, lname, salary);
    }
}

We have a list of User records. We filter the users by their salary.

var filtered = users.stream().filter(e -> e.salary() > 2000)
        .collect(Collectors.toList());

We filter out all users whose salary is bigger than 2000.

$ java Main.java
[User[fname=Ben, lname=Walter, salary=2050], User[fname=Robin, lname=Brown, salary=2300]]

Source

Java Record - language reference

In this article we have worked with Java record type.

Author

My name is Jan Bodnar, and I am a passionate programmer with extensive programming experience. I have been writing programming articles since 2007. To date, I have authored over 1,400 articles and 8 e-books. I possess more than ten years of experience in teaching programming.

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