Java Keep Equality and Hashing Consistent

Understand the idea

The default Object equals normally represents identity. A value type may instead define equality from stable components. If you override equals, provide a compatible hashCode and satisfy reflexivity, symmetry, transitivity and consistency. Do not base a durable hash key on mutable state that changes after insertion. Hash collisions are expected possibilities, not proof that equals is broken.

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.Set;
import java.util.HashSet;

public class Main {
    record BookingKey(String session, int groupNumber) {}

    public static void main(String[] args) {
        Set<BookingKey> keys = new HashSet<>();
        keys.add(new BookingKey("Clay", 4));
        keys.add(new BookingKey("Clay", 4));
        System.out.println(keys.size());
        System.out.println(keys.contains(new BookingKey("Clay", 4)));
    }
}

Expected output

1
true

The record supplies value-oriented equals and hashCode from both components. The set recognises the second value as equal to the first even though new created a different object. The string and int components are stable for this example, making the record an appropriate key representation.