EqualsVerifier

EqualsVerifier can be used in Java unit tests to verify whether the contract for the equals and hashCode methods in a class is met.

Getting Started

EqualsVerifier's Maven coordinates are:

<dependency>
    <groupId>nl.jqno.equalsverifier</groupId>
    <artifactId>equalsverifier</artifactId>
    <version>3.15.6</version>
    <scope>test</scope>
</dependency>

(Note that there's also a 'fat' jar with no transitive dependencies with artifactId equalsverifier-nodep.)

Now you can write a test:

@Test
public void equalsContract() {
    EqualsVerifier.forClass(Foo.class).verify();
}

EqualsVerifier is an opinionated library, which means that it can be quite strict. If you feel it's too much, you can make it more lenient:

@Test
public void equalsContract() {
    EqualsVerifier.simple().forClass(Foo.class).verify();
}

This way, EqualsVerifier will throw less errors at you. However, it's usually better to just fix the errors: EqualsVerifier throws them for a reason!

For more information, please see the project's website.

Prefer to watch a short video?

Video by Tom Cools

A note on equality

EqualsVerifier cares about bug-free equality, in Java and in real life. The place where a person happens to be born, the colour of their skin, their gender, or the person they happen to love, must not affect the way they are treated in life. If it does, that's a bug and it should throw an error.

Don't allow bugs in your equality.

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Contribution

Pull requests are welcome! If you plan to open one, please also register an issue or send a message to the Google Group, so we can discuss it first. It would be a shame to put in a lot of work on something that isn't a good fit for the project. Also, I can help you by giving pointers on where to find certain things.

Development

Build

To build EqualsVerifier, you need Maven. Just call mvn from the command-line, without any parameters, and you're done. Alternatively, you can use any IDE with Maven support.

There are several Maven profiles that can be enabled or disabled:

Formatting

EqualsVerifier uses Prettier-Java through prettier-maven-plugin to format Java files. You can call it using mvn clean verify, or run mvn prettier:write to only run the formatter.

Modules

This project is a multi-module project to make it easier to deal with shading and multi-release jar files. See this question on StackOverflow for the rationale behind it.

Here's a description of the modules:

Signed JAR

The lib/ folder in the equalsverifier-test-core module contains a local Maven repository containing a signed JAR, used to test potential ClassLoader issues. Here's how to install a JAR into it:

mvn org.apache.maven.plugins:maven-install-plugin:2.3.1:install-file \
                        -Dfile=<path-to-file> -DgroupId=<myGroup> \
                        -DartifactId=<myArtifactId> -Dversion=<myVersion> \
                        -Dpackaging=<myPackaging> -DcreateChecksum=true \
                        -DlocalRepositoryPath=equalsverifier-test-core/lib

The signed JAR itself can be found in this repo.

Website

To generate the website

• Using Docker: start the server by running docker-compose up or docker compose run jekyll serve.
• Using Jekyll: install the Ruby 3.x toolchain and run bundle exec jekyll serve --watch

Note that thepage uses the TilburgsAns font but references it from the main site at jqno.nl. In development, it will fall back to a sans-serif font. See the font license here.

Disclaimer

Copyright 2009-2024 Jan Ouwens