Introduction

If you want to check class equivalence in Java, you need to implement ʻequals () and hashCode () `properly [^ 1]. Normally, I don't implement it myself, so I think there is no mistake, but I'm worried about not testing it.

When I was looking for an efficient method, I found a library called EqualsVerifier [].

Installation

Getting Started describes how to set in Maven. For other tools, see Mvn Repository.

Maven Repository: nl.jqno.equalsverifier » equalsverifier

How to use

For example, there is the following Name class. These ʻequals () and hashCode () `are automatically generated by IntelliJ IDEA.

import java.util.Objects;

public final class Name {
    private final String name;

    public Name of(String name) {
        return new Name(name);
    }

    public Name(String name) {
        this.name = name;
    }

    @Override
    public boolean equals(Object o) {
        if (this == o) return true;
        if (o == null || getClass() != o.getClass()) return false;
        Name name1 = (Name) o;
        return Objects.equals(name, name1.name);
    }

    @Override
    public int hashCode() {
        return Objects.hash(name);
    }
}

Here's a test for this (using JUnit 4.12.

import nl.jqno.equalsverifier.EqualsVerifier;
import org.junit.Test;

import java.sql.Timestamp;

public final class EqualsVerifierTest {
    @Test
    public void test_name() {
        EqualsVerifier.forClass(Name.class).verify();
    }
}

Only this. It's easy.

Of course, I don't know if I really tested it, so for example, I test it with the Timestamp [] class, which does not follow the general contract of ʻequals ()`.

EqualsVerifier.forClass(Timestamp.class).verify();

I got the following error.

java.lang.AssertionError: Overloaded: More than one equals method found. Signature should be: public boolean equals(Object obj) For more information, go to: http://www.jqno.nl/equalsverifier/errormessages

If you want to exclude certain fields

It can be found in Ignoring fields, but can be excluded, for example: See the documentation for details.

Pass the field names to ignore to the withIgnoredFields method
Use the Java keyword transient

I think it's better to use transient for myself.

Operating principle

It is written at the bottom of Why, what, how?, but it is roughly as follows. It's a little black magic.

Like the mocking framework, create an instance without calling a constructor.
Use reflection to set the value in the field.
Check if ʻequals () and hashCode () `get the expected results for these.
Examine the signature of the ʻequals () `method to make sure you are using overrides instead of overloads.

in conclusion

I thought it was important to have a feeling of comfort (´ω`)

[^ 1]: Effective Java 2nd Edition See "Item 8 Follow the general contract when overriding equals" and "Item 9 Always override hashCode when overriding equls".