EqualsBuilder: an API for object comparison in Java

What is EqualsBuilder?

Compare objects in Java using the following DSL:

@Override
public boolean equals(Object o) {
    return EqualsBuilder.test(this, o)
            .comparing(Human::getName)
            .comparing(Human::getSurname)
            .comparing(Human::getAge)
            .areEqual();
}

Motivation

In Java there are three methods inherited from Object class that are often overloaded:

• java boolean equals(Object o)
• int hashCode()
• String toString()

The most simple implementation for toString, generated by an IDE might look as follows:

@Override
public String toString() {
    return "Address{" +
            "house=" + house +
            ", street='" + street + '\'' +
            ", city='" + city + '\'' +
            '}';
}

This code is not exactly what is considered 'pretty' by most developers. A more appealing implementation, using a helper from Guava would look like:

@Override
public String toString() {
    return MoreObjects.toStringHelper(this)
            .add("house", house)
            .add("street", street)
            .add("city", city)
            .toString();
}

With a better readability and maintainability, it is usually favored by the developers more.

For a .hashCode() method overloads in Java 7 an Objects.hash(Object...) method was introduced, which allowed developers to write the following code:

@Override
public int hashCode() {
    return Objects.hash(house, street, city);
}

At the same time Objects.equals(a, b) was also introduced, making .equals overloads somewhat more readable.

@Override
public boolean equals(Object o) {
    if (this == o) return true;
    if (o == null || getClass() != o.getClass()) return false;
    Address that = (Address) o;
    return Objects.equals(house, that.house) &&
            Objects.equals(street, that.street) &&
            Objects.equals(city, that.city);
}

Still, this implementation has some nasty boilerplate code like null checks, type checks, and type casts.

As an alternative one might consider using EqualsBuilder class from Apache Commons library:

@Override
public boolean equals(Object o) {
    if (o == null) { return false; }
    if (o == this) { return true; }
    if (o.getClass() != getClass()) {
        return false;
    }
    Address rhs = (Address) o;
    return new EqualsBuilder()
            .appendSuper(super.equals(o))
            .append(house, rhs.house)
            .append(street, rhs.street)
            .append(city, rhs.city)
            .isEquals();
}

Unfortunately, not only does this implementation not help with casts and type checks, but it also takes more space on your screen and attempts to fix English language by introducing a method named "isEquals".

To compliment already existing helpers for overloading .hashCode and .toString, an attempt was made to create a DSL for comparing two Java objects.

Goals

The goals for this project are:

• Make a simple yet good DSL for comparing objects by different fields
• Reduce boilerplate code required to use this DSL to zero
• Reduce text repetition (.append(street, other.street) might look like .append(street))
• Make it comparable to a naive implementation generated by an IDE performance-wise