Java: The Object Equality Problem

I was writing some Java test code when I faced up the voracity of the

equals

method. It presents, despite its ostensible simplicity, a tricky problem.

I'd like to emphasize that this is not a specific Java issue. For example C# has a homologous way.

A bit of Java context

The

Object

class is the root of every class. It defines various methods and

equals

is one of them. By default this method has a simple behavior: an object x is only equals to itself. Any other object is different.

Obviously the

equals

method has the common properties of an equivalence relation. It is reflexive: x is equal to x; It is symmetric: if x is equal to y, then y is equal to x; and so on...

Furthermore a logic relationship links

equals

and the

hashCode

method. The latter returns the object hash. In this context it's an integer representation of the object. So if two object are equal, then their hash should be equal too.

The problem

I'll use a simple code example to highlight the main issue. Here is the starting point:

interface Book {
  String title();

  String author();
}

final class DbBook implements Book {
  ...
}

We can ask to a

Book

instance its title and its author. A

DbBook

instance represents a book stored in a database.

As I said the

Object

class is the root of every class. This means that

DbBook

also inherits the

equals

method with its default behavior. So we should override

equals

to implement a custom equivalence relation. To respect the aforesaid logic implicatio we should also override the

hashCode

method.

Now suppose that in our context two books are equal if they have the same title. This seems the

equals

goal and here is a common implementation (I generated it with the IDE):

final class DbBook implements Book {
  ...
  @Override
  public boolean equals(final Object o) {
    if (this == o) return true;
    if (!(o instanceof DbBook)) return false;
    final DbBook dbBook = (DbBook) o;
    return title().equals(dbBook.title());
  }

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

However because the

instanceof

check,

aDbBook

is only comparable to

anotherDbBook

. This means that an

AnotherBookImplementation

instance is always different from

aDbBook

, though having the same title.

The problem seems the

instanceof

check. So we can weak it a bit:

final class DbBook implements Book {
  ...
  @Override
  public boolean equals(final Object o) {
    if (this == o) return true;
    if (!(o instanceof Book)) return false;
    final Book book = (Book) o;
    return title().equals(book.title());
  }
  ...
}

In this way we are restricting

to be a

Book

instance.

The effect of this change is the destruction of our software. As I anticipated the

equals

method should be reflexive. This means that every

Book

implementation must exhibit this

equals

behavior. In other words: every

Book

implementation is high-coupled with each other. Definitely it's a really bad approach.

A different approach

The main issue of the

equals

approach is that responsabilities aren't decoupled correctly. There should be another object responsible of the comparison. There should be another object that represents the comparison.

There may be various implementation of this approach. I'll suggest one called representation-based and another behavior-based.

Representation-based equality

The first one is derives from this article. Basically an object (like

aDbBook

) can gives us a representation of itself. Then a

Comparison<R>

object represents a comparison between two

representation. In this way a representation is similar to the hash returned by

hashCode

. But it's more generic because it could be based on bytes, strings and so on...

However this means that

aCat

could be equal to

aDog

, if they have the same representation. I consider this as the main drawback of this approach.

Behavior-based equality

The behavior-based is born from an observation. I think that the only valid discriminating factor about objects is their behavior. It's exposed through the methods. Or, more formally, through the messages the object supports. The protocol or interface is the collection of the supported messages.

For this reason the first step to define equality should be based on interfaces. Then an

Equality

object will represent the equality between two objects with the same interface.

In this way

aCat

will be always different from

aDog

because the different interfaces. Presumably the former implements a

Cat

interface, the latter a

Dog

interface. Nonetheless, thanks to polymorphism, if they both implement a

Pet

interface, then they could be equal. This could be possibile with

anEquality

limited to the

Pet

interface.

Here is an example related to the initial

Book

example. I defined two

Equality

classes to stress out that equality is not a

Book

responsability.

interface Equality {
  Boolean equals();
}

final class TitleBasedEquality implements Equality {
  TitleBasedEquality(final Book book, final Book anotherBook) {
    this.book = book;
    this.anotherBook = anotherBook;
  }

  @Override
  public Boolean equals() {
    return book.title().equals(anotherBook.title());
  }

  private final Book book;
  private final Book anotherBook;
}

final class PrefixBasedEquality implements Equality {
  PrefixBasedEquality(final Book book, final Book anotherBook, final Integer length) {
    this.book = book;
    this.anotherBook = anotherBook;
    this.length = length;
  }

  @Override
  public Boolean equals() {
    var first = book.title().substring(length);
    var second = anotherBook.title().substring(length);
    return first.equals(second);
  }

  private final Book book;
  private final Book anotherBook;
  private final Integer length;
}

So a

TitleBasedEquality

object compares the full title. A

PrefixBasedEquality

compares only a prefix.

We gather a lot of flexibility. And we can choose the correct equality comparison based on the context. This is possible thanks to the responsability decoupling.

However, as you can see, I'm using

String.equals

. I could replace it with

StringEquality

. But I consider this case as a reasonable compromise forced by the programming language.

A possible drawback of this approach regards an interface with only void methods. In this case each pair of instances are always equal. But this means that these type of objects are only and always comparated on their interface. I find it coherent and I find it respectful towards the objects.

Conclusion

Definitely the object equality problem is a tough problem. I think that the major issue is that we think equality in terms of data. But objects are not data. This is the reason because I support the idea of some sort of behavior-based comparison. After all the exhibited behavior is what distinguishes one object from another one. Nothing more.