Equivalence and Equality

There is an interesting debate going on over at The Eternal Issue About Object.hashCode() in Java. The root of the problem in the debate is that the participants are not distinguishing between equivalence and equality (aka identity). In computing we make a distinction between the two. Any two instances are equivalent if their values are the same. Equality is a stronger equivalence. Ie, it is not enough that the values are the same, but the values must be the same instances. Eg, A = "joe", B = "joe", and C = A then A, B, and C  are equivalent while only A and C are equal.

Java has made a mess of this distinction because its Object class 1) implements an equals() method and 2) the implementation is equality. So, by default, no two instance are ever equivalent. Most people would consider "joe" and "joe" to be equal, but not Java. (Java should have defined equals() and hashCode() as abstract or, better yet, define them in an Equatable interface.)

My reading of the Eternal Issue article is that there is a contract that base-class instances would use a value, provided during construction, to distinguish themselves. This is equivalence. Eg

class Baseclass {
   String name;
   Baseclass(String name) { 
      this.name = name; 
   }
   boolean equals(Baseclass that) { 
      this.name.equals(that.name); 
   }
}

The sub-class would depend on this, in part, for its determination of equivalence, eg

class Subclass extends Baseclass {
   int age;
   Subclass(String name, int age) { 
      super(name); 
      this.age = age; 
   }
   boolean equals(Subclass that) { 
      super.equals(that) && this.age == that.age; 
   }
}

Now, what happens when you remove Baseclass's equals() method? Doing that changes Baseclass's distinguishing behavior from equivalence to equality as instances now default to using Object.equals(). This is a extraordinary change to the contract between the classes. Subclass equivalence will immediately start to fail because no two Subclass instances will ever be at the same memory location.

Never mix equivalence and equality and an inheritance hierarchy.

A consequence of the rule is that since you decided not to use name to distinguish Baseclass instances then all instances are equivalent. Ie, they are indistinguishable. The only correct change that adheres to the contract would be not to remove equals() but to to replace it with

boolean equals(Baseclass that) { true; }

Of course, any change to Baseclass is likely a bad idea and I would never recommend that you respond to this change with anything less than tar and feathers. But, at least, make sure the changer of the Baseclass understands what they did vis a vis the contract.