Declarative Object Identity Using Relation Types

Object-oriented languages define the identity of an object to be an address-based object identifier. The programmer may customize the notion of object identity by overriding the equals() and hashCode() methods following a specified contract. This customization often introduces latent errors, since the contract is unenforced, and at times impossible to satisfy. Notably, equals() may refer to mutable state, which allows object identity to change during execution, breaking standard library invariants.

We propose a programming model based on a relational view of the heap which defines identity declaratively, obviating the need for equals() and hashCode() methods. Each element in the heap (called a tuple) belongs to a relation type and relates an immutable identity to mutable state. The model entails a stricter contract: identity never changes during an execution. Objects, values, and singletons arise as special cases of tuples. We formalize the model as an adaptation of Featherweight Java, and implement it by extending Java with relation types. Experiments on a set of Java programs show that the majority of classes that override equals() can be refactored into relation types, and that most of the remainder are buggy or fragile.