Equality and Hashing for (almost) Free: Generating Implementations From Abstraction Functions

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Executive Summary

In an object-oriented language such as Java, every class requires implementations of two special methods, one for determining equality and one for computing hash codes. Although the specification of these methods is usually straightforward, they can be hard to code (due to sub-classing, delegation, cyclic references, and other factors) and often harbor subtle faults. A technique is presented that simplifies this task. Instead of writing code for the methods, the programmer gives, as a brief annotation, an abstraction function that defines an abstract view of an object's representation, and sometimes an additional observer in the form of an iterator method. Equality and hash codes are then computed in library code that uses reflection to read the annotations.

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