Contracts in Practice

TL;DR

This study empirically analyzes how software contracts are used, evolve, and relate to implementation changes across multiple projects, revealing stable contract usage and minimal inheritance impact.

Contribution

It provides the first large-scale empirical analysis of contract usage and evolution in real-world software projects across multiple programming languages.

Findings

Over 33% of program elements include contracts, stable over time.

No strong preference for specific contract element types.

Contracts are more stable than implementation code.

Abstract

Contracts are a form of lightweight formal specification embedded in the program text. Being executable parts of the code, they encourage programmers to devote proper attention to specifications, and help maintain consistency between specification and implementation as the program evolves. The present study investigates how contracts are used in the practice of software development. Based on an extensive empirical analysis of 21 contract-equipped Eiffel, C#, and Java projects totaling more than 260 million lines of code over 7700 revisions, it explores, among other questions: 1) which kinds of contract elements (preconditions, postconditions, class invariants) are used more often; 2) how contracts evolve over time; 3) the relationship between implementation changes and contract changes; and 4) the role of inheritance in the process. It has found, among other results, that: the percentage of program elements that include contracts is above 33% for most projects and tends to be stable over time; there is no strong preference for a certain type of contract element; contracts are quite stable compared to implementations; and inheritance does not significantly affect qualitative trends of contract usage.