An Empirical Comparison between API Tutorials and API Crowd Documentation

API documentation is critical for developers to learn APIs. However, it is unclear whether different types of API documentation have distinct effects on the API learnability for developers. Thus, we conduct an empirical study to compare the API learnability between official API tutorials and API crowd documentation based on three aspects, i.e., the coverage of APIs, the concerns for APIs, and the presentations. Based on the quantitative analyses, we find that: (i) API crowd documentation can be regarded as a supplement to the official API tutorials in some extent; (ii) the concerns for frequently used APIs between official API tutorials and API crowd documentation show a huge mismatch, which may prevent developers from deeply understanding the usages of APIs through only one type of API documentation; (iii) official API tutorials can help developers seek API information on a long page and API crowd documentation could provide long codes for a particular programming task. These findings may help developers determine which API documentation could be selected for learning APIs and find the useful information they need.