An Empirical Comparison Between Tutorials and Crowd Documentation of Application Programming Interface

API (application programming interface) documentation is critical for developers to learn APIs. However, it is unclear whether API documentation indeed improves the API learnability for developers. In this paper, we focus on two types of API documentation, i.e., official API tutorials and API crowd documentation. First, we analyze API coverage and check API consistencies in API documentation based on the API traceability. Then, we conduct a survey and extract several characteristics to analyze which API documentation can help developers learn APIs. Our findings show that: 1) API crowd documentation can be regarded as a supplement to the official API tutorials to some extent; 2) the concerns for frequently-used APIs between different types of API documentation show a huge mismatch, which may prevent developers from deeply understanding the usages of APIs through only one type of API documentation; 3) 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 select the suitable API documentation and find the useful information they need.