In recent years, with the continuous expansion and deepening of the application of software technology in various industries and fields, as well as the development of software architecture, services computing, etc., the software industry has emerged with feature-rich and large-scale third-party APIs or Libraries. Software developers are increasingly relying on these APIs when implementing software functions. However, learning the usage of these APIs is very difficult and time-consuming. There are two main reasons: 1) missing or wrong documents; 2) few sample codes for API usage. Therefore, designing automatic API completion methods to help developers use the API correctly and quickly has great application value. However, most of the existing API automatic completion methods regard the code segments to be completed as plain text, ignore the impact of the object types of APIs. Therefore, this study explores the role of the object types in completing APIs. Besides, inspired by the object state diagram, an concrete API completion method is designed and implemented that uses the types of the objects as a novel feature. Specifically, the subsequence of the same object type is first extracted from the API call sequence and a deep learning model is used to encode the state of each object. Then, the objects’ states is used to generate a state representation of the entire method block. In order to evaluate the proposed method, comprehensive experiments are conducted on six popular java projects. The experimental results prove that the proposed API completion method achieves significantly higher predicting accuracy than the baseline approaches.