We are pleased to present Alibaba Java Coding Guidelines, which consolidates the best programming practices over the years from Alibaba Group's technical teams. A vast number of Java programming teams impose demanding requirements on code quality across projects as we encourage reuse and better understanding of each other's programs. We have seen many programming problems in the past. For example, defective database table structures and index designs may cause software architecture flaws and performance risks. Yet as another example, confusing code structures make it difficult to maintain. Furthermore, vulnerable code without authentication is prone to hackers’ attacks. To address those kinds of problems, we developed this document for Java developers in Alibaba.[Mandatory] Using a deprecated class or method is prohibited. Note: For example, decode(String source, String encode) should be used instead of the deprecated method decode(String encodeStr). Once an interface has been deprecated, the interface provider has the obligation to provide a new one. At the same time, client programmers have the obligation to check out what its new implementation is.