Feel the characteristics of Java data object JDO 2.0 query language (1)

The improvement of query language is jdo2 0 specification. This paper expounds jdo2 from a higher level Some new features provided by 0. Due to jdo2 0 specification has not entered the state of public draft. At present, nothing has been finalized, and everything may face changes. However, jdo2 0 will soon enter the final stage, and the query feature mentioned here is jdo2 The 0 expert group takes the most time and is relatively stable. Therefore, I have enough reason to believe that the final specification will be basically consistent with the description here. If readers feel that this article misses some important features, they are recommended to go to the JDO forum immediately（ http://www.jdocentral.com/forums/index.php?showforum=10 ）To propose and discuss. Here we need to thank jdo2 0 specification leader Craig Russell authorized me to disclose these jdo2 New features in the 0 query language. The query results are first introduced from the most in-depth improvements. In jdo1 0, the query result is always the collection of instances of the class you specify. Consider the following UML class diagram, which expresses the four classes a, B, C and D and their relationships: