It really doesn’t resemble Java at all. C++ looks more like Java than Rules DSL does. Core Java classes are available in it though, as they are available in all programming languages that run on the Java Runtime Environment, but the syntax bears almost nothing in common with Java beyond what is common among most programming languages.
To quote from the docs,
The rule syntax is based on Xbase and as a result it is sharing many details with Xtend, which is built on top of Xbase as well.
“Sharing many details” doesn’t mean “the same as” though. For example, Xtend supports arrays, classes, functions, and data structures. Rules DSL supports none of those. The definition of a Rule is wholly unique to Rules DSL as is the overall structure of a .rules file. So for reference, the Rules documentation page and Xtend - Classes and Members is as close as we get to a reference. But keep in mind that not everything present will necessarily apply in Rules DSL. And it is not a complete reference. That’s why I wrote Design Pattern: Working with Groups in Rules which covers a lot of the missing filter/map/reduce List operations that are missing from the Xtend docs.