Through Java's evolution keywords have been added. While code that uses those words as identifiers may be compilable under older versions of Java, it will not be under modern versions.
Following keywords are marked as invalid identifiers
||Keyword||Added||
|``++_++``|9|
|``++enum++``|5.0|
``++assert++`` and ``++strictfp++`` are another example of valid identifiers which became keywords in later versions, but are not supported by this rule.