Besides null being a potential type for any reference and causing a  ton of issues I do not think most Java programmers regularly care about the lack of abstractions like higher-kinded or union types. Would be nice to have, but never have I felt like I needed them.<p>Enum types didn&#x27;t exist in Java when the Comparable interface was implemented. That&#x27;s why some API&#x27;s look old-school by comparison because newer language features didn&#x27;t exist yet.