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The documentation for java.time.temporal.Temporal contains the following note:
Implementation Requirements: [...] All implementations must be Comparable.
Why does Temporal not just extend Comparable?
Background: I want to work with comparable temporals (not with subtypes like LocalDateTime etc.) and have to resort to a somewhat illegible type <T extends Temporal & Comparable<T>> which also messes up NetBeans' auto-complete feature.
Edit: I want to implement a temporal interval. The obvious implementations for contains(Interval i), contains(Temporal t), overlaps(...), adjoins(...) etc. use Comparable::compareTo(Comparable c) to compare the start and end points, but for interoperability (toDuration(), parse(CharSequence cs)) I need e.g. Duration::between(Temporal s, Temporal e) or SubtypeOfTemporal::parse(CharSequence cs) (yielding Temporal).
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Java For Testers A temporal field is a field of date-time, such as month-of-year or hour-of-minute. These fields are represented by the TemporalField interface and the ChronoField class implements this interface.
The Java Date Time API was added from Java version 8. instant() method of Clock class returns a current instant of Clock object as Instant Class Object. Instant generates a timestamp to represent machine time. So this method generates a timestamp for clock object.
If it implemented Comparable<Temporal>, every suclass instance would have to be comparable with any other subclass instance. And comparing an Instant with a LocalDate, for example, doesn't make sense.
Given that the contract mandates that they are comparable, you can cast T to Comparable<T> and safely ignore the compiler warning.
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