Java Enum Definition: Understanding Generics with Type Parameters
Java's Enum class employs a peculiar type parameter that may initially seem confusing:
class Enum<e extends enum>></e>
This parameter means that an enum type must derive from another enum type with the same type argument. In practice, the type argument is the enum type itself, as seen in this example:
public enum StatusCode extends Enum<statuscode></statuscode>
In this scenario, E is equivalent to StatusCode. Thus, E extends Enum
This construct allows the Enum API to refer to itself, such as in Enum
A similar concept is observed in C# with ProtocolBuffers' interfaces:
public interface IBuilder<tmessage tbuilder> where TMessage : IMessage<tmessage tbuilder> where TBuilder : IBuilder<tmessage tbuilder> public interface IMessage<tmessage tbuilder> where TMessage : IMessage<tmessage tbuilder> where TBuilder : IBuilder<tmessage tbuilder></tmessage></tmessage></tmessage></tmessage></tmessage></tmessage>
This ensures that messages and builders can be paired appropriately.
It's worth noting that the type parameter does not prevent inconsistencies. For example, the following types would be valid:
public class First extends Enum<first> {} public class Second extends Enum<first> {}</first></first>
In this case, Second would implement Comparable
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