How Does Java\'s Enum Generic Type Parameter `E extends Enum` Work?

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>

In this scenario, E is equivalent to StatusCode. Thus, E extends Enum, fulfilling the parameter's requirement.

This construct allows the Enum API to refer to itself, such as in Enum implements Comparable. The base class can perform comparisons while ensuring that only the appropriate enums are compared.

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>

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> {}

In this case, Second would implement Comparable instead of Comparable, which can lead to unexpected behavior. However, the intention of the type parameter is to provide benefits in valid cases, not necessarily to enforce consistent usage.

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