class Enum<K extends Enum<K>>

In Java, the java.lang.Enum class is declared as:

public abstract class Enum<E extends Enum<E>>

Why is the type parameter declared as <E extends Enum<E>>? What would happen if it were declared as <E extends Enum> instead?
Solution


This pattern is used when a generic base class needs to reference the type of the subclass. For example, suppose we had:

public enum CardSuit { HEARTS, CLUBS, DIAMONDS, SPADES }

This gets compiled as something like:

public class CardSuit extends Enum<CardSuit>....

which then means that every occurrence of type parameter E in Enum's declaration becomes CardSuit. Thus, the superclass (Enum) is able to reference subclass types in its method signatures and return types this way through the type parameter.

If just <E extends Enum> were used instead, that would still be the case, but some information would be lost. The type parameter E is mentioned in three places in Enum:

1. implements Comparable<E>
2. public int compareTo(E o)
3. public Class<E> getDeclaringClass()

If the class declaration just had E extends Enum, then the Comparable interface is only aware of Comparable<Enum> instead of Comparable<CardSuit>. So E extends Enum<E> contains some extra information, making it more typesafe.