How can I require a generic parameter to be an enum that implements an interface?

Solution 1:

Use an '&' instead:

public class MyWidget<T extends Enum<T> & MyInterface> {
    ...
}

The JLS calls this an "intersection type", but I can find no mention of it in the Java tutorials. I'll just say that it does exactly what you were wishing that "extends" would do.

Also, I should mention that you can have as many types as you want in the intersection type. So if you wanted, you could do:

public class MyWidget<T extends Enum<T> & MyInterface & Serializable & Cloneable> {
    ...
}

[Note: this code sample should not be construed as an endorsement of the Cloneable interface; it was merely handy at the time.]

Solution 2:

The JSR 203 (new new IO) stuff for JDK 7 is making a lot of use of enums that implement interfaces (for example: http://openjdk.java.net/projects/nio/javadoc/java/nio/file/FileVisitOption.html) to allow them some wiggle room in the future for future additional sets of enum options. So that is a feasible approach and obviously one that was chosen after a lot of thought in one large Sun project.