Generics using Enum in Java

Question

I have an enum

public enum Days {
SUNDAY, MONDAY, TUESDAY, WEDNESDAY,
THURSDAY, FRIDAY, SATURDAY 

}

I want to make a class which can take values of type Days. So I used Java Generics

public class State<T extend Days>

But there is an error

The type parameter T should not be bounded by the final type Days. 
Final types cannot be further extended

How can I resolve this?

Answer 1

enums are final types, which means, you can not extend from them

a generic like wants a Class as Parameter which is Days or an extended class, but the extended class is not possible

so the only parameter possible is Days and you don't need a generic, if only one value is possible

Answer 2

Don't use a generics bound. There's no need. Just use an unbounded type, like this:

public class State<T> {
    public State(T startState) {
        // whatever
    } 
}

And to use it:

State<Days> dayState = new State<Days>(Days.SUNDAY);

This is a straightforward typed class that doesn't need a bound.

The only bound that might make sense is a bound to an enum:

public class State<T extends Enum<T>> {
    public State(T startState) {
        // whatever
    } 
} 

This version requires that the generic parameter be an enum. With this version the above usage example would still compile, but this would not compile (for example):

State<String> dayState = new State<String>("hello");

because String is not an enum.