A DESCRIPTION OF THE REQUEST :
Enums do not take type parameters.
The following code doesn't compile:

class Property<T> {
  public abstract <T> T accept(Visitor<T> v);
  public static enum Type<T> {
    STRING<String>,
    DOUBLE<Double>;
  }
  public static Property<T> newInstance(Type<T> type, T initValue) {
  switch(type) {
    case STRING:
      return new StringProperty(initValue);
    case DOUBLE:
      return new DoubleProperty(initValue);
    default:
      throw new RuntimeException("invalid enum constant");
    }
  }
}

As well as this code:
public enum UserInfo<T> {
  FirstName<String>,
  LastName<String>,
  UserSince<Date>,
  LastLogin<Date>;
}

public <T> void setUserInfo( UserInfo<T> infoType, T infoValue ) {}


JUSTIFICATION :
Allowing enums to be parameterized allows writing type-safe methods that enforce compile-time consistency between their parameters;
for example, the method:
public <T> void setUserInfo( UserInfo<T> infoType, T infoValue ) {}
which would compile when called on (FirstName, "Mathew"), but issue an unchecked warning or refuse to compile when called with (UserSince, "31/07/2004").

This allows code to be written in a type-safe way when it was impossible before.


EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
The code above compiles without warnings.
ACTUAL -
The above code doesn't compile; the type parameter is rejected by the compiler.