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Java Forum / General / May 2007values() of an Enumeration type that is a generic parameter?
I'm trying to write a generic class using an enumeration as a type parameter, but I ran into trouble trying to iterate over the enumeration values: class Foo<E extends Enum<E>> { public void doSomething () { for (E index: E.values ()) { // do stuff here } } } The compiler says: Foo.java:55: cannot find symbol symbol : method values() location: class java.lang.Enum<E> for (E index: E.values ()) ^ 1 error Does E not inherit the values() method of the enumeration type? Is there some other way I can do this? I'm also rather unclear on why various JDK classes use <E extends Enum<E>> for their parameter; I can't quite seem to grasp what it means. I tried just using <E extends Enum>, but the compiler had the same complaint. Any hints will be much appreciated! Thanks! Eric > Foo.java:55: cannot find symbol > symbol : method values() [quoted text clipped - 4 lines] > > Does E not inherit the values() method of the enumeration type? There is no values method in java.lang.Enum. > Is there some other way I can do this? Pass a Class<E extends Enum<E>> as enumType to your constructor. Replace "E.values()" with enumType.getEnumConstants(). > I'm also rather unclear on why various JDK classes use > <E extends Enum<E>> for their parameter; I can't quite seem to > grasp what it means. I tried just using <E extends Enum>, but the > compiler had the same complaint. Enum has generic parameter, so that needs to be supplied. Enum requires a generic parameter so that it can implement Comparable correctly, and declare its getDeclaringClass method. Tom Hawtin > I'm trying to write a generic class using an enumeration as a type > parameter, but I ran into trouble trying to iterate over the enumeration > values: > > class Foo<E extends Enum<E>> ... > for (E index: E.values ()) ... > The compiler says: > [quoted text clipped - 6 lines] > > Does E not inherit the values() method of the enumeration type? No. First of all, static methods are never inherited. Second, E here is a type variable with Enum as an upper bound, so at best it can be used to access static methods on the Enum class. Enum has no method called "values". The only static method is "valueOf(Class<T>,String)". > Is there some other way I can do this? No. The code you have displayed have no chance of knowing the type parameter "E" at runtime. You need to have access to an actual value of that type, or the class object for it, in order to use reflection. > I'm also rather unclear on why various JDK classes use > <E extends Enum<E>> for their parameter; I can't quite seem to > grasp what it means. I tried just using <E extends Enum>, but the > compiler had the same complaint. That's because Enum itself is parameterized. It's a hack that attempts to introduce self-types to Java classes. It allows inherited methods to have the correct type for subtypes. For Enum, the method signature for "compareTo(E)" is inherited, but each concrete Enum type must specify their own type for "E" to be comparable to each other. This is achieved by having, e.g., enum Foo { BAR, BAZ} be implemented by a class of type "Foo extends Enum<Foo>". This way, the subclass passes back its own type to the superclass signature it inherits. /L  SignatureLasse Reichstein Nielsen - lrn@hotpop.com DHTML Death Colors: <URL:http://www.infimum.dk/HTML/rasterTriangleDOM.html> 'Faith without judgement merely degrades the spirit divine.' I wrote: > I'm also rather unclear on why various JDK classes use > <E extends Enum<E>> for their parameter; I can't quite seem to > grasp what it means. I tried just using <E extends Enum>, but the > compiler had the same complaint. > That's because Enum itself is parameterized. It's a hack that attempts > to introduce self-types to Java classes. [quoted text clipped - 7 lines] > the subclass passes back its own type to the superclass signature > it inherits. Thanks for the explanation. I don't think I understand it 100%, but I'm closer. I can understand parameterized types of the form <E1 extends SomeClass<E2>>. It's just <E1 extends SomeClass<E1>> that seems quite confusing. Eric > No. The code you have displayed have no chance of knowing the > type parameter "E" at runtime. You need to have access to an > actual value of that type, or the class object for it, in order > to use reflection. OK. So EnumMap is able to figure out the range of the Enum type parameter only because the constructors get the class object at runtime? And from the class object, it has to use reflection to get the enumeration values? Thanks, Eric > OK. So EnumMap is able to figure out the range of the Enum type > parameter only because the constructors get the class object > at runtime? Yes. > And from the class object, it has to use reflection to get the > enumeration values? Actually, the Class<T> object has the method "T[] getEnumConstants()" that returns the values of an enum type (or null, if the Class is not an enum), so reflection isn't needed anyway. I hadn't noticed that method before today :) /L SignatureLasse Reichstein Nielsen - lrn@hotpop.com DHTML Death Colors: <URL:http://www.infimum.dk/HTML/rasterTriangleDOM.html> 'Faith without judgement merely degrades the spirit divine.' > Actually, the Class<T> object has the method "T[] getEnumConstants()" > that returns the values of an enum type (or null, if the Class is not > an enum), so reflection isn't needed anyway. That /is/ reflection. SignatureLew > > Actually, the Class<T> object has the method "T[] getEnumConstants()" > > that returns the values of an enum type (or null, if the Class is not > > an enum), so reflection isn't needed anyway. > > That /is/ reflection. Hummmm... not exactly. getEnumConstants() is only available (i.e. returns something other than null) when the class *is* an Enum class. Not really the behavior you expect from reflection, that should be independant from hypotheses on the nature of the class, and report type-checking failures through a few, well-defined runtime exceptions. In my opinion, this is rather a hack to implement inheritance-aware static fields, in the particular case of <E extends Enum<E>> E valueOf(Class<E>,String). Look at the implementation of Enum-related methods in Class... you could do the same thing with a static Map<Class<? extends Enum>,Enum[]>, that would let you implement <E extends Enum<E>> E[] values(Class<E>), but with much less performance. And what object is Class-common, but updated/able whith inheritance? The Class object, of course! -- YLS Free MagazinesGet these publications absolutely FREE for up to 12 months. There are no hidden fees and no obligation. 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