Java: Generic accepting only enum types

Question

Assuming that I have a basic enum like:

public enum Color { Red, Green, Blue}

How can one write a generic class which only accepts "enum classes" so that a concrete instantiation of that generic class might look like MyClass<Color>?

Edit:

What a actually want to do is to write a generic abstract class containing a function returning all enum "entries" as list:

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

    public List<E> getEnumList() {
        return Arrays.asList(E.values());
    }

}

While Day.values() is available E.values() is not. What i am doing wrong here?

No its implementation should not be limited to one concrete enum type but to arbitrary enum types. I just used Day to describe more concretely what i want. — Sebastian Hoffmann
– Sebastian Hoffmann, Commented Jul 15, 2012 at 11:49

Istvan Devai · Accepted Answer · 2012-07-15 11:51:36Z

7

public class EnumAcceptor<E extends Enum<E>> {
    ...
}

Use E as a type inside your class.

answered Jul 15, 2012 at 11:51

Istvan Devai

4,02226 silver badges22 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

Tom Hawtin - tackline · Accepted Answer · 2012-07-15 12:53:20Z

4

See Istvan Devai for answer to original question.

For the follow up, methods like values() are static methods, so you're out of luck trying to get that from a generic parameter. As a poor solution, you can pass the enum's Class object into the constructor. and use Class.getEnumConstants. But you might as well pass MyEnum.values() into the constructor rather than the class, and so avoid reflection altogether. It's a real shame there isn't a sensible enum metaclass.

answered Jul 15, 2012 at 12:53

Tom Hawtin - tackline

148k30 gold badges222 silver badges314 bronze badges

Comments

Eric · Accepted Answer · 2012-07-15 11:52:24Z

1

An enum really declares a class derived from Enum. As such, you can use:

public class MyClass<T extends Enum> { }

edited Jul 15, 2012 at 11:52

answered Jul 15, 2012 at 11:50

Eric

98.1k54 gold badges257 silver badges389 bronze badges

1 Comment

Tom Hawtin - tackline Over a year ago

You're mixing generics and raw types.

OldCurmudgeon · Accepted Answer · 2012-11-30 14:45:38Z

1

Note that @Istvan's solution can only accept elements of the enum, which is fine if that is all you want.

Although you cannot pass the enum itself as a parameter (because it does not actually have an object equivalent) you can specify that you must receive the class of the enum in your constructor and derive the enum's details from that:

public class EnumAcceptor<E extends Enum<E>> {
  public EnumAcceptor(Class<E> c) {
    // Can get at the enum constants through the class.
    E[] es = c.getEnumConstants();
  }

  enum ABC {
    A, B, C;
  }

  public static void main(String args[]) {
    EnumAcceptor<ABC> abcAcceptor = new EnumAcceptor<ABC>(ABC.class);
  }
}

answered Nov 30, 2012 at 14:45

OldCurmudgeon

66k18 gold badges126 silver badges220 bronze badges

1 Comment

OldCurmudgeon Over a year ago

Oops - duplicate of @Tom's post - sorry Tom :(

Andrew Aylett · Accepted Answer · 2012-11-30 14:58:49Z

0

You can't use E.values() due to type erasure -- the type of E is not available at run-time.

For the specific case in your question, you're probably better off using Guava's Lists.newArrayList:

List<Color> days = Lists.newArrayList(Color.values());

answered Nov 30, 2012 at 14:58

Andrew Aylett

41.1k6 gold badges71 silver badges99 bronze badges

Collectives™ on Stack Overflow

Java: Generic accepting only enum types

5 Answers 5

Comments

Comments

1 Comment

1 Comment

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

5 Answers 5

Comments

Comments

1 Comment

1 Comment

Comments

Your Answer

Sign up or log in

Post as a guest

Related