Why can't I create an enum in an inner class in Java?

Question

What I try to do is this:

public class History {
    public class State {
        public enum StateType {

Eclipse gives me this compile error on StateType: The member enum StateType must be defined inside a static member type.

The error disappears when I make the State class static. I could make State static, but I don't understand why I cannot declare an enum in an inner class.

please take a look at this: http://stackoverflow.com/questions/700831/java-local-enums — s_bei, Feb 13 '13 at 16:23
Unless you wanted State to access methods in History, make State a [static class](http://stackoverflow.com/a/1353326/2054731). — ArneHugo, Apr 15 '14 at 12:26

Joachim Sauer · Accepted Answer · 2013-02-13T16:39:47.890

97

enum types that are defined as nested types are always implicitly static (see JLS §8.9. Enums)

You can't have a static nested type inside a non-static one (a.k.a an "inner class", see JLS §8.1.3. Inner Classes and Enclosing Instances).

Therefore you can't have an enum inner type inside a non-static nested type.

edited Feb 13 '13 at 16:39

answered Feb 13 '13 at 16:23

Joachim Sauer

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Is there a certain reason that nested `enum` types are implicitly `static`? – mrbela Apr 21 '21 at 12:46
1

@mrbela: if non-static nested `enum` types were allowed, then what enclosing class instance should the enum values reference? That idea is incompatible with the basic tenents of an `enum` type (namely that there's a finite, well-defined set of instances). – Joachim Sauer Apr 21 '21 at 12:53

score 19 · Answer 2 · answered Oct 05 '17 at 21:29

If you declared an enum like this:

enum Suit {SPADES, HEARTS, CLUBS, DIAMONDS}

The Java compiler would synthetically generate the following class for you:

final class Suit extends java.lang.Enum<Suit> {
  public static final Suit SPADES;
  public static final Suit HEARTS;
  public static final Suit CLUBS;
  public static final Suit DIAMONDS;
  private static final Suit[] $VALUES;
  public static Suit[] values();
  public static Suit valueOf(java.lang.String);
  private Suit();
}

There is no intention to create other instances of this class other than those static fields already defined in it (as you could infer from its private constructor), but most importantly, and as mentioned in the accepted answer, a inner class cannot have static members (JLS §8.1.3. Inner Classes and Enclosing Instances), and since the enum synthetic class does, it makes it unacceptable as inner class.

Shouldn't it be ```final static class Suit extends java.lang.Enum``` and not ```final class Suit extends java.lang.Enum``` because enum has static variables (```SPADES, CLUBS etc```) and an inner (nested non-static) class can't have static declarations? — Marko Cain, Aug 01 '20 at 18:05

score 6 · Answer 3 · answered Jan 30 '16 at 10:08

Already enough information from +Joachim Sauer, I am just adding some extra details.

You can define inner enum only if your inner class is static nested inner class. See below

private static class DbResource {

    public enum DB {
        MERGE_FROM, MERGE_TO, MAIN;
    }
}

score 2 · Answer 4 · answered Mar 08 '19 at 17:40

This worked for my use-case:

public class History {

    public interface HConstants{
         public enum StateType { PAST,CURRENT,FUTURE}
    }

    //Inner class
    public class State implements HConstants{
        public StateType stateField = StateType.PAST;

Why can't I create an enum in an inner class in Java?

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