How to define static constants in a Java enum?

java enums constants

70,864

Solution 1

As IntelliJ IDEA suggest when extracting constant - make static nested class. This approach works:

@RequiredArgsConstructor
public enum MyEnum {
    BAR1(Constants.BAR_VALUE),
    FOO("Foo"),
    BAR2(Constants.BAR_VALUE),
    ...,
    BARn(Constants.BAR_VALUE);



    @Getter
    private final String value;

    private static class Constants {
        public static final String BAR_VALUE = "BAR";
    }
}

Solution 2

public enum MyEnum {
    BAR1(MyEnum.BAR_VALUE);

    public static final String BAR_VALUE = "Bar";

works fine

Solution 3

public enum MyEnum {
//  BAR1(       foo),   // error: illegal forward reference
//  BAR2(MyEnum.foo2),  // error: illegal forward reference
    BAR3(MyEnum.foo);   // no error

  public static final int foo =0;
  public static       int foo2=0;
  MyEnum(int i) {}

  public static void main(String[] args) { System.out.println("ok");}
}

This can be done without an inner class for the constant.

70,864

Author by

jilt3d

Updated on July 05, 2022

Comments

jilt3d almost 2 years
Is there any way to define static final variables (effectively constants) in a Java enum declaration?

What I want is to define in one place the string literal value for the BAR(1...n) values:
```
@RequiredArgsConstructor
public enum MyEnum {
    BAR1(BAR_VALUE),
    FOO("Foo"),
    BAR2(BAR_VALUE),
    ...,
    BARn(BAR_VALUE);

    private static final String BAR_VALUE = "Bar";

    @Getter
    private final String value;
}
```
I got the following error message for the code above: Cannot reference a field before it is defined.
DaShaun about 7 years

I think it should be noted that the @RequiredArgsConstructor is from the lombok library.
Bruno Leite almost 7 years

Although this one does not working for using the 'constants' into annotations values.
BeUndead about 6 years

@BrunoLeite This does work for using the constants in annotation values if you make the Constants class non- private.
Snickbrack over 4 years

I don't see any aspect that this answer adds to the existing and accepted answer. Could you please clarify your answer any further by providing a explanation?
user207421 over 4 years

'Static inner' is a contradiction in terms.
user207421 over 4 years

@Snickbrack I do. It eliminates the static nested class. Good answer.
Maciej Dobrowolski over 4 years

@user207421 Static nested it is. Thanks :)
borjab about 4 years

Could the upper case syntax cause confusion between enum values and constants?
Bentaye about 4 years

@borjab yes if you use that method, you can't have a constant and an enum with the same name
gillesB almost 4 years

Do you have an explanation why this works? I am wondering why explicitly using MyEnum makes a difference, because implicitly it is already there.
raspacorp over 3 years

Tried this and gives the same error: illegal forward reference in compilation time
bit_cracker007 almost 3 years

Should fail during runtime!
Guilherme Taffarel Bergamin almost 3 years

@bit_cracker007, I've just tried this and it works. Great for using in annotations. At first thought, seems better than the accepted answer
Guilherme Taffarel Bergamin almost 3 years

@gillesB, if you don't use MyEnum, it gives you an Illegal forward reference error. I think that you need to call the constant as if from another place because if not, in practice you would be using an attribute from an object that doesn't exist yet, so you need to statically access that value "from outside".