Java generic Types for method signature on enums implementing

Question

I basically want to have an enum with specific methods, e.g.

public interface PropertyTypeMethods {
    public int getTypeId();

    public void setTypeId(Integer typeId);
}

and something like

public enum BasePropertyTypes implements PropertyTypeMethods {

ONE, TWO;

private int typeId;

@Override
public int getTypeId() {
    return typeId;
}

@Override
public void setTypeId(Integer typeId) {
    this.typeId = typeId;
}

}

as well as an extended version of this enum, e.g.

public enum ExtendedPropertyTypes implements PropertyTypeMethods {
HUNDRED, THOUSEND; 
// same as above
}

which would result in ONE.setTypeId(1) ==> ONE.getTypeId()==1 //true. That is the basic concept. But now I want to call a generic method like e.g.

private <E extends Enum<E> & PropertyTypeMethods> void initEnum(Enum<E> curType) {
   // the below approach does not work :-/
   curType.setTypeId(1); // or any other reasonable value....

But somehow I cannot figure out what the correct method signature would be. Following this question I figured at least some part of the puzzle - but still don't get it for the method signature. Still it remains unclear how to specify curType properly in the signature to execute an appropriate call.

Why <E extends Enum<E> & PropertyTypeMethodes>? To set the type, <E extends PropertyTypeMethods> is enough. — daniu
– daniu, Commented Feb 16, 2018 at 12:36
Enums are supposed to be constant. If you want a mutable "enum", make it a plain old class. — Andy Turner
– Andy Turner, Commented Feb 16, 2018 at 12:37
Just create a class, give it a private or protected constructor and add constant instances. Voila, it's basically an mutable and extendable enum. You may also add a static method to provide all available instances of that class like values() on enums. — Lukas Körfer
– Lukas Körfer, Commented Feb 16, 2018 at 12:46
Like Andy said, enum values should be constants. I think what you really want is an EnumMap. — VGR
– VGR, Commented Feb 16, 2018 at 15:30
Well I need to review the other suggestion .... About the Why? The main problem is this enum is already cluttered all over the code. And now there is a breaking change to set the ids manually - instead of hard coded (as it was). Therefore the request... I would love to have it differently - from the start — LeO
– LeO, Commented Feb 17, 2018 at 13:07

ttzn · Accepted Answer · 2018-02-16 13:06:25Z

2

This will work :

private <E extends Enum<E> & PropertyTypeMethods> void initEnum(E curType) {
  curType.setTypeId(1);
}

However, I don't think it's a good idea to make mutable Enums (they're meant to be labels over constant values, not state-carrying singletons). Furthermore, you shouldn' write methods that require an Enum parameter when all they should care about is its interface :

// This will be easier to work with down the road
private void initEnum(PropertyTypeMethods curType) {
  curType.setTypeId(1);
}

answered Feb 16, 2018 at 13:06

ttzn

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2 Comments

GhostCat Over a year ago

Upvote for pointing out the pointlessness of the whole thing ;-)

LeO Over a year ago

Thx the hint with the signature solved this issue. I don't see my situation as ideal - nevertheless it works with the existing code.

M.F · Accepted Answer · 2018-02-16 13:03:22Z

1

The correct signature would simply be

private void initEnum(PropertyTypeMethods onject) {
    object.setTypeId(1);
}

But as Andy Turner mentioned, enums are expected to be immutable, i.e. only final immutable field. Therefore, enums also have a constructor Enum Types.

If you have more complex enums, it is a common way to implement them as follows

public enum BasePropertyTypes implements PropertyTypeMethods {
   ONE (new PropertyTypeMethods() {
          @Override
          public int getTypeId() {
              return 1;
          }
     });

    private final PropertyTypeMethods m;

    BasePropertyTypes(PropertyTypeMethods m) {
       this.m = m;
    }

   @Override
   public int getTypeId()
   {
      return this.m.getTypeId();
   }
}

But from your example, I would suggest to review your actual issue. Propably enums are not the proper way at all.

answered Feb 16, 2018 at 13:03

M.F

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1 Comment

LeO Over a year ago

Yeah, I need to review - but anyway this enum is used all over the code - and a breaking change reg. ID needs some other measures - which are not as intended at the very beginning.... See as well my other comment..

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