How to instantiate an object tied to java enum

Question

I want to instantiate object for an enum. For example:

public enum Type {

  TYPE1("type_name_1", new TypeObj1()), 
  TYPE2("type_name_2", new TypeObj2());

  TypeInterface typeObj;
  String value;
}

public class TypeObj1 implements TypeInterface {
  ...
}

public class TypeObj2 implements TypeInterface {
  ...
}

Is this a correct way to do it? I want a single object of TypeInterface attached to enum. Please suggest alternatives.

What do you mean by tied to an enum? Can `TYPE1` have `TypeObj2` for example? — davidalayachew, May 24 '23 at 15:31
It is very unclear what you are trying to achieve. Does this question about using generics with enum help? https://stackoverflow.com/a/11490505/721855 — aled, May 24 '23 at 15:38
@aled I wanted to know if the approach I am using is correct or not. I would like to instantiate object of class implementing TypeInterface in Enum constructor. Is this right practice? — Pandey, May 24 '23 at 16:01
Then my next question, you said that `TYPE1` can only interact with `TypeObj1`. Is this something that you want ***enforced*** by the compiler? Or do you just want to map the relationship and see if there is a better way to do it? — davidalayachew, May 24 '23 at 16:05

Basil Bourque · Accepted Answer · 2023-05-24T23:57:36.493

Yes, you are headed in the right direction.

You’ll need a constructor to assign the passed objects to your member fields.

Make your member fields final to prevent their reassignment to another object reference.

public enum Type {
  // Enum objects.
  TYPE1("type_name_1", new TypeObj1()), 
  TYPE2("type_name_2", new TypeObj2());

  // Member fields.
  final TypeInterface typeObj;
  final String value;

  // Constructor.
  Type( final String value , final TypeInterface typeInterface ) {
    this.typeObj = typeInterface ;
    this.value = value ;
  }
}

Tip: This kind of thought exercise is more clear and productive if you devise a simple but semi-realistic scenario rather than your amorphous “TypeInterface” and “value”.

public enum Pet {
  // Enum objects.
  FLUFFY( "tuxedo" , new Cat() ), 
  ROVER( "chocolate Lab" , new Dog() );
  
  // Member fields.
  final Animal animal;
  final String description;

  // Constructor.
  Type( final String desc , final Animal animal ) {
    this.animal = animal ;
    this.description = desc ;
  }
}

score 0 · Answer 2 · answered May 24 '23 at 15:49

Create a constructor for your enum, which matches the arguments you're specifying.
There is no access-modifier; and private is redundant.

Enum Types (The Java™ Tutorials > Learning the Java Language > Classes and Objects).

public enum Type {
    TYPE1("type_name_1", new TypeObj1()),
    TYPE2("type_name_2", new TypeObj2());

    final TypeInterface typeObj;
    final String value;
    
    Type(String value, TypeInterface typeObj) {
        this.value = value;
        this.typeObj = typeObj;
    }
}

How to instantiate an object tied to java enum

2 Answers2