Enum with array/list values

Question

if (isProductDeliverable) {
  REQUIRED_FIELDS = Arrays.asList(new String[] { Fields.NAME, Fields.EMAIL, Fields.ADDRESS });
} else {
  REQUIRED_FIELDS = Arrays.asList(new String[] { Fields.NAME, Fields.EMAIL });
}

Instead of this, I want to have a predefined enum with two fields - REQUIRED_FIELDS_FOR_DELIVERABLE_PRODUCTS and REQUIRED_FIELDS_FOR_DOWNLOADABLE_PRODUCTS

I know the theory of enums but I've never used them so I cant figure out a way how to do this.

Or maybe a way to ask for the required fields by passing this "isProductDeliverable" boolean and get the correct array of fields?

Have you tried anything yet? Note that it's unconventional to have a variable with a shouty name like REQUIRED_FIELDS... — Jon Skeet
– Jon Skeet, Commented Feb 22, 2015 at 12:06
Yes I've seen the tutorial but I dont know how to apply it in this particular case. — Kaloyan Roussev
– Kaloyan Roussev, Commented Feb 22, 2015 at 12:09

Teddy · Accepted Answer · 2015-02-22 12:46:59Z

19

Enums can have data and behaviour much like classes. Something like this should work...

public enum RequiredFields {
    REQUIRED_FIELDS_FOR_DELIVERABLE_PRODUCTS( Fields.NAME, Fields.EMAIL, Fields.ADDRESS ),
    REQUIRED_FIELDS_FOR_DOWNLOADABLE_PRODUCTS( Fields.NAME, Fields.EMAIL );

    private List<String> fields;
    private RequiredFields(String... fields){
        this.fields = Arrays.asList(fields);
    }
    public List<String> getFields(){
        return fields;
    }
}

Further improvement:

In above code, the fields property is still mutable. Someone could do REQUIRED_FIELDS_FOR_DELIVERABLE_PRODUCTS.getFields().add(..) which would beat the whole purpose of having the enum in the first place.

Better implementation for the constructor would be:

private RequiredFields(String... fields){
    this.fields = ImmutableList.copyOf(fields); //com.google.common.collect.ImmutableList
}

edited Feb 22, 2015 at 12:46

answered Feb 22, 2015 at 12:26

Teddy

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6 Comments

Kaloyan Roussev Over a year ago

So whenever I need the list of fields, I just do REQUIRED_FIELDS_FOR_DOWNLOADABLE_PRODUCTS.getFields(), right? Is there a way to pass "isDeliverable" to the enum so that it returns the correct type, so I can avoid an if else construct?

Teddy Over a year ago

'Which enum to return' logic is probably better kept outside the enum, in the class where you are using the enum.

Kaloyan Roussev Over a year ago

Ok but is it at least possible?

Adrian Leonhard Over a year ago

Collections.unmodifiableList(Arrays.asList(fields)) works too and has no external dependencies.

Adrian Leonhard Over a year ago

@J.K. Enum is very similar to a standard class, you can just add a static method with an if/else.

|

ArunSelvam P M · Accepted Answer · 2021-04-23 08:40:00Z

0

For creating immutable list of objects, from Java 9 you can use List.of static method. So you can create something like:

   Enum1("a","b"),
   Enum2("c","d")
   private String fields;
   EnumClassName(String...args) {
    this.fields = List.of(args);
   }
  }

In the calling method you can call like list = Enum1.fields; Hoping this clears your understanding.

answered Apr 23, 2021 at 8:40

ArunSelvam P M

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