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?

Answer 1

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
}

Answer 2

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.