sebastiandaschner blog

EnumSet for enum collections

friday, january 19, 2018

In the last blog post we discovered EnumMaps for mappings with enum keys. You might have observed that there is also a specialized Set that is optimized for enums: EnumSet.

We again define a CoffeeType enum:

public enum CoffeeType {

Now we can create sets of this enum type, by using the EnumSet implementation:

Set<CoffeeType> favoriteCoffeeTypes = EnumSet.of(ESPRESSO, POUR_OVER, LATTE);

assertThat(favoriteCoffeeTypes).containsOnly(ESPRESSO, POUR_OVER, LATTE);

The favoriteCoffeeTypes still acts like any Set, that is, adding duplicates won’t change its contents:


assertThat(favoriteCoffeeTypes).containsOnly(ESPRESSO, POUR_OVER, LATTE);

Interesting side note: If you look into the JDK, you’ll see that EnumSet is implemented by both RegularEnumSet and JumboEnumSet; the number of enum elements determines the implementation being used. If you’re interested in how the EnumSet implementation manage to be highly efficient, I challenge you to have a look into these classes. Hint: Bitwise operations :-)

This post was reposted from my newsletter issue 018.


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