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EnumSet for enum collections

#java 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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