Mapping enum keys with EnumMaps

#java friday, january 19, 2018

Here’s a type that has been around in the JDK for a while and that comes in handy when we want to define maps with enum types as keys: An EnumMap is a such as specialized Map.

We’ll create a map for a given enum:

public enum CoffeeType {
    ESPRESSO, POUR_OVER, FRENCH_PRESS
}

The EnumMap needs to be aware of the enum class at creation time:

Map<CoffeeType, String> favoriteCoffeeOrigins = new EnumMap<>(CoffeeType.class);

favoriteCoffeeOrigins.put(CoffeeType.ESPRESSO, "Ethiopia");
favoriteCoffeeOrigins.put(CoffeeType.POUR_OVER, "Colombia");
favoriteCoffeeOrigins.put(CoffeeType.FRENCH_PRESS, "Indonesia");

assertThat(favoriteCoffeeOrigins.get(CoffeeType.ESPRESSO)).isEqualTo("Ethiopia");

An EnumMap is much more efficient compared to a HashMap implementation. All basic map operations of this Java implementation are executed in constant time.

This post was reposted from my newsletter issue 017.

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