Enum Collections in Java

What are EnumSet and EnumMap, and when should you use them?

Introduction

Recently I had the opportunity to rediscover one of my favourite things about Java, that there are so many different collection implementations to choose from! If you know what implementations are available, and how to use them (both of which a quick Google search can help with), you can write much more efficient code.

Everyone who has used Java should be familiar with the common collections, and their standard implementations, such as: Set and HashSet, and Map and HashMap. These are general-purpose implementations of the Set and Map data structures, which all developers should be familiar with. They can generally be used everywhere, however, there exist other implementations which are much  better suited for specific use cases.

In this blog post I'll discuss a few of the Enum-optimised implementations, and when to use them. I assume that you're already familiar with the standard implementations of these collections, and with the concept of an Enum.

EnumSet and EnumMap

The two enum-specific implementations that I discuss here are  EnumSet and EnumMap. These implementations are fairly straightforward and easy to understand.

EnumSet and EnumMap were added to Java in version 1.5 (2004), and while they've been around for a long time now (18 years!), in my experience, not many developers know about or use them. Developers, especially those who have moved to Java from other languages, will often just use the standard collection implementations and not give it a second thought. This is generally okay, but we can do better!

EnumSet

Simply put, an EnumSet is a concrete implementation of the Set interface, which is optimised for the storage of a specific enum type. Given the static nature of an enum, and the fact that they are generally fairly small, consisting of only a few constants, it is possible to use a very efficient implementation to store them, while still implementing the Set interface.

If you ever find yourself needing to store enums in a Set, you should use the EnumSet implementation, instead of the standard HashSet, unless you have a very specific reason not to. EnumSet is not thread-safe, it keeps elements in their natural order, and it does not accept null values. As with other collections, you could use the Collections.synchronizedSet() wrapper if you require a thread-safe set.

Without going in to too much of the implementation detail, it is good enough to know that EnumSet is backed by a bit vector. For up to 64 constant long enums (which is more than enough for almost every use case), they are simply stored in a long data field (or an array of longs if there are more than 64 constants). This makes EnumSet extremely memory efficient, and similarly computationally efficient for all operations, as they are implemented using arithmetic bitwise operations. All of the basic operations will run in constant time. This is much more efficient than in a HashSet, where the hashCode of the value will always have to be calculated first.

As an example, assume we have the following enum:

public enum Direction {
	NORTH, SOUTH, EAST, WEST
}
Direction enum definition

Then, creating an EnumSet to hold values from the Direction enum can be accomplished in two ways:

// Create a new EnumSet containing all constants in the enum
Set<Direction> directions = EnumSet.allOf(Direction.class);

// Create a new empty EnumSet
Set<Direction> directions = EnumSet.noneOf(Direction.class);
EnumSet definitions

All of the usual Set methods can be used with an EnumSet, with the most common being: add(), contains(), forEach(), and remove(). There are a few other useful methods, all of which can be found by visiting the Java reference.

Note: As EnumSet is an Abstract class, it cannot be instantiated using the new operator, one of the above static factory methods must be used instead.

EnumMap

Similarly, EnumMap is a concrete implementation of the Map interface, which is optimised for use with an enum key. As with EnumSet, given the characteristics of an enum, it is possible to create a much more efficient implementation than the general-purpose HashMap.

If you ever find yourself using an enum as the key for a map, you should seriously consider using an EnumMap. EnumMap is not thread-safe, it displays natural order sorting (the order in which the enum keys are defined), and does not accept null keys, but does accept null values. As with other collections, you could use the Collections.synchronizedMap() wrapper if you require a thread-safe map.

Again, without going into much of the implementation detail, it is good enough to know that EnumMap is backed by a simple Java array. This makes it much more memory and computationally efficient than a HashMap, as it does not need to account for hashing a generic key, nor the possibility of hash collisions.  This makes all storage and retrieval operations much faster, and all basic operations run in constant time.

As an example, assuming we have the same Direction enum defined earlier, an EnumMap can be created and added to as follows:

Map<Direction, String> directions = new EnumMap<>(Direction.class);
directions.put(Direction.NORTH, "Go north");
directions.get(Direction.NORTH);
EnumMap definition and usage

As with any other implementation of the Map interface, all of the usual operations can be performed on an EnumMap.

Benchmarks

In order to get some idea of the practical performance improvements of using EnumSet and EnumMap over HashSet and HashMap, I wrote a small microbenchmark that tests the creation, insertion, contains, and removal operations on each implementation, over many iterations. The code can be found here: https://github.com/yestinj/EnumCollections

I ran these microbenchmarks on my M1 Mac Mini (2020, 8GB) running macOS Monterey 12.4, using JMH version 1.35, on OpenJDK 64-bit Server VM 18.0.1.1+0.

Enum collection microbenchmark results

First, looking at HashSet versus EnumSet; when using HashSet the benchmark performed at  ~14.64m operations per second, while the EnumSet implementation performed at a whopping ~425.27m operations per second! This makes EnumSet on average 29x faster than HashSet!

Next, considering the maps; HashMap performs at ~15.54m operations per second, and EnumMap performs at ~70.51m operations per second. This makes EnumMap, on average, 4.5x faster than HashMap!

These microbenchmarks are synthetic, and don't represent a real use case, however, it should still be obvious how much more computationally efficient it is to use EnumSet and EnumMap! And, this doesn't even take into account the memory efficiency improvements.

Conclusion

In summary, if you ever find yourself using an enum as the key for a map, or as the object you're storing in a set, be sure to use EnumMap and EnumSet if possible, and benefit from their improved memory efficient and computational performance.

As always, 'micro-optimisations' such as this can be counterproductive, I wouldn't recommend refactoring an existing codebase just for this. However, if you're having performance issues, or writing new code, there's really no reason not to consider using these implementations. Just make sure that you've done your homework 😊

Additional Reading

  1. https://www.baeldung.com/java-enum-map
  2. https://www.geeksforgeeks.org/enummap-class-java-example/
  3. https://www.baeldung.com/java-enumset
  4. https://www.geeksforgeeks.org/enumset-class-java/