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Memory consumption of popular Java data types – part 1

by Mikhail Vorontsov

This article will give you an overview of some popular Java data types memory consumption. This article follows An overview of memory saving techniques in Java article earlier published in this blog. I strongly recommend you to read the previous article before reading this one.

Enum, EnumMap, EnumSet

Enums were added to Java 1.5. Technically speaking, each enum is an Object and has all memory consumption properties of objects described in the previous article. Luckily, they have several useful properties:

All enum objects are singletons. This is guaranteed by Java language – you can create an instance of enum only by its definition. This means that you pay for enum object once and then you only spend 4 bytes per its reference (in this article I will assume that object references occupy 4 bytes). But what is the benefit of enums if they consume as much memory as int variables and 4 times more than byte variables, besides type safety and better support in IDE?

The answer is the ordinal() method implemented by every enum. It is an increasing number starting from 0 and ending at the number of values in the given enum minus 1. Such enum property allows us to use arrays for enum to any other object mapping: a value related to the first enum value will be stored in array[0], second enum will be mapped to array[1] and so on according to Enum.ordinal() method result. By the way, it was a short description of JDK EnumMap class implementation.

If you need to implement a map from Object into enum, you won’t have a tailored JDK implementation at hand. Of course, you can use any JDK Object to Object map, but it wouldn’t be that efficient. The easy way here is to use Trove TObjectByteMap (or TObjectIntMap in rare cases when you have more than 128 values in your enum) and map from Object key into Enum.ordinal() value. You will need a decoding method for getters in order to convert byte into an enum. This method will require 1 byte per entry, which is the best we can do without paying CPU algorithmic penalty (of course, we can use less than 1 byte per enum, if there are less than 128, 64, 32, etc elements in the enum, but it may make your code more complicated for a very little memory gain).

With all this knowledge at hand, you may now realize that EnumSet is implemented similarly to BitSet. There are 2 EnumSet implementations in JDK: RegularEnumSet and JumboEnumSet. The former is used for enums having less than 65 values (it covers 99,9999% of real-world enums), the latter is used for larger enumerations.

RegularEnumSet utilizes the knowledge of the fact that there are less or equal to 64 values in the enum. It allows RegularEnumSet to use a single long to store all “enum present in the set” flags, rather than using long[] (utilized by JumboEnumSet or BitSet). Using a single long instead of long[1] allows to save 4 bytes on long[] reference and 16 bytes on long value (long occupies 8 bytes, long[1] needs 12 bytes for header, 8 bytes for a single long and 4 bytes for alignment).

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