enums.md

Enums in Java

Enum implementation in Java needs to jump through a few hoops to work with the JVM. Consider the use of the AgeRange enum as used in Person.java:

enum AgeRange {
    AGE_0_9,
    AGE_10_19,
    AGE_20_39,
    AGE_40_64,
    AGE_65_79,
    AGE_80_99,
    AGE_100_PLUS
}

AgeRange is implemented as a class deriving from the java.lang.Enum class which is part of the JVM standard runtime. An Enum can be seen as a class with a name (of type String) which represents the default label (e.g. AGE_10_19) and an ordinal (of time Int) which represents the value index (from 0 to 6 in the present example).

However, when building Person.java, the Java compiler also creates a class Person$1.class. This class contains the following:

• A static member int[] $SwitchMap$AgeRange
• A static initializer which populates $SwitchMap$AgeRange to map each value used in Person.java to an index starting at 1. So if Person.java only use, say, AGE_10_19, AGE_20_39 and AGE_100_PLUS, $SwitchMap$AgeRange would be initialized to [0, 1, 2, 0, 0, 0, 3].

When the code in Person.java needs to look at the Person instance ageRange object:

switch (this.ageRange) {
    case AGE_10_19:
        System.out.println("Teen");
        break;
    case AGE_20_39:
        ...

the bytecode first calls this.ageRange.ordinal() which returns the enum's index, then get the mapping from the Person$1.$SwitchMap$AgeRange array which should be between 1 and 3. This allows a compact tableswitch instruction with only 3 entries.