Understanding the syntax of this enum statement

Question

I came across the following Kotlin code for an enum:

enum class Section(val position: Int, val textKey: Int, val fragment: Fragment) {

    GUIDE(0, R.string.main_pager_guide, QotGuideFragment()),
    LEARN(1, R.string.main_pager_learn, QotLearnFragment()),
    ME(2, R.string.main_pager_me, QotToBeVisionFragment()),
    PREPARE(3, R.string.main_pager_prepare, QotPrepareFragment()),
    ;
}

However, when I review the Kotlin docs on enums, I don't see anything in it that shows this kind of syntax. The line:

GUIDE(0, R.string.main_pager_guide, QotGuideFragment())

I don't understand how these 3 parameters are used. Also, the enum class Section shows 3 constructor parameters that don't appear to be used.

The official docs on enum are at:

https://kotlinlang.org/docs/reference/enum-classes.html

the Section constructor has 3 params, and the GUIDE() call passes 3 params of matching types, that should ring a bell. The same syntax is used in the 2nd example of the docs you link to, it's just a slighlty simplified version — Tim, Jan 08 '19 at 10:14
**And this is possible in Java too**. An example here - https://stackoverflow.com/a/8811869/816416 — Vishnu Haridas, Jan 08 '19 at 19:31

score 1 · Accepted Answer · answered Jan 08 '19 at 10:24

1

From https://kotlinlang.org/docs/reference/enum-classes.html:

Each enum constant is an object

so GUIDE is an instance of Section class, meaning an object initialized as

GUIDE(0, R.string.main_pager_guide, QotGuideFragment())

You can get the values that initialized GUIDE, like this:

val guidePosition = Section.GUIDE.position
val guideTextKey = Section.GUIDE.textKey
val guideFragment = Section.GUIDE.fragment

answered Jan 08 '19 at 10:24

forpas

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"GUIDE is an instance of Section class" is the most important take-away. That isn't exactly how the docs word it. So your explanation is better. Thanks. – Johann Jan 08 '19 at 10:40

score 0 · Answer 2 · answered Jan 08 '19 at 10:30

usually your enums will be like that

enum class Section() {
    GUIDE,
    LEARN,
    ME,
    PREPARE
}

without any parameters

but in your example the base constructor of the enum has parameters that are set as properties also

enum class Section(**val** position: Int, **val** textKey: Int, **val** fragment: Fragment)

with the keyword val in the constructor you set as property of the class

then it has

GUIDE(0, R.string.main_pager_guide, QotGuideFragment()),
LEARN(1, R.string.main_pager_learn, QotLearnFragment()),
ME(2, R.string.main_pager_me, QotToBeVisionFragment()),
PREPARE(3, R.string.main_pager_prepare, QotPrepareFragment())

so for GUIDE 0 -> position, R.string.main_pager_guide -> textKey and QotGuideFragment -> fragment

Understanding the syntax of this enum statement

2 Answers2