Enum with properties and methods in Kotlin

Introduction

Enums help group related values together. Adding properties and methods makes each value more useful and descriptive.

When you want to represent a fixed set of related constants with extra details.

When each option needs its own behavior or data.

When you want to organize code clearly instead of using many separate variables.

When you want to avoid mistakes by limiting choices to predefined options.

When you want to add functions that work differently for each enum value.

Syntax

Kotlin

enum class EnumName(val propertyName: PropertyType) {
    VALUE1(propertyValue1) {
        override fun methodName() {
            // code for VALUE1
        }
    },
    VALUE2(propertyValue2) {
        override fun methodName() {
            // code for VALUE2
        }
    };

    abstract fun methodName()
}

Each enum value can have its own property values and method implementations.

The abstract method must be implemented by all enum values.

Examples

This enum represents directions with degrees and a description method.

Kotlin

enum class Direction(val degrees: Int) {
    NORTH(0) {
        override fun description() = "Upwards"
    },
    EAST(90) {
        override fun description() = "Rightwards"
    },
    SOUTH(180) {
        override fun description() = "Downwards"
    },
    WEST(270) {
        override fun description() = "Leftwards"
    };

    abstract fun description(): String
}

This enum models traffic lights with color and an action method.

Kotlin

enum class Light(val color: String) {
    RED("Red") {
        override fun action() = "Stop"
    },
    YELLOW("Yellow") {
        override fun action() = "Get Ready"
    },
    GREEN("Green") {
        override fun action() = "Go"
    };

    abstract fun action(): String
}

Sample Program

This program defines planets with mass and radius. Each planet has its own surface gravity method. The main function prints details for each planet.

Kotlin

enum class Planet(val mass: Double, val radius: Double) {
    EARTH(5.972e24, 6371.0) {
        override fun surfaceGravity() = 9.807
    },
    MARS(0.64171e24, 3389.5) {
        override fun surfaceGravity() = 3.721
    };

    abstract fun surfaceGravity(): Double
}

fun main() {
    for (planet in Planet.values()) {
        println("${planet.name} has mass ${planet.mass} kg and surface gravity ${planet.surfaceGravity()} m/s²")
    }
}

Important Notes

Remember to put a semicolon (;) before defining methods if enum values have bodies.

Properties can be used like normal class properties.

Methods can be abstract or concrete inside the enum class.

Summary

Enums group fixed related values with extra data and behavior.

Each enum value can have its own properties and methods.

This helps write clear and organized code for fixed sets of options.

Practice

1. What is the main purpose of using an enum with properties and methods in Kotlin?

easy

A. To group related fixed values with additional data and behavior

B. To create a list of unrelated variables

C. To define functions without any fixed values

D. To replace all classes in Kotlin

Solution

1. Step 1: Understand enum basics

   Enums group fixed related values, like days or colors.

2. Step 2: Recognize added properties and methods

   Adding properties and methods lets each enum value hold extra data and behavior.

3. Final Answer:

   To group related fixed values with additional data and behavior -> Option A

4. Quick Check:

   Enums = fixed values + extra data [OK]

Hint: Enums bundle fixed options with data and actions [OK]

Common Mistakes:

• Thinking enums are for unrelated variables
• Confusing enums with regular classes
• Assuming enums can't have methods

2. Which of the following is the correct syntax to declare an enum with a property color of type String in Kotlin?

easy

A. enum class Fruit { APPLE(color = "Red"), BANANA(color = "Yellow") }

B. enum Fruit { APPLE: String = "Red", BANANA: String = "Yellow" }

C. enum class Fruit(val color: String) { APPLE("Red"), BANANA("Yellow") }

D. enum class Fruit { val color: String = "Red"; APPLE, BANANA }

Solution

1. Step 1: Check enum class declaration with property

   In Kotlin, enum classes can have constructor parameters like val color: String.

2. Step 2: Verify enum values initialization

   Each enum value must pass the property value in parentheses, e.g., APPLE("Red").

3. Final Answer:

   enum class Fruit(val color: String) { APPLE("Red"), BANANA("Yellow") } -> Option C

4. Quick Check:

   Enum with constructor and values = enum class Fruit(val color: String) { APPLE("Red"), BANANA("Yellow") } [OK]

Hint: Use constructor parameters and pass values in parentheses [OK]

Common Mistakes:

• Using colon syntax instead of constructor parameters
• Trying to assign properties inside enum body incorrectly
• Omitting parentheses for enum values

3. What will be the output of the following Kotlin code?

enum class Size(val label: String) {
    SMALL("S"), MEDIUM("M"), LARGE("L");
    fun description() = "Size is $label"
}

fun main() {
    println(Size.MEDIUM.description())
}

medium

A. Size is M

B. Size is MEDIUM

C. MEDIUM

D. Size.MEDIUM

Solution

1. Step 1: Understand enum property and method

   Each enum value has a label property: MEDIUM has "M".

2. Step 2: Evaluate the description() method call

   The method returns "Size is $label", so for MEDIUM it returns "Size is M".

3. Final Answer:

   Size is M -> Option A

4. Quick Check:

   Method uses label property = "Size is M" [OK]

Hint: Method uses property value, not enum name [OK]

Common Mistakes:

• Printing enum name instead of property
• Confusing property label with enum constant name
• Ignoring method call output

4. Identify the error in this Kotlin enum declaration:

enum class Direction(val degrees: Int) {
    NORTH(0),
    EAST(90),
    SOUTH(180),
    WEST
}

medium

A. Semicolon missing after enum constants

B. Cannot have properties in enum classes

C. Enum constants must be lowercase

D. Missing property value for WEST enum constant

Solution

1. Step 1: Check enum constructor parameters

   The enum class requires an Int parameter degrees for each constant.

2. Step 2: Verify each enum constant initialization

   All constants except WEST provide a value; WEST is missing (value).

3. Final Answer:

   Missing property value for WEST enum constant -> Option D

4. Quick Check:

   All enum constants must pass constructor values [OK]

Hint: All enum constants must provide constructor arguments [OK]

Common Mistakes:

• Forgetting to pass constructor arguments for all constants
• Assuming semicolon is mandatory after constants
• Thinking enum constants must be lowercase

5. Given this enum with a method:

enum class TrafficLight(val duration: Int) {
    RED(30) {
        override fun signal() = "Stop"
    },
    YELLOW(5) {
        override fun signal() = "Caution"
    },
    GREEN(25) {
        override fun signal() = "Go"
    };
    abstract fun signal(): String
}

fun main() {
    val lights = TrafficLight.values()
    val signals = lights.map { it.signal() }
    println(signals)
}

What is the output when running main()?

hard

A. [30, 5, 25]

B. ["Stop", "Caution", "Go"]

C. [RED, YELLOW, GREEN]

D. ["Go", "Caution", "Stop"]

Solution

1. Step 1: Understand enum with abstract method

   Each enum constant overrides signal() to return a specific string.

2. Step 2: Map enum values to their signals

   The map calls signal() on each constant, producing ["Stop", "Caution", "Go"].

3. Final Answer:

   ["Stop", "Caution", "Go"] -> Option B

4. Quick Check:

   Abstract method overridden per constant = correct signals [OK]

Hint: Abstract method overridden per enum constant returns correct string [OK]

Common Mistakes:

• Printing enum names instead of method results
• Confusing property values with method outputs
• Ignoring abstract method override requirement