Enum with properties and methods in Kotlin - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of enum with property and method

What is the output of this Kotlin code?

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
}

fun main() {
    val dir = Direction.EAST
    println("${dir.name} is at ${dir.degrees} degrees and means ${dir.description()}")
}

A EAST is at 90 degrees and means Rightwards

B EAST is at 90 degrees and means Upwards

C EAST is at 180 degrees and means Rightwards

D Compilation error due to abstract method

💡 Hint

Check how each enum constant overrides the abstract method description().

Each enum constant provides its own implementation of the abstract method description(). EAST returns "Rightwards" and has degrees 90.

❓ Predict Output

intermediate

Enum method call result

What does this Kotlin program print?

Kotlin

enum class Light(val duration: Int) {
    RED(30),
    GREEN(45),
    YELLOW(5);

    fun isSafeToGo() = this == GREEN
}

fun main() {
    val current = Light.YELLOW
    println(current.isSafeToGo())
}

A true

B Runtime exception

C Compilation error: missing method body

D false

💡 Hint

Check which enum constant returns true for isSafeToGo().

Only GREEN returns true for isSafeToGo(). YELLOW returns false.

🔧 Debug

advanced

Identify the error in enum with method override

This Kotlin enum tries to override a method but causes a compilation error. What is the error?

Kotlin

enum class Status {
    SUCCESS {
        override fun message() = "Operation succeeded"
    },
    FAILURE {
        override fun message() = "Operation failed"
    };

    fun message(): String = "Unknown"
}

A Cannot override non-abstract method message() without marking it abstract

B Missing semicolon after enum constants

C Abstract method message() must be declared abstract in enum

D Enum constants cannot have method bodies

💡 Hint

Check if the method message() is abstract or not.

You cannot override a non-abstract method in enum constants. To override, message() must be abstract.

❓ Predict Output

advanced

Output of enum with companion object method

What is the output of this Kotlin code?

Kotlin

enum class Planet(val mass: Double, val radius: Double) {
    EARTH(5.972e24, 6371.0),
    MARS(0.64171e24, 3389.5);

    companion object {
        fun heaviest(): Planet = values().maxByOrNull { it.mass } ?: EARTH
    }
}

fun main() {
    println(Planet.heaviest())
}

A Compilation error: companion object syntax

B MARS

C EARTH

D Runtime exception: null pointer

💡 Hint

Check which planet has the greater mass.

EARTH has a larger mass than MARS, so heaviest() returns EARTH.

🧠 Conceptual

expert

Number of enum constants with overridden methods

Given this Kotlin enum, how many constants override the abstract method `sound()`?

Kotlin

enum class Animal {
    DOG {
        override fun sound() = "Bark"
    },
    CAT {
        override fun sound() = "Meow"
    },
    COW {
        override fun sound() = "Moo"
    },
    FISH;

    abstract fun sound(): String
}

A 0

B 3

C 1

D 4

💡 Hint

Check which constants provide a method body for sound().

DOG, CAT, and COW override sound(). FISH does not and will cause a compilation error if used.

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