Enum class declaration in Kotlin - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of enum property access

What is the output of this Kotlin code?

Kotlin

enum class Direction {
    NORTH, SOUTH, EAST, WEST
}

fun main() {
    val dir = Direction.EAST
    println(dir.name)
}

A EAST

B Direction.EAST

C Error: Cannot access enum property

D east

💡 Hint

The 'name' property of an enum returns the exact name as declared.

In Kotlin, each enum constant has a 'name' property that returns its name as a string exactly as declared, including case.

❓ Predict Output

intermediate

Enum ordinal value output

What will this Kotlin program print?

Kotlin

enum class Color {
    RED, GREEN, BLUE
}

fun main() {
    println(Color.GREEN.ordinal)
}

A Error: ordinal not accessible

B 2

C 0

D 1

💡 Hint

Ordinal starts counting from zero for the first enum constant.

The 'ordinal' property returns the position of the enum constant starting at 0. RED=0, GREEN=1, BLUE=2.

🔧 Debug

advanced

Identify the error in enum constructor usage

Which option correctly fixes the error in this Kotlin enum declaration?

Kotlin

enum class Size(val cm: Int) {
    SMALL(30), MEDIUM, LARGE(50)
}

A Add a constructor argument to MEDIUM: MEDIUM(40)

B Remove constructor arguments from all constants

C Change val cm to var cm

D Add default value to constructor: val cm: Int = 0

💡 Hint

All enum constants must provide constructor arguments if the enum class has a constructor.

When an enum class has a constructor, all constants must call it with appropriate arguments. MEDIUM lacks an argument causing a compile error.

🧠 Conceptual

advanced

Behavior of enum with overridden methods

What will this Kotlin code print when run?

Kotlin

enum class Mood {
    HAPPY {
        override fun feeling() = "Joy"
    },
    SAD {
        override fun feeling() = "Tears"
    };
    abstract fun feeling(): String
}

fun main() {
    println(Mood.HAPPY.feeling())
    println(Mood.SAD.feeling())
}

A HAPPY\nSAD

B feeling\nfeeling

C Joy\nTears

D Error: abstract method not implemented

💡 Hint

Each enum constant can override abstract methods individually.

The enum declares an abstract method 'feeling'. Each constant overrides it with its own implementation, so the calls print their respective strings.

❓ Predict Output

expert

Output of enum with companion object and custom method

What is the output of this Kotlin program?

Kotlin

enum class Level(val code: Int) {
    LOW(1), MEDIUM(5), HIGH(10);

    companion object {
        fun fromCode(code: Int) = values().find { it.code == code } ?: LOW
    }
}

fun main() {
    println(Level.fromCode(5))
    println(Level.fromCode(7))
}

A MEDIUM\nnull

B MEDIUM\nLOW

C 5\n1

D Error: companion object method not accessible

💡 Hint

The method returns LOW if no matching code is found.

The 'fromCode' method searches for a Level with matching code. 5 matches MEDIUM, 7 matches none so returns LOW by default.

Practice

1. What is the main purpose of an enum class in Kotlin?

easy

A. To define a function that returns multiple values

B. To group a fixed set of related constants under one type

C. To create a class that can be instantiated multiple times

D. To declare a variable that can hold any data type

Solution

1. Step 1: Understand enum class purpose

   An enum class is designed to hold a fixed set of constants that belong together logically.

2. Step 2: Compare options with enum usage

   Only To group a fixed set of related constants under one type correctly describes grouping related constants under one type, which is the main use of enum classes.

3. Final Answer:

   To group a fixed set of related constants under one type -> Option B

4. Quick Check:

   Enum class purpose = group constants [OK]

Hint: Enums group fixed constants for clarity and safety [OK]

Common Mistakes:

• Thinking enums are for creating multiple instances
• Confusing enums with regular classes
• Believing enums can hold any data type variable

2. Which of the following is the correct syntax to declare an enum class named Direction with constants NORTH, SOUTH, EAST, and WEST?

easy

A. enum class Direction { NORTH, SOUTH, EAST, WEST }

B. class enum Direction { NORTH, SOUTH, EAST, WEST }

C. enum Direction { NORTH; SOUTH; EAST; WEST }

D. enum class Direction = (NORTH, SOUTH, EAST, WEST)

Solution

1. Step 1: Recall Kotlin enum class syntax

   Kotlin enum classes start with enum class followed by the name and curly braces containing constants separated by commas.

2. Step 2: Check each option's syntax

   enum class Direction { NORTH, SOUTH, EAST, WEST } matches the correct syntax. Options A, C, and D have incorrect keywords, punctuation, or structure.

3. Final Answer:

   enum class Direction { NORTH, SOUTH, EAST, WEST } -> Option A

4. Quick Check:

   Correct enum syntax = enum class Direction { NORTH, SOUTH, EAST, WEST } [OK]

Hint: Use 'enum class Name { CONST1, CONST2 }' syntax [OK]

Common Mistakes:

• Using 'class enum' instead of 'enum class'
• Separating constants with semicolons instead of commas
• Assigning enum constants with '=' instead of listing

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

enum class Color { RED, GREEN, BLUE }
fun main() {
    val c = Color.GREEN
    println(c.name)
}

medium

A. green

B. Color.GREEN

C. 1

D. GREEN

Solution

1. Step 1: Understand enum constant property 'name'

   Each enum constant has a built-in name property that returns its exact name as a string.

2. Step 2: Analyze the code output

   The code prints c.name where c is Color.GREEN, so it prints "GREEN" exactly as declared.

3. Final Answer:

   GREEN -> Option D

4. Quick Check:

   Enum constant name property = GREEN [OK]

Hint: Enum.name returns constant name as string [OK]

Common Mistakes:

• Expecting lowercase output
• Printing enum constant directly without .name
• Confusing ordinal number with name

4. Identify the error in this Kotlin enum class declaration:

enum class Size {
    SMALL,
    MEDIUM
    LARGE
}

medium

A. Cannot declare enum class without constructor

B. Enum constants must be lowercase

C. Missing comma after MEDIUM

D. Enum class must have at least 5 constants

Solution

1. Step 1: Check enum constants separation

   Enum constants must be separated by commas except the last one. Here, MEDIUM is missing a comma after it.

2. Step 2: Validate other options

   Constants can be uppercase, no constructor is required, and no minimum count is needed. So only missing comma is the error.

3. Final Answer:

   Missing comma after MEDIUM -> Option C

4. Quick Check:

   Enum constants separated by commas [OK]

Hint: Separate enum constants with commas except last [OK]

Common Mistakes:

• Omitting commas between constants
• Using lowercase for constants (not required)
• Thinking constructor is mandatory

5. How can you add a property rgb to an enum class Color that stores the color's RGB integer value, and access it for Color.RED? Choose the correct code snippet.

hard

A. enum class Color(val rgb: Int) { RED(0xFF0000), GREEN(0x00FF00), BLUE(0x0000FF) }
fun main() { println(Color.RED.rgb) }

B. enum class Color { RED = 0xFF0000, GREEN = 0x00FF00, BLUE = 0x0000FF }
fun main() { println(Color.RED.rgb) }

C. enum class Color { RED(0xFF0000), GREEN(0x00FF00), BLUE(0x0000FF) }
fun main() { println(Color.RED.rgb) }

D. enum class Color { RED, GREEN, BLUE }
fun main() { println(Color.RED.rgb) }

Solution

1. Step 1: Understand enum property declaration

   To add a property, declare a constructor parameter in the enum class and pass values to each constant.

2. Step 2: Check each option for correct syntax

   enum class Color(val rgb: Int) { RED(0xFF0000), GREEN(0x00FF00), BLUE(0x0000FF) } fun main() { println(Color.RED.rgb) } correctly declares val rgb: Int in constructor and assigns values. enum class Color { RED = 0xFF0000, GREEN = 0x00FF00, BLUE = 0x0000FF } fun main() { println(Color.RED.rgb) } uses invalid assignment syntax. enum class Color { RED(0xFF0000), GREEN(0x00FF00), BLUE(0x0000FF) } fun main() { println(Color.RED.rgb) } misses constructor declaration. enum class Color { RED, GREEN, BLUE } fun main() { println(Color.RED.rgb) } has no property defined.

3. Final Answer:

   enum class Color(val rgb: Int) { RED(0xFF0000), GREEN(0x00FF00), BLUE(0x0000FF) } fun main() { println(Color.RED.rgb) } -> Option A

4. Quick Check:

   Enum property via constructor = enum class Color(val rgb: Int) { RED(0xFF0000), GREEN(0x00FF00), BLUE(0x0000FF) } fun main() { println(Color.RED.rgb) } [OK]

Hint: Add properties via constructor parameters in enum class [OK]

Common Mistakes:

• Trying to assign values with '=' inside enum constants
• Not declaring constructor parameters in enum class
• Accessing property that was never declared