Why enums constrain values in Kotlin - See It in Action

Why enums constrain values

📖 Scenario: Imagine you are building a simple app that tracks the status of orders in a store. Each order can only have one of a few fixed statuses, like Pending, Shipped, or Delivered. You want to make sure the status can never be set to something invalid by mistake.

🎯 Goal: You will create an enum class in Kotlin to hold the allowed order statuses. Then you will use this enum to assign and print the status of an order, showing how enums help keep values safe and limited.

📋 What You'll Learn

Create an enum class called OrderStatus with values PENDING, SHIPPED, and DELIVERED

Create a variable called currentStatus and set it to OrderStatus.PENDING

Change currentStatus to OrderStatus.SHIPPED

Print the value of currentStatus to show the current order status

💡 Why This Matters

🌍 Real World

Enums are used in apps to keep data clean and predictable, like order statuses, user roles, or traffic light colors.

💼 Career

Understanding enums is important for writing safe, clear code in Kotlin, which is widely used for Android apps and backend services.

1

Create the enum class for order statuses

Create an enum class called OrderStatus with the values PENDING, SHIPPED, and DELIVERED.

Kotlin

// Your code here

Hint

Use enum class OrderStatus { PENDING, SHIPPED, DELIVERED } to define the fixed set of statuses.

2

Create a variable for the current order status

Create a variable called currentStatus and set it to OrderStatus.PENDING.

Kotlin

enum class OrderStatus {
    PENDING,
    SHIPPED,
    DELIVERED
}

// Your code here

Hint

Use var currentStatus = OrderStatus.PENDING to start with the pending status.

3

Change the current status to shipped

Change the variable currentStatus to OrderStatus.SHIPPED.

Kotlin

enum class OrderStatus {
    PENDING,
    SHIPPED,
    DELIVERED
}

var currentStatus = OrderStatus.PENDING

// Your code here

Hint

Assign OrderStatus.SHIPPED to currentStatus to update the status.

4

Print the current order status

Write a println statement to print the value of currentStatus.

Kotlin

enum class OrderStatus {
    PENDING,
    SHIPPED,
    DELIVERED
}

var currentStatus = OrderStatus.PENDING
currentStatus = OrderStatus.SHIPPED

// Your code here

Hint

Use println(currentStatus) to show the current status on the screen.

Practice

1. Why do Kotlin enums constrain values to a fixed set?

easy

A. To prevent invalid values and reduce errors

B. To allow any value to be assigned

C. To make the code run faster

D. To increase the size of the program

Solution

1. Step 1: Understand enum purpose

   Enums define a limited set of named values to represent options clearly.

2. Step 2: Identify why limiting values helps

   Limiting values prevents invalid or unexpected inputs, reducing bugs.

3. Final Answer:

   To prevent invalid values and reduce errors -> Option A

4. Quick Check:

   Enums limit values = prevent errors [OK]

Hint: Enums fix allowed values to avoid mistakes [OK]

Common Mistakes:

• Thinking enums allow any value
• Believing enums improve speed directly
• Confusing enums with variables

2. Which of the following is the correct syntax to declare an enum in Kotlin?

easy

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

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

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

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

Solution

1. Step 1: Recall Kotlin enum syntax

   Kotlin enums use 'enum class' followed by the name and curly braces with values separated by commas.

2. Step 2: Check each option

   enum class Direction { NORTH, SOUTH, EAST, WEST } matches correct syntax; others have wrong keywords or separators.

3. Final Answer:

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

4. Quick Check:

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

Hint: Remember: 'enum class Name { VALUES }' [OK]

Common Mistakes:

• Using 'class enum' instead of 'enum class'
• Separating values with semicolons
• Assigning enum like a variable

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

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

medium

A. false\nfalse

B. false\ntrue

C. true\ntrue

D. true\nfalse

Solution

1. Step 1: Understand enum comparison

   Enum values are compared by identity. c is assigned Color.GREEN.

2. Step 2: Evaluate println statements

   c == Color.GREEN is true; c == Color.RED is false.

3. Final Answer:

   true\nfalse -> Option D

4. Quick Check:

   Enum equality matches assigned value [OK]

Hint: Enum comparisons check exact named value [OK]

Common Mistakes:

• Assuming enums compare by string content
• Confusing true/false order
• Using == incorrectly with enums

4. Identify the error in this Kotlin enum usage:

enum class Size { SMALL, MEDIUM, LARGE }
fun main() {
    val s: Size = Size.EXTRA_LARGE
    println(s)
}

medium

A. Cannot print enum values directly

B. Size.EXTRA_LARGE does not exist in the enum

C. Enum values must be lowercase

D. Missing semicolon after enum values

Solution

1. Step 1: Check enum declaration

   Enum Size has only SMALL, MEDIUM, LARGE values defined.

2. Step 2: Analyze usage error

   Code tries to assign Size.EXTRA_LARGE which is not defined, causing error.

3. Final Answer:

   Size.EXTRA_LARGE does not exist in the enum -> Option B

4. Quick Check:

   Using undefined enum value = error [OK]

Hint: Use only defined enum values [OK]

Common Mistakes:

• Assuming enums accept any value
• Thinking semicolons are required
• Believing enum values must be lowercase

5. You want to restrict user input to only three modes: EASY, MEDIUM, HARD using Kotlin enums. Which approach best ensures invalid modes cannot be used in your program?

hard

A. Define enum class Mode { EASY, MEDIUM, HARD } and use Mode type for input

B. Use strings "EASY", "MEDIUM", "HARD" and check with if-else

C. Use integers 1, 2, 3 to represent modes without enum

D. Allow any string input and convert to uppercase

Solution

1. Step 1: Understand enum benefits for input restriction

   Enums limit values to a fixed set, preventing invalid inputs.

2. Step 2: Compare approaches

   Define enum class Mode { EASY, MEDIUM, HARD } and use Mode type for input uses enum type, ensuring only EASY, MEDIUM, HARD are valid. Others allow invalid inputs or require manual checks.

3. Final Answer:

   Define enum class Mode { EASY, MEDIUM, HARD } and use Mode type for input -> Option A

4. Quick Check:

   Enums enforce valid values automatically [OK]

Hint: Use enums to restrict allowed options [OK]

Common Mistakes:

• Using strings without validation
• Using integers without clear meaning
• Allowing any input without checks