Constant values with const val in Kotlin - Mini Project: Build & Apply

Using Constant Values with const val in Kotlin

📖 Scenario: You are creating a simple program to calculate the area of a rectangle. To keep your code clean and easy to update, you want to use a constant value for the number of sides of a rectangle.

🎯 Goal: Learn how to declare and use constant values with const val in Kotlin by creating a constant for the number of sides of a rectangle and using it in a calculation.

📋 What You'll Learn

Declare a constant using const val

Use the constant in a calculation

Print the result

💡 Why This Matters

🌍 Real World

Constants are used in real-world programs to store values that should not change, like configuration settings, fixed measurements, or limits.

💼 Career

Understanding how to use constants helps you write safer and clearer code, which is important in software development jobs.

1

Create a constant for the number of sides

Write a const val named SIDES and set it to 4.

Kotlin

/* Your code here */

Hint

Use const val to declare a constant value that does not change.

2

Create variables for length and width

Create two variables: length with value 5 and width with value 3.

Kotlin

const val SIDES = 4
/* Your code here */

Hint

Use val to create variables that hold the rectangle's dimensions.

3

Calculate the area of the rectangle

Create a variable area and set it to the product of length and width.

Kotlin

const val SIDES = 4
val length = 5
val width = 3
/* Your code here */

Hint

Multiply length and width to get the area.

4

Print the number of sides and the area

Print the text "A rectangle has " followed by the constant SIDES, then " sides and area " followed by the variable area.

Kotlin

const val SIDES = 4
val length = 5
val width = 3
val area = length * width
/* Your code here */

Hint

Use println and string templates with $ to include variables in the output.

Practice

1. What is the main purpose of using const val in Kotlin?

easy

A. To declare a constant value that cannot change during program execution

B. To create a variable that can be changed anytime

C. To define a function that returns a constant

D. To declare a class property that is mutable

Solution

1. Step 1: Understand what const val means

   const val is used to declare constants in Kotlin, which are values fixed at compile time and cannot be changed later.

2. Step 2: Compare options with the definition

   To declare a constant value that cannot change during program execution correctly states that const val declares a constant value that never changes. Other options describe variables or functions, which are not correct.

3. Final Answer:

   To declare a constant value that cannot change during program execution -> Option A

4. Quick Check:

   const val = fixed value [OK]

Hint: Remember: const val means fixed value, no changes allowed [OK]

Common Mistakes:

• Thinking const val can be changed like a variable
• Confusing const val with regular val or var
• Assuming const val can hold complex objects

2. Which of the following is the correct syntax to declare a constant integer named MAX_COUNT with value 100 in Kotlin?

easy

A. val MAX_COUNT = 100

B. var MAX_COUNT = 100

C. const MAX_COUNT = 100

D. const val MAX_COUNT = 100

Solution

1. Step 1: Recall the correct syntax for constants

   In Kotlin, constants are declared using const val followed by the name and value.

2. Step 2: Check each option

   const val MAX_COUNT = 100 uses const val MAX_COUNT = 100, which is the correct syntax. val MAX_COUNT = 100 misses const, C misses val, and B uses var which is mutable.

3. Final Answer:

   const val MAX_COUNT = 100 -> Option D

4. Quick Check:

   const val + name + = + value = correct syntax [OK]

Hint: const val always together for constants [OK]

Common Mistakes:

• Omitting const keyword
• Using var instead of val
• Writing const without val

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

const val PI = 3.14
fun main() {
    println("Value of PI: $PI")
}

medium

A. Compilation error

B. Value of PI: $PI

C. Value of PI: 3.14

D. Value of PI: 3

Solution

1. Step 1: Understand string interpolation in Kotlin

   The println statement uses $PI which means it will replace $PI with the value of the constant PI.

2. Step 2: Check the constant value and output

   PI is declared as 3.14, so the output will be "Value of PI: 3.14" exactly.

3. Final Answer:

   Value of PI: 3.14 -> Option C

4. Quick Check:

   String interpolation replaces $PI with 3.14 [OK]

Hint: Look for $variable inside strings for value substitution [OK]

Common Mistakes:

• Thinking $PI prints literally as $PI
• Expecting integer output instead of float
• Assuming compilation error due to const val usage

4. Identify the error in the following Kotlin code:

const val greeting = "Hello"
fun main() {
    greeting = "Hi"
    println(greeting)
}

medium

A. const val must be declared inside a function

B. Cannot reassign a value to a const val

C. Missing semicolon after greeting declaration

D. Variable greeting is not initialized

Solution

1. Step 1: Understand const val immutability

   const val declares a constant that cannot be changed after initialization.

2. Step 2: Analyze the reassignment

   The code tries to assign a new value to greeting which is a constant. This causes a compilation error.

3. Final Answer:

   Cannot reassign a value to a const val -> Option B

4. Quick Check:

   const val = fixed, no reassignment allowed [OK]

Hint: const val values cannot be changed after declaration [OK]

Common Mistakes:

• Trying to assign new value to const val
• Thinking semicolon is required in Kotlin
• Declaring const val inside functions (allowed only at top-level or object)

5. You want to create a constant string APP_NAME that can be used throughout your Kotlin app. Which of the following is the best way to declare it to ensure it is a compile-time constant accessible everywhere?

hard

A. Declare const val APP_NAME = "MyApp" at the top level or inside an object

B. Declare val APP_NAME = "MyApp" inside main() function

C. Declare var APP_NAME = "MyApp" inside a class constructor

D. Declare const var APP_NAME = "MyApp" inside a class

Solution

1. Step 1: Understand where const val can be declared

   In Kotlin, const val must be declared at the top level or inside an object to be a compile-time constant accessible everywhere.

2. Step 2: Evaluate each option

   Declare const val APP_NAME = "MyApp" at the top level or inside an object correctly declares const val APP_NAME = "MyApp" at the top level or inside an object. Declare val APP_NAME = "MyApp" inside main() function uses val inside a function, which is not compile-time constant. Declare var APP_NAME = "MyApp" inside a class constructor uses var which is mutable. Declare const var APP_NAME = "MyApp" inside a class uses invalid syntax const var.

3. Final Answer:

   Declare const val APP_NAME = "MyApp" at the top level or inside an object -> Option A

4. Quick Check:

   const val + top-level/object = global constant [OK]

Hint: const val must be top-level or in object for global constant [OK]

Common Mistakes:

• Declaring const val inside functions
• Using var with const keyword
• Using const without val