Val for immutable references in Kotlin - Mini Project: Build & Apply

Val for immutable references

📖 Scenario: Imagine you are creating a simple program to store information about a book. You want to make sure that the book's title does not change once it is set.

🎯 Goal: Learn how to use val in Kotlin to create an immutable reference for a variable.

📋 What You'll Learn

Create a variable using val to hold a book title

Assign the exact string "Kotlin Basics" to the variable

Try to change the value of the variable (this will be explained)

Print the value of the variable

💡 Why This Matters

🌍 Real World

In real apps, using <code>val</code> helps keep important data safe and predictable, like user names or IDs that should not change.

💼 Career

Understanding <code>val</code> is essential for Kotlin developers to write reliable and bug-free code.

1

Create an immutable variable with val

Create a variable called bookTitle using val and assign it the string "Kotlin Basics".

Kotlin

// Create a val variable called bookTitle and assign "Kotlin Basics"
// Your code here

Hint

Use val to make the variable immutable. For example: val name = "value".

2

Try to change the val variable

Add a line below that tries to assign "Advanced Kotlin" to bookTitle.

Kotlin

val bookTitle = "Kotlin Basics"
// Try to change bookTitle to "Advanced Kotlin" here
// Your code here

Hint

Try writing bookTitle = "Advanced Kotlin". Kotlin will not allow this because bookTitle is immutable.

3

Print the value of the val variable

Write a println statement to print the value of bookTitle.

Kotlin

val bookTitle = "Kotlin Basics"
// bookTitle = "Advanced Kotlin" // This line will cause an error if uncommented
// Print bookTitle below
// Your code here

Hint

Use println(bookTitle) to show the value on the screen.

4

Run and observe the output

Run the program and observe the output. It should print Kotlin Basics.

Kotlin

val bookTitle = "Kotlin Basics"
// bookTitle = "Advanced Kotlin" // This line will cause an error if uncommented
println(bookTitle)
// Run the program to see the output

Hint

The program prints the value of bookTitle. Since it is immutable, it stays as Kotlin Basics.

Practice

1. What does the val keyword do in Kotlin?

easy

A. Defines a class

B. Creates an immutable reference that cannot be reassigned

C. Declares a function

D. Creates a mutable variable that can be reassigned

Solution

1. Step 1: Understand the role of val

   val declares a variable whose reference cannot be changed after assignment.

2. Step 2: Compare with other keywords

   var allows reassignment, but val does not.

3. Final Answer:

   Creates an immutable reference that cannot be reassigned -> Option B

4. Quick Check:

   val = immutable reference [OK]

Hint: Remember: val means fixed reference, var means changeable [OK]

Common Mistakes:

• Confusing val with var
• Thinking val makes the object itself immutable
• Assuming val declares a function or class

2. Which of the following is the correct syntax to declare an immutable integer variable named count with value 10 in Kotlin?

easy

A. val count = 10

B. var count = 10

C. int val count = 10

D. val count : Int =

Solution

1. Step 1: Identify correct keyword and syntax

   val is used for immutable variables, and Kotlin infers type if not specified.

2. Step 2: Check each option

   var count = 10 uses var (mutable), C has invalid syntax, D is incomplete.

3. Final Answer:

   val count = 10 -> Option A

4. Quick Check:

   Immutable variable syntax = val count = 10 [OK]

Hint: Use val for fixed values, var for changeable ones [OK]

Common Mistakes:

• Using var instead of val for immutable variables
• Incorrect Kotlin type declarations
• Leaving declarations incomplete

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

val list = mutableListOf(1, 2, 3)
list.add(4)
println(list)

medium

A. Runtime error

B. [1, 2, 3]

C. Compilation error

D. [1, 2, 3, 4]

Solution

1. Step 1: Understand val with mutable objects

   val means the reference to the list cannot change, but the list itself can be modified.

2. Step 2: Analyze code behavior

   Adding 4 to the mutable list changes its content, so printing shows [1, 2, 3, 4].

3. Final Answer:

   [1, 2, 3, 4] -> Option D

4. Quick Check:

   val reference + mutable object = modified list [OK]

Hint: val fixes reference, not object content if mutable [OK]

Common Mistakes:

• Thinking val prevents modifying the list content
• Expecting compilation or runtime errors
• Confusing immutable reference with immutable object

4. Identify the error in the following Kotlin code:

val number = 5
number = 10
println(number)

medium

A. No error, prints 10

B. Runtime error: reassignment not allowed

C. Compilation error: val cannot be reassigned

D. Syntax error: missing semicolon

Solution

1. Step 1: Check reassignment of val variable

   val variables cannot be reassigned after initialization.

2. Step 2: Identify error type

   Trying to assign 10 to number causes a compilation error.

3. Final Answer:

   Compilation error: val cannot be reassigned -> Option C

4. Quick Check:

   val reassignment = compilation error [OK]

Hint: val variables cannot change after first assignment [OK]

Common Mistakes:

• Assuming val variables can be reassigned
• Confusing runtime errors with compile errors
• Ignoring Kotlin's no-semicolon rule

5. Given the following Kotlin code, what will be the output?

val map = mutableMapOf("a" to 1, "b" to 2)
map["a"] = 3
println(map)

Explain why this works even though map is declared with val.

hard

A. {a=3, b=2}

B. {a=1, b=2}

C. Compilation error: cannot modify val map

D. Runtime error

Solution

1. Step 1: Understand val with mutable collections

   val means the reference to the map cannot change, but the map's content can be modified.

2. Step 2: Analyze the map update

   Updating key "a" to 3 changes the map content, so printing shows {a=3, b=2}.

3. Final Answer:

   {a=3, b=2} -> Option A

4. Quick Check:

   val reference + mutable map = updated map content [OK]

Hint: val fixes reference, mutable map content can change [OK]

Common Mistakes:

• Thinking val prevents modifying map content
• Expecting compilation or runtime errors
• Confusing immutable reference with immutable object