String type and immutability in Kotlin - Mini Project: Build & Apply

Understanding String Type and Immutability in Kotlin

📖 Scenario: Imagine you are creating a simple program that stores a user's name and tries to change it. This will help you understand how strings work in Kotlin and why they cannot be changed once created.

🎯 Goal: You will create a string variable, try to change it, and then print the results to see how string immutability works in Kotlin.

📋 What You'll Learn

Create a string variable with a specific value

Create a new string by modifying the original string

Understand that the original string does not change

Print both the original and the new string

💡 Why This Matters

🌍 Real World

Understanding string immutability helps when working with user input, text processing, and data that should not change accidentally.

💼 Career

Many programming jobs require handling text data safely and efficiently, and knowing string immutability prevents bugs and improves code quality.

1

Create a string variable

Create a string variable called originalName and set it to the value "Alice".

Kotlin

// Create a string variable called originalName and set it to "Alice"
// Your code here

Hint

Use val to create a string variable and assign the value with double quotes.

2

Create a new string by adding text

Create a new string variable called newName by adding the text " Smith" to originalName.

Kotlin

val originalName = "Alice"
// Create a new string variable called newName by adding " Smith" to originalName
// Your code here

Hint

Use the + operator to join strings in Kotlin.

3

Show that original string is unchanged

Use println to print originalName and newName on separate lines.

Kotlin

val originalName = "Alice"
val newName = originalName + " Smith"
// Print originalName and newName on separate lines
// Your code here

Hint

Use println twice to print each string on its own line.

4

Observe string immutability

Run the program and observe that originalName remains "Alice" while newName is "Alice Smith". This shows strings cannot be changed once created.

Kotlin

val originalName = "Alice"
val newName = originalName + " Smith"
println(originalName)
println(newName)
// Run the program to see the output

Hint

The output shows the original string did not change after adding text.

Practice

1. What does it mean that a String in Kotlin is immutable?

easy

A. Strings can only hold numbers, not letters.

B. You can change characters inside the String directly.

C. Once created, the text inside a String cannot be changed.

D. Strings automatically update when variables change.

Solution

1. Step 1: Understand immutability concept

   Immutability means the object cannot be changed after creation.

2. Step 2: Apply to Kotlin String

   Kotlin Strings cannot be modified once created; any change creates a new String.

3. Final Answer:

   Once created, the text inside a String cannot be changed. -> Option C

4. Quick Check:

   String immutability = cannot change text [OK]

Hint: Remember: Strings are like printed paper, not editable [OK]

Common Mistakes:

• Thinking you can change characters inside the same String
• Confusing immutability with variable reassignment
• Believing Strings hold only numbers

2. Which of the following is the correct way to concatenate two Strings str1 and str2 in Kotlin?

easy

A. str1 & str2

B. str1 + str2

C. str1.join(str2)

D. str1.append(str2)

Solution

1. Step 1: Recall Kotlin String concatenation syntax

   Kotlin uses the + operator to join two Strings.

2. Step 2: Check each option

   Only str1 + str2 is valid Kotlin syntax for concatenation.

3. Final Answer:

   str1 + str2 -> Option B

4. Quick Check:

   Use + to join Strings [OK]

Hint: Use + to combine Strings, like adding words together [OK]

Common Mistakes:

• Using methods like append or join which don't exist on Kotlin String
• Using & which is not a Kotlin operator for Strings
• Trying to modify Strings directly

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

val text = "Hello"
val newText = text + " World"
println(text)

medium

A. Hello

B. Hello World

C. World

D. HelloWorld

Solution

1. Step 1: Understand String immutability and concatenation

   The original String text remains unchanged after concatenation.

2. Step 2: Analyze the print statement

   Only text is printed, which is still "Hello".

3. Final Answer:

   Hello -> Option A

4. Quick Check:

   Original String unchanged = Hello [OK]

Hint: Printing original String shows it stays the same [OK]

Common Mistakes:

• Assuming original String changes after + operation
• Confusing newText with text in print
• Ignoring immutability of Strings

4. Identify the error in this Kotlin code snippet:

var greeting = "Hi"
greeting[0] = 'B'
println(greeting)

medium

A. println cannot print Strings.

B. Variable greeting should be val, not var.

C. Single quotes cannot be used for characters.

D. You cannot change characters in a String by index.

Solution

1. Step 1: Check String mutability rules

   Kotlin Strings are immutable; you cannot assign to an index.

2. Step 2: Analyze the code line causing error

   Line greeting[0] = 'B' tries to change a character, which is invalid.

3. Final Answer:

   You cannot change characters in a String by index. -> Option D

4. Quick Check:

   Strings immutable = no index assignment [OK]

Hint: Strings are like frozen text, can't change letters inside [OK]

Common Mistakes:

• Thinking var allows character change inside String
• Confusing char literals with strings
• Believing println can't print Strings

5. Given a list of words, how can you create a new list with each word appended by "!" without changing the original words?

val words = listOf("cat", "dog", "bird")
val excitedWords = ???

Which Kotlin code correctly does this considering String immutability?

hard

A. val excitedWords = words.map { it + "!" }

B. val excitedWords = words.apply { this[0] += "!" }

C. val excitedWords = words.forEach { it + "!" }

D. val excitedWords = words.replace("", "!")

Solution

1. Step 1: Understand immutability and list transformations

   Strings cannot be changed, so create new Strings with + "!".

2. Step 2: Use map to transform each word into a new String

   map returns a new list with transformed elements without modifying original.

3. Final Answer:

   val excitedWords = words.map { it + "!" } -> Option A

4. Quick Check:

   Use map + + operator for new list [OK]

Hint: Use map to create new list with modified Strings [OK]

Common Mistakes:

• Trying to modify list elements directly with apply or forEach
• Using replace on list instead of Strings
• Assuming original list changes