String type and immutability in Kotlin - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this Kotlin code?

Consider the following Kotlin code snippet:

val original = "hello"
val modified = original.replace('h', 'j')
println(original)
println(modified)

What will be printed?

Kotlin

val original = "hello"
val modified = original.replace('h', 'j')
println(original)
println(modified)

A hello\njello

B hello\nhello

C jello\njello

D jello\nhello

💡 Hint

Remember that Kotlin strings are immutable. The replace function returns a new string.

The replace function does not change the original string. It returns a new string with replacements. So original remains "hello" and modified is "jello".

🧠 Conceptual

intermediate

Why are Kotlin strings immutable?

Which of the following is the main reason Kotlin strings are immutable?

A To allow strings to be changed in place without creating new objects

B To make strings mutable and faster to change

C To prevent strings from being used in collections

D To allow strings to be safely shared between threads without synchronization

💡 Hint

Think about thread safety and performance.

Immutable strings can be shared safely between threads without locks, improving performance and safety.

🔧 Debug

advanced

What error does this Kotlin code produce?

Examine this Kotlin code:

var text = "hello"
text[0] = 'j'
println(text)

What error will this code cause?

Kotlin

var text = "hello"
text[0] = 'j'
println(text)

A Error: Val cannot be reassigned

B Error: Unresolved reference to 'text'

C Error: String cannot be assigned to by index

D No error, prints 'jello'

💡 Hint

Think about whether Kotlin strings allow changing characters by index.

Kotlin strings are immutable. You cannot assign a character to a specific index. This causes a compilation error.

🚀 Application

advanced

How to create a new string with a changed first character?

You want to change the first character of a Kotlin string str to 'J'. Which code snippet correctly does this?

A val newStr = "J" + str.substring(1)

B str[0] = 'J'; val newStr = str

C val newStr = str.replaceAt(0, 'J')

D val newStr = str.setCharAt(0, 'J')

💡 Hint

Remember strings are immutable, so you must create a new string.

Concatenating 'J' with the substring from index 1 creates a new string with the first character changed.

❓ Predict Output

expert

What is the output of this Kotlin code involving string references?

Consider this Kotlin code:

val a = "test"
val b = a
val c = b.replace('t', 'b')
println(a)
println(b)
println(c)

What will be printed?

Kotlin

val a = "test"
val b = a
val c = b.replace('t', 'b')
println(a)
println(b)
println(c)

A best\nbest\nbesb

B test\ntest\nbesb

C test\ntest\ntest

D test\nbest\nbesb

💡 Hint

Remember that b is just a reference to a, and strings are immutable.

Both a and b refer to the same string "test". The replace creates a new string c with replacements. So a and b print "test", c prints "besb".

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