StringBuilder for performance in Kotlin - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this Kotlin code using StringBuilder?

Consider the following Kotlin code snippet. What will it print?

Kotlin

fun main() {
    val sb = StringBuilder()
    for (i in 1..3) {
        sb.append(i)
        sb.append(",")
    }
    println(sb.toString())
}

A 1,2,3,

B 1, 2, 3,

C 1,2,3

D 123,

💡 Hint

Look carefully at what is appended inside the loop.

The loop appends the number and then a comma each time, so the output ends with a comma after the last number.

❓ Predict Output

intermediate

What is the final string length after these StringBuilder operations?

Given this Kotlin code, what is the length of the string in sb after execution?

Kotlin

fun main() {
    val sb = StringBuilder("Hello")
    sb.append(" World")
    sb.delete(5, 6)
    println(sb.length)
}

A 6

B 11

C 5

D 10

💡 Hint

Deleting one character reduces length by one.

Initial string "Hello" has length 5, appending " World" adds 6 characters (including space), total 11. Deleting one character reduces length to 10.

🔧 Debug

advanced

Why does this Kotlin code throw an exception?

This Kotlin code throws an exception. What is the cause?

Kotlin

fun main() {
    val sb = StringBuilder("abc")
    sb.delete(1, 5)
    println(sb.toString())
}

A No exception, output is "a"

B StringIndexOutOfBoundsException because end index is greater than string length

C NullPointerException because sb is null

D IllegalArgumentException because start index is greater than end index

💡 Hint

Check the valid range for delete method indices.

The delete method throws StringIndexOutOfBoundsException if the end index is greater than the current length of the StringBuilder.

🧠 Conceptual

advanced

Why is StringBuilder preferred over String concatenation in loops?

Which option best explains why using StringBuilder is better than using String concatenation inside loops in Kotlin?

A StringBuilder modifies the string in place, avoiding creating many temporary String objects

B String concatenation is faster because it uses immutable strings

C String concatenation uses less memory than StringBuilder

D StringBuilder automatically sorts the characters for better performance

💡 Hint

Think about how strings are stored and changed in memory.

Strings are immutable in Kotlin, so concatenation creates new objects each time. StringBuilder changes the string content without creating new objects, improving performance.

❓ Predict Output

expert

What is the output of this Kotlin code using StringBuilder and insert?

Analyze this Kotlin code and select the exact output it produces.

Kotlin

fun main() {
    val sb = StringBuilder("Kotlin")
    sb.insert(3, " is")
    sb.insert(0, "I think")
    println(sb.toString())
}

A I thinkK isotlin

B I thinkKotlin is

C I thinkKot islin

D si niltoKkniht I

💡 Hint

Remember insert shifts characters to the right.

First insert adds " is" at index 3: "Kot islin". Then insert "I think" at index 0 shifts everything right, resulting in "I thinkKot islin".