Building blocks of type-safe builders in Kotlin - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a builder function that returns a StringBuilder.

Kotlin

fun buildString(): StringBuilder = [1]()

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AList

BStringBuilder

CString

DMutableList

Attempts:

3 left

💡 Hint

Common Mistakes

Using String instead of StringBuilder

Using List which is unrelated to string building

The function should return a new instance of StringBuilder to build strings safely.

2fill in blank

medium

Complete the code to add a function literal with receiver to the builder.

Kotlin

fun buildString(block: StringBuilder.() -> Unit): String {
    val sb = StringBuilder()
    sb.[1](block)
    return sb.toString()
}

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Aapply(block)

Brun(block)

Clet(block)

Dwith(block)

Attempts:

3 left

💡 Hint

Common Mistakes

Using run which returns the block result, not the receiver

Using let which passes the receiver as an argument, not as receiver

The apply function executes the block with sb as the receiver and returns sb.

3fill in blank

hard

Fix the error in the builder function to correctly build and return the string.

Kotlin

fun buildString(block: StringBuilder.() -> Unit): String {
    val sb = StringBuilder()
    sb.apply [1]
    return sb.toString()
}

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A{ block() }

B(block)

Cblock

Dblock()

Attempts:

3 left

💡 Hint

Common Mistakes

Passing block without braces causes a syntax error

Calling block() directly without lambda syntax

The apply function expects a lambda with receiver, so we pass block directly to invoke the block on sb.

4fill in blank

hard

Fill both blanks to create a type-safe builder that adds items to a list.

Kotlin

class ListBuilder<T> {
    private val items = mutableListOf<T>()
    fun add(item: T) { items.[1](item) }
    fun build(): List<T> = items.[2]()
}

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Aadd

BtoList

Cremove

Dclear

Attempts:

3 left

💡 Hint

Common Mistakes

Using remove instead of add

Returning the mutable list directly without toList

Use add to add items and toList to return an immutable list.

5fill in blank

hard

Fill all three blanks to define a type-safe builder function that uses the ListBuilder.

Kotlin

fun <T> buildList(block: ListBuilder<T>.() -> Unit): List<T> {
    val builder = ListBuilder<T>()
    builder.[1](block)
    return builder.[2]()
}

fun main() {
    val list = buildList<String> {
        add("Hello")
        add("World")
    }
    println(list.[3])
}

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Aapply

Bbuild

CjoinToString()

Drun

Attempts:

3 left

💡 Hint

Common Mistakes

Calling block without apply

Returning builder instead of builder.build()

Printing list directly without joinToString

Use apply to run the block on builder, build to get the list, and joinToString() to print the list nicely.