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]()

Drag options to blanks, or click blank then click option'

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()
}

Drag options to blanks, or click blank then click option'

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()
}

Drag options to blanks, or click blank then click option'

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]()
}

Drag options to blanks, or click blank then click option'

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])
}

Drag options to blanks, or click blank then click option'

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.

Practice

1. What is the main purpose of type-safe builders in Kotlin?

easy

A. To create complex objects step-by-step with safety and clarity

B. To write faster loops for collections

C. To handle exceptions automatically

D. To convert data types implicitly

Solution

1. Step 1: Understand the concept of type-safe builders

   Type-safe builders allow building complex objects in a structured and safe way using Kotlin's DSL features.

2. Step 2: Identify the main purpose

   The main goal is to create objects step-by-step with clear rules, avoiding errors and improving readability.

3. Final Answer:

   To create complex objects step-by-step with safety and clarity -> Option A

4. Quick Check:

   Type-safe builders = safe, clear object creation [OK]

Hint: Type-safe builders help build objects safely step-by-step [OK]

Common Mistakes:

• Confusing builders with loops or exception handling
• Thinking they convert data types automatically

2. Which Kotlin syntax correctly defines a type-safe builder function named html that takes a lambda with receiver?

easy

A. fun html(block: (Html) -> Unit) { /*...*/ }

B. fun html(block: () -> Html) { /*...*/ }

C. fun html(block: Html.() -> Unit) { /*...*/ }

D. fun html(block: () -> Unit) { /*...*/ }

Solution

1. Step 1: Identify lambda with receiver syntax

   Type-safe builders use lambdas with receiver, which have the form ReceiverType.() -> ReturnType.

2. Step 2: Match the correct function signature

   The function should accept a lambda with receiver Html and return Unit, so Html.() -> Unit is correct.

3. Final Answer:

   fun html(block: Html.() -> Unit) { /*...*/ } -> Option C

4. Quick Check:

   Lambda with receiver = Html.() -> Unit [OK]

Hint: Look for lambda with receiver syntax ReceiverType.() -> Unit [OK]

Common Mistakes:

• Using regular lambda (Html) -> Unit instead of lambda with receiver
• Omitting the receiver type in the lambda
• Returning Html instead of Unit

3. Given the following Kotlin code, what is the output?

class Html {
    val children = mutableListOf<String>()
    fun body(block: Body.() -> Unit) {
        val body = Body()
        body.block()
        children.addAll(body.contents)
    }
    override fun toString() = children.joinToString(", ")
}

class Body {
    val contents = mutableListOf<String>()
    fun p(text: String) {
        contents.add("
$text
")
    }
}

fun html(block: Html.() -> Unit): Html {
    val html = Html()
    html.block()
    return html
}

fun main() {
    val result = html {
        body {
            p("Hello")
            p("World")
        }
    }
    println(result)
}

medium

A. Hello, World

B. <p>Hello</p>, <p>World</p>

C. <body><p>Hello</p><p>World</p></body>

D. Compilation error

Solution

1. Step 1: Understand the builder structure

   The html function creates an Html object, calls body which creates a Body object, then calls p twice adding <p>Hello</p> and <p>World</p> to Body's contents.

2. Step 2: Check how children are collected and printed

   The Html object's children list gets Body's contents added, then toString() joins them with ", ". So output is "<p>Hello</p>, <p>World</p>".

3. Final Answer:

   <p>Hello</p>, <p>World</p> -> Option B

4. Quick Check:

   Children list joined = <p>Hello</p>, <p>World</p> [OK]

Hint: Look for toString join of children list for output [OK]

Common Mistakes:

• Assuming HTML tags are printed as is without escaping
• Expecting full HTML tags like <body> in output
• Thinking code causes compilation error

4. What is the error in this Kotlin builder code snippet?

class Html {
    fun body(block: Body.() -> Unit) {
        val body = Body()
        block()
    }
}

class Body {
    fun p(text: String) { /*...*/ }
}

fun main() {
    val html = Html()
    html.body {
        p("Hello")
    }
}

medium

A. The lambda is called without receiver, so 'p' is unresolved

B. Body class is missing a constructor

C. Html class should inherit from Body

D. The 'body' function should return a value

Solution

1. Step 1: Check how the lambda is called inside body()

   The lambda block is called as block() without receiver, so inside the lambda 'p' is not recognized.

2. Step 2: Identify correct lambda invocation

   It should be called as body.block() to set the receiver to the Body instance, allowing 'p' to be resolved.

3. Final Answer:

   The lambda is called without receiver, so 'p' is unresolved -> Option A

4. Quick Check:

   Lambda call without receiver = unresolved 'p' [OK]

Hint: Call lambda with receiver: use receiver.lambda() syntax [OK]

Common Mistakes:

• Calling lambda without receiver inside builder
• Assuming inheritance fixes builder scope
• Expecting return value from builder function

5. How can you extend a type-safe builder to support nested elements, for example adding a div inside body, while keeping type safety?

hard

A. Use a global div function outside the builder classes

B. Add a div property of type String to Body

C. Make Body inherit from Div

D. Define a div function inside Body that takes a lambda with receiver of a new Div class

Solution

1. Step 1: Understand nested builder pattern

   To support nested elements, define functions inside builder classes that accept lambdas with receivers of nested element classes.

2. Step 2: Apply to 'div' inside 'body'

   Inside Body, define a div function taking Div.() -> Unit lambda, creating a Div instance and applying the lambda to it.

3. Final Answer:

   Define a div function inside Body that takes a lambda with receiver of a new Div class -> Option D

4. Quick Check:

   Nested builders = functions with lambdas with receiver [OK]

Hint: Nested builders use functions with lambdas with receiver inside classes [OK]

Common Mistakes:

• Using simple properties instead of nested builder functions
• Trying inheritance instead of composition
• Defining builder functions outside the class scope