Chaining scope functions in Kotlin - Mini Project: Build & Apply

Chaining scope functions

📖 Scenario: You are organizing a small library system. You want to create a book record, update its details, and then display the final information.

🎯 Goal: Build a Kotlin program that creates a Book data class instance, uses chaining of scope functions to update its properties, and then prints the updated book details.

📋 What You'll Learn

Create a Book data class with title, author, and year properties

Create an instance of Book with initial values

Use chaining of scope functions apply and also to update the book's year and print a message

Print the final updated book details

💡 Why This Matters

🌍 Real World

Chaining scope functions helps write clean and readable code when updating objects and performing related actions, common in app development.

💼 Career

Understanding Kotlin scope functions and chaining is valuable for Android developers and Kotlin programmers to write concise and maintainable code.

1

Create the Book data class and initial book

Create a Kotlin data class called Book with properties title (String), author (String), and year (Int). Then create a variable myBook and assign it a Book instance with title "The Kotlin Guide", author "Anna", and year 2020.

Kotlin

data class Book(val title: String, val author: String, var year: Int)

// Create myBook instance here
// Your code here

Hint

Remember to use var for year because it will change later.

2

Add a variable for the new publication year

Create a variable called newYear and set it to 2023.

Kotlin

data class Book(val title: String, val author: String, var year: Int)

val myBook = Book("The Kotlin Guide", "Anna", 2020)

// Create newYear variable here
// Your code here

Hint

Just create a simple val with the number 2023.

3

Chain apply and also to update and print

Use chaining of scope functions on myBook: first use apply to update the year property to newYear, then use also to print the message "Updated book year to 2023".

Kotlin

data class Book(val title: String, val author: String, var year: Int)

val myBook = Book("The Kotlin Guide", "Anna", 2020)

val newYear = 2023

// Chain apply and also on myBook here
// Your code here

Hint

Use apply to change year, then also to print the message.

4

Print the final updated book details

Write a println statement to print myBook so it shows the updated book details.

Kotlin

data class Book(val title: String, val author: String, var year: Int)

val myBook = Book("The Kotlin Guide", "Anna", 2020)

val newYear = 2023

myBook.apply {
    year = newYear
}.also {
    println("Updated book year to $newYear")
}

// Print myBook here
// Your code here

Hint

Just print myBook to see the updated details.

Practice

1. What does chaining scope functions in Kotlin help you achieve?

easy

A. Perform multiple operations on one object without repeating its name

B. Create multiple objects at once

C. Write loops faster

D. Automatically convert types

Solution

1. Step 1: Understand the purpose of scope functions

   Scope functions let you run code blocks on an object, often returning the object or a result.

2. Step 2: Recognize chaining benefit

   Chaining lets you call many scope functions one after another on the same object without repeating its name.

3. Final Answer:

   Perform multiple operations on one object without repeating its name -> Option A

4. Quick Check:

   Chaining scope functions = clean, repeated operations [OK]

Hint: Chaining avoids repeating object names in multiple calls [OK]

Common Mistakes:

• Thinking chaining creates new objects
• Confusing chaining with loops
• Believing chaining changes data types automatically

2. Which of the following is the correct syntax to chain apply and also on an object obj?

easy

A. obj.apply { }.also { }

B. obj.apply() also()

C. apply(obj) { } also { }

D. obj.apply { } also { }

Solution

1. Step 1: Recall correct chaining syntax

   In Kotlin, scope functions are called with dot notation and lambda blocks inside curly braces.

2. Step 2: Check each option

   obj.apply { }.also { } uses correct dot chaining with braces. obj.apply() also() misses dots and braces. apply(obj) { } also { } uses wrong function call style. obj.apply { } also { } misses dot before also.

3. Final Answer:

   obj.apply { }.also { } -> Option A

4. Quick Check:

   Dot + braces = correct chaining syntax [OK]

Hint: Use dot and braces for each scope function call [OK]

Common Mistakes:

• Omitting dots between functions
• Using parentheses instead of braces for lambdas
• Calling scope functions like regular functions without object

3. What is the output of this Kotlin code?

val result = "hello".also { println(it.uppercase()) }.length
println(result)

medium

A. hello\n5

B. 5\nHELLO

C. HELLO\nhello

D. HELLO\n5

Solution

1. Step 1: Understand also behavior

   also runs the lambda with the object as it, prints uppercase "HELLO", then returns the original string "hello".

2. Step 2: Calculate result

   After also, length is called on "hello", which is 5. Then println(result) prints 5.

3. Final Answer:

   HELLO 5 -> Option D

4. Quick Check:

   also prints uppercase, length returns 5 [OK]

Hint: also returns original object, so length is on original string [OK]

Common Mistakes:

• Thinking also returns uppercase string
• Confusing print order
• Expecting length to be printed before uppercase

4. Find the error in this Kotlin code chaining scope functions:

val text = "kotlin"
text.let { it.uppercase() }.apply { println(this) }

medium

A. apply cannot be called after let because let returns a different type

B. Missing lambda braces in apply

C. let does not return the original object, so apply prints wrong value

D. No error, code works fine

Solution

1. Step 1: Analyze let return value

   let returns the lambda result, here it.uppercase(), a new String "KOTLIN".

2. Step 2: Check apply receiver

   apply is called on the uppercase string, so this inside apply is "KOTLIN" and println(this) prints "KOTLIN".

3. Step 3: Identify the problem

   The original text is "kotlin", but apply prints the uppercase string, not the original. This may be unexpected if you want to print original.

4. Final Answer:

   let does not return the original object, so apply prints wrong value -> Option C

5. Quick Check:

   let returns lambda result, not original object [OK]

Hint: Remember let returns lambda result, not original object [OK]

Common Mistakes:

• Assuming let returns original object
• Thinking apply needs braces
• Believing no error exists

5. Given a mutable list val nums = mutableListOf(1, 2, 3), which chained scope function call correctly adds 4, prints the list, and returns the updated list?

hard

A. nums.let { it.add(4) }.also { println(it) }

B. nums.apply { add(4) }.also { println(it) }

C. nums.also { add(4) }.apply { println(this) }

D. nums.run { add(4); println(this) }

Solution

1. Step 1: Understand return values of scope functions

   apply returns the original object after running the block, also returns the original object too, let returns lambda result, run returns lambda result.

2. Step 2: Check each option

   nums.apply { add(4) }.also { println(it) } adds 4 inside apply, then prints list in also, returning the updated list. nums.let { it.add(4) }.also { println(it) }'s let returns Unit from add, so also prints Unit. nums.also { add(4) }.apply { println(this) } has syntax error ('add' unresolved in also block, needs it.add(4)). nums.run { add(4); println(this) } returns Unit from run.

3. Final Answer:

   nums.apply { add(4) }.also { println(it) } -> Option B

4. Quick Check:

   apply returns object, also prints it [OK]

Hint: Use apply to modify and also to print, both return original object [OK]

Common Mistakes:

• Using let which returns Unit from add
• Expecting run to return the list
• Confusing order of operations in chaining