With function behavior and use cases in Kotlin - Mini Project: Build & Apply

Using the Kotlin 'with' Function for Cleaner Code

📖 Scenario: You are creating a simple program to manage a book's details. You want to update and display the book's information in a clean and readable way.

🎯 Goal: Build a Kotlin program that uses the with function to update and print the details of a book object.

📋 What You'll Learn

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

Create an instance of Book with initial values

Use the with function to update the book's year and print all details inside the with block

Print the book details outside the with block to confirm changes

💡 Why This Matters

🌍 Real World

Using the <code>with</code> function helps keep your code clean and readable when working with objects that need multiple operations.

💼 Career

Understanding <code>with</code> is useful for Kotlin developers to write concise and maintainable code, especially in Android app development and backend services.

1

Create the Book data class and an instance

Create a Kotlin data class called Book with properties title (String), author (String), and year (Int). Then create a variable myBook of type Book with title as "Kotlin Basics", author as "Jane Doe", and year as 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 an Int variable called newYear and set it to 2023.

Kotlin

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

val myBook = Book("Kotlin Basics", "Jane Doe", 2020)

// Create newYear variable here
// Your code here

Hint

Use val newYear = 2023 to create the variable.

3

Use the with function to update and print book details

Use the Kotlin with function with myBook as the receiver. Inside the with block, update the year property to newYear and print the book details in the format: "Title: [title], Author: [author], Year: [year]".

Kotlin

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

val myBook = Book("Kotlin Basics", "Jane Doe", 2020)

val newYear = 2023

// Use with function to update year and print details
// Your code here

Hint

Inside with(myBook) { }, you can access properties directly without myBook.

4

Print the updated book details outside the with block

Print the book details again outside the with block using println in the same format: "Title: [title], Author: [author], Year: [year]" to confirm the update.

Kotlin

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

val myBook = Book("Kotlin Basics", "Jane Doe", 2020)

val newYear = 2023

with(myBook) {
    year = newYear
    println("Title: $title, Author: $author, Year: $year")
}

// Print book details outside with block
// Your code here

Hint

Use println("Title: ${myBook.title}, Author: ${myBook.author}, Year: ${myBook.year}") to print outside the with block.

Practice

1. What is the main purpose of the with function in Kotlin?

easy

A. To define a new class inside a function

B. To create a new thread for concurrent execution

C. To execute multiple operations on the same object without repeating its name

D. To handle exceptions in a concise way

Solution

1. Step 1: Understand the role of with

   The with function groups multiple calls on the same object, so you don't repeat the object name.

2. Step 2: Compare with other options

   Options A, B, and D describe unrelated Kotlin features like class definition, threading, and exception handling.

3. Final Answer:

   To execute multiple operations on the same object without repeating its name -> Option C

4. Quick Check:

   with groups calls on one object = C [OK]

Hint: Remember: with avoids repeating object name [OK]

Common Mistakes:

• Confusing with with threading or concurrency
• Thinking with creates new classes
• Assuming with handles exceptions

2. Which of the following is the correct syntax to use with in Kotlin?

easy

A. with(object) { /* code block */ }

B. with { object -> /* code block */ }

C. with object { /* code block */ }

D. with(object) -> { /* code block */ }

Solution

1. Step 1: Recall the syntax of with

   The correct syntax is with(object) { /* code block */ }, where the object is passed as an argument and the block follows in braces.

2. Step 2: Identify incorrect syntax

   Options B, C, and D misuse arrow notation or omit parentheses, which are syntax errors.

3. Final Answer:

   with(object) { /* code block */ } -> Option A

4. Quick Check:

   Correct with syntax uses parentheses and braces = A [OK]

Hint: Use parentheses for object, braces for block in with [OK]

Common Mistakes:

• Omitting parentheses around the object
• Using arrow notation incorrectly
• Placing the block outside braces

3. What is the output of the following Kotlin code?

data class Person(var name: String, var age: Int)

fun main() {
    val person = Person("Alice", 25)
    val result = with(person) {
        age += 1
        "${name} is now $age years old"
    }
    println(result)
}

medium

A. Alice is now 25 years old

B. Runtime error due to modifying age

C. Compilation error due to string interpolation

D. Alice is now 26 years old

Solution

1. Step 1: Analyze the with block behavior

   The with function uses person as the context. Inside, age is incremented by 1, changing it from 25 to 26.

2. Step 2: Evaluate the returned string

   The last expression in the block is the string interpolation "${name} is now $age years old", which becomes "Alice is now 26 years old".

3. Final Answer:

   Alice is now 26 years old -> Option D

4. Quick Check:

   Age incremented inside with = D [OK]

Hint: Remember last expression in with is the return value [OK]

Common Mistakes:

• Forgetting that age is incremented inside with
• Thinking string interpolation causes error
• Assuming modifying properties causes runtime error

4. Identify the error in this Kotlin code using with:

fun main() {
    val list = listOf(1, 2, 3)
    with(list) {
        add(4)
        println(this)
    }
}

medium

A. Cannot call add on an immutable list

B. Missing parentheses in with call

C. Incorrect use of this inside with

D. Print statement syntax error

Solution

1. Step 1: Check the list type and mutability

   The list is created with listOf, which returns an immutable list that does not support add.

2. Step 2: Understand the error cause

   Calling add(4) on an immutable list causes a compile-time error because the method is not available.

3. Final Answer:

   Cannot call add on an immutable list -> Option A

4. Quick Check:

   Immutable list blocks add method = A [OK]

Hint: Use mutableListOf to add elements, not listOf [OK]

Common Mistakes:

• Assuming with changes mutability
• Thinking this is incorrect inside with
• Ignoring immutability of listOf

5. Given a mutable map of student names to scores, how can you use with to increase all scores by 5 points and return the updated map? Choose the correct Kotlin code snippet.

hard

A. with(students) { keys.forEach { it += 5 }; this }

B. with(students) { for (key in keys) this[key] = this[key]!! + 5; this }

C. with(students) { mapValues { it.value + 5 } }

D. with(students) { forEach { it.value += 5 }; this }

Solution

1. Step 1: Understand map mutability and with usage

   The map is mutable, so we can update values by keys inside with. with(students) { for (key in keys) this[key] = this[key]!! + 5; this } correctly loops keys and updates values.

2. Step 2: Analyze other options for correctness

   with(students) { keys.forEach { it += 5 }; this } tries to add 5 to keys (strings), which is invalid. with(students) { mapValues { it.value + 5 } } returns a new map but does not update original. with(students) { forEach { it.value += 5 }; this } tries to modify it.value in forEach, but map entries are immutable in this context.

3. Final Answer:

   with(students) { for (key in keys) this[key] = this[key]!! + 5; this } -> Option B

4. Quick Check:

   Update map values by keys inside with = B [OK]

Hint: Use keys to update mutable map values inside with [OK]

Common Mistakes:

• Trying to modify keys instead of values
• Assuming mapValues mutates original map
• Modifying map entries directly in forEach causes error