Mutable vs immutable interfaces in Kotlin - Hands-On Comparison

Mutable vs Immutable Interfaces in Kotlin

📖 Scenario: Imagine you are building a simple contact list app. You want to keep a list of contacts where some parts of your program can only read the contacts, while other parts can add or remove contacts.

🎯 Goal: You will create a Kotlin program that shows the difference between mutable and immutable interfaces by using List and MutableList. You will start by creating a list of contacts, then add a helper variable, use the mutable interface to add a contact, and finally print the updated list.

📋 What You'll Learn

Create an immutable list of contacts using List

Create a mutable list variable from the immutable list

Add a new contact to the mutable list

Print the final list of contacts

💡 Why This Matters

🌍 Real World

Many apps need to protect data from accidental changes by using immutable interfaces, while allowing controlled changes with mutable interfaces.

💼 Career

Understanding mutable vs immutable collections is important for writing safe and clear Kotlin code in Android development and backend services.

1

Create an immutable list of contacts

Create a variable called contacts that holds an immutable list of strings with these exact values: "Alice", "Bob", "Charlie" using listOf().

Kotlin

# Create an immutable list called contacts with "Alice", "Bob", and "Charlie"
# Your code here

Need a hint?

Use val contacts: List = listOf("Alice", "Bob", "Charlie") to create an immutable list.

2

Create a mutable list from the immutable list

Create a variable called mutableContacts that holds a mutable list of strings by copying the contacts list using toMutableList().

Kotlin

val contacts: List = listOf("Alice", "Bob", "Charlie")
# Create a mutable list called mutableContacts from contacts
# Your code here

Need a hint?

Use toMutableList() on contacts to create a mutable copy.

3

Add a new contact to the mutable list

Add the string "Diana" to the mutableContacts list using the add() method.

Kotlin

val contacts: List = listOf("Alice", "Bob", "Charlie")
val mutableContacts: MutableList = contacts.toMutableList()
# Add "Diana" to mutableContacts
# Your code here

Need a hint?

Use mutableContacts.add("Diana") to add the new contact.

4

Print the updated list of contacts

Print the mutableContacts list using println() to show all contacts including the new one.

Kotlin

val contacts: List = listOf("Alice", "Bob", "Charlie")
val mutableContacts: MutableList = contacts.toMutableList()
mutableContacts.add("Diana")
# Print mutableContacts
# Your code here

Need a hint?

Use println(mutableContacts) to display the list.