Null safety in collections in Kotlin - Mini Project: Build & Apply

Null Safety in Collections

📖 Scenario: You are building a simple contact list app. Some contacts may have missing phone numbers, so you need to handle null values safely in your list of contacts.

🎯 Goal: Create a list of contacts where some phone numbers can be null. Then, filter out the contacts without phone numbers using Kotlin's null safety features.

📋 What You'll Learn

Create a list of contacts with names and nullable phone numbers

Create a variable to hold the filtered list of contacts with non-null phone numbers

Use Kotlin's safe call operator and filtering to keep only contacts with phone numbers

Print the filtered list of contacts

💡 Why This Matters

🌍 Real World

Handling contact lists or any data where some information might be missing is common in apps like phonebooks, messaging, or customer management.

💼 Career

Understanding null safety in collections helps prevent crashes and bugs in real-world Kotlin applications, a key skill for Android developers and backend Kotlin programmers.

1

Create a list of contacts with nullable phone numbers

Create a list called contacts with these exact entries: "Alice" to "123-4567", "Bob" to null, "Charlie" to "987-6543", and "Diana" to null. Use a list of pairs where the second value can be null.

Kotlin

# Create the list called contacts with the specified entries
# Your code here

Need a hint?

Use listOf with pairs like "Name" to phoneNumber. Phone numbers can be null.

2

Create a variable to hold filtered contacts

Create a variable called contactsWithPhone and set it to an empty list of pairs with String and nullable String types.

Kotlin

val contacts = listOf(
    "Alice" to "123-4567",
    "Bob" to null,
    "Charlie" to "987-6543",
    "Diana" to null
)

// Create an empty list called contactsWithPhone
# Your code here

Need a hint?

Use var contactsWithPhone: List> = listOf() to create an empty list with the right type.

3

Filter contacts to keep only those with phone numbers

Set contactsWithPhone to a filtered list from contacts that keeps only pairs where the phone number (second value) is not null. Use filter and check it.second != null.

Kotlin

val contacts = listOf(
    "Alice" to "123-4567",
    "Bob" to null,
    "Charlie" to "987-6543",
    "Diana" to null
)

var contactsWithPhone: List> = listOf()

// Filter contacts to keep only those with non-null phone numbers
# Your code here

Need a hint?

Use contacts.filter { it.second != null } to keep only contacts with phone numbers.

4

Print the filtered contacts

Print contactsWithPhone using println to show the contacts that have phone numbers.

Kotlin

val contacts = listOf(
    "Alice" to "123-4567",
    "Bob" to null,
    "Charlie" to "987-6543",
    "Diana" to null
)

var contactsWithPhone: List> = listOf()

contactsWithPhone = contacts.filter { it.second != null }

// Print the contactsWithPhone list
# Your code here

Need a hint?

Use println(contactsWithPhone) to display the filtered list.