Nullable types with ? suffix in Kotlin - Mini Project: Build & Apply

Nullable types with ? suffix

📖 Scenario: Imagine you are creating a simple contact list app. Some contacts have phone numbers, but some do not. You want to store phone numbers in a way that allows missing numbers without causing errors.

🎯 Goal: Learn how to use nullable types with the ? suffix in Kotlin to safely handle values that might be missing.

📋 What You'll Learn

Create a variable with a nullable type using the ? suffix

Assign a value and null to the nullable variable

Use safe call operator ?. to access the nullable variable

Print the results to see how Kotlin handles nullable types

💡 Why This Matters

🌍 Real World

Nullable types help apps handle missing or optional data safely, like missing phone numbers in contacts.

💼 Career

Understanding nullable types is essential for Kotlin developers to avoid crashes and write safe, reliable code.

1

Create a nullable variable

Create a variable called phoneNumber of type String? and assign it the value "123-456-7890".

Kotlin

# Create a variable called phoneNumber of type String? and assign "123-456-7890"
# Your code here

Hint

Use String? to make the variable nullable.

2

Assign null to the nullable variable

Change the value of phoneNumber to null.

Kotlin

var phoneNumber: String? = "123-456-7890"
# Assign null to phoneNumber
# Your code here

Hint

Just assign null to the variable.

3

Use safe call operator to access nullable variable

Create a variable called length and assign it the length of phoneNumber using the safe call operator ?..

Kotlin

var phoneNumber: String? = "123-456-7890"
phoneNumber = null
# Create variable length using phoneNumber?.length
# Your code here

Hint

Use ?. to safely get length without error if phoneNumber is null.

4

Print the length variable

Print the value of length using println.

Kotlin

var phoneNumber: String? = "123-456-7890"
phoneNumber = null
val length = phoneNumber?.length
# Print length
# Your code here

Hint

Just print length to see the result.

Practice

1. What does adding a ? after a type in Kotlin mean?

easy

A. The variable is immutable.

B. The variable can hold a null value.

C. The variable is a constant.

D. The variable is a function.

Solution

1. Step 1: Understand Kotlin type suffix

   In Kotlin, adding ? after a type means the variable can hold null.

2. Step 2: Compare with non-nullable types

   Without ?, variables cannot be null and will cause errors if assigned null.

3. Final Answer:

   The variable can hold a null value. -> Option B

4. Quick Check:

   Nullable type = variable can be null [OK]

Hint: Remember: ? means nullable type in Kotlin [OK]

Common Mistakes:

• Thinking ? makes variable immutable
• Confusing ? with constant declaration
• Assuming ? means function type

2. Which of the following is the correct syntax to declare a nullable String variable in Kotlin?

easy

A. var name: String?

B. var name: ?String

C. var name: String!

D. var name: nullable String

Solution

1. Step 1: Recall Kotlin nullable syntax

   Kotlin uses ? after the type name to mark it nullable, e.g., String?.

2. Step 2: Check each option

   var name: String? uses String? correctly. Options A, B, and D use invalid syntax.

3. Final Answer:

   var name: String? -> Option A

4. Quick Check:

   Nullable syntax = type followed by ? [OK]

Hint: Put ? right after the type name for nullable [OK]

Common Mistakes:

• Placing ? before the type
• Using ! instead of ?
• Writing 'nullable' keyword explicitly

3. What will be the output of this Kotlin code?

var text: String? = null
println(text?.length ?: "No length")

medium

A. 0

B. null

C. No length

D. Throws NullPointerException

Solution

1. Step 1: Understand safe call and Elvis operator

   text?.length safely accesses length if text is not null; else returns null. The Elvis operator ?: provides "No length" if left side is null.

2. Step 2: Evaluate given code

   Since text is null, text?.length is null, so the expression prints "No length".

3. Final Answer:

   No length -> Option C

4. Quick Check:

   Safe call + Elvis = fallback value [OK]

Hint: Safe call ?. returns null safely; ?: provides default [OK]

Common Mistakes:

• Expecting NullPointerException on null safe call
• Thinking null?.length returns 0
• Ignoring Elvis operator fallback

4. Identify the error in this Kotlin code snippet:

var number: Int? = 10
var result: Int = number + 5

medium

A. Variable 'result' should be nullable.

B. Variable 'number' cannot be nullable.

C. Missing semicolon after declaration.

D. Cannot add Int? and Int directly.

Solution

1. Step 1: Check type compatibility

   number is nullable Int?, but result expects non-null Int. Adding nullable and non-null directly is not allowed.

2. Step 2: Understand Kotlin's null safety

   You must safely unwrap number before adding, e.g., using number ?: 0 or safe call.

3. Final Answer:

   Cannot add Int? and Int directly. -> Option D

4. Quick Check:

   Nullable + non-null Int needs safe unwrap [OK]

Hint: Nullable Int can't be used directly in arithmetic [OK]

Common Mistakes:

• Assuming nullable Int can be used like Int
• Ignoring need for safe call or Elvis operator
• Thinking semicolon is required in Kotlin

5. You have a nullable list of nullable Strings: val items: List?. How do you safely print the length of the first item or print 0 if the list or first item is null?

hard

A. println(items?.get(0)?.length ?: 0)

B. println(items[0].length)

C. println(items!!.get(0)!!.length)

D. println(items?.get(0).length ?: 0)

Solution

1. Step 1: Analyze nullable types

   items can be null, and its elements can be null. So both list and first item need safe calls.

2. Step 2: Use safe calls and Elvis operator

   items?.get(0)?.length ?: 0 safely accesses first item length or returns 0 if null.

3. Final Answer:

   println(items?.get(0)?.length ?: 0) -> Option A

4. Quick Check:

   Safe calls chain + fallback = safe nullable access [OK]

Hint: Chain ?. for each nullable and use ?: for default [OK]

Common Mistakes:

• Using !! which throws exception if null
• Missing safe call on first item
• Assuming list or item can't be null