Non-nullable types by default in Kotlin - Mini Project: Build & Apply

Non-nullable types by default

📖 Scenario: You are creating a simple Kotlin program to store and display a person's name and age. Kotlin treats types as non-nullable by default, which means variables cannot hold null unless explicitly allowed.

🎯 Goal: Build a Kotlin program that defines a non-nullable name variable and a non-nullable age variable, then prints them.

📋 What You'll Learn

Create a non-nullable name variable of type String with the value "Alice".

Create a non-nullable age variable of type Int with the value 30.

Print the name and age variables in the format: Name: Alice, Age: 30.

💡 Why This Matters

🌍 Real World

Non-nullable types help prevent errors by ensuring variables always have valid values, which is important in apps that handle user data.

💼 Career

Understanding Kotlin's type system is essential for Android developers and anyone working with Kotlin to write safe and reliable code.

1

Create non-nullable variables

Create a non-nullable variable called name of type String and set it to "Alice".

Kotlin

# Your code here

Hint

Use val to declare a read-only variable and specify the type String without a question mark.

2

Add a non-nullable age variable

Add a non-nullable variable called age of type Int and set it to 30 below the name variable.

Kotlin

val name: String = "Alice"
# Your code here

Hint

Remember to declare age as Int without a question mark to keep it non-nullable.

3

Create a message string

Create a variable called message of type String that uses an f-string (string template) to combine name and age in the format: Name: Alice, Age: 30.

Kotlin

val name: String = "Alice"
val age: Int = 30
# Your code here

Hint

Use Kotlin string templates with $variableName inside double quotes.

4

Print the message

Print the message variable using println.

Kotlin

val name: String = "Alice"
val age: Int = 30
val message: String = "Name: $name, Age: $age"
# Your code here

Hint

Use println(message) to display the message on the screen.

Practice

1. In Kotlin, what happens if you declare a variable without a ? after its type?

easy

A. The variable cannot hold null values.

B. The variable can hold null values.

C. The variable is automatically initialized to null.

D. The variable must be declared as lateinit.

Solution

1. Step 1: Understand Kotlin's default type behavior

   Kotlin types are non-nullable by default, meaning variables cannot hold null unless specified.

2. Step 2: Check the effect of missing ?

   Without ?, the variable type is non-nullable, so it cannot hold null.

3. Final Answer:

   The variable cannot hold null values. -> Option A

4. Quick Check:

   Non-nullable type = no null allowed [OK]

Hint: No ? means no null allowed [OK]

Common Mistakes:

• Thinking variables are nullable by default
• Assuming variables initialize to null automatically
• Confusing lateinit with nullability

2. Which of the following Kotlin variable declarations is correct to allow null values?

easy

A. val name: String? = null

B. val name: String?

C. val name: String = null

D. val name: String = "null"

Solution

1. Step 1: Identify nullable type syntax

   Adding ? after the type allows the variable to hold null.

2. Step 2: Check initialization

   Nullable variables must be initialized or declared with null explicitly, so val name: String? = null is correct.

3. Final Answer:

   val name: String? = null -> Option A

4. Quick Check:

   Nullable type with ? and initialized to null [OK]

Hint: Use ? and initialize nullable variables [OK]

Common Mistakes:

• Declaring nullable without ?
• Not initializing nullable variables
• Assigning null to non-nullable types

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

fun main() {
    val text: String? = null
    println(text?.length ?: "No text")
}

medium

A. 0

B. Compilation error

C. null

D. No text

Solution

1. Step 1: Understand safe call operator usage

   The text?.length safely accesses length only if text is not null.

2. Step 2: Evaluate the Elvis operator

   Since text is null, text?.length is null, so the Elvis operator ?: returns "No text".

3. Final Answer:

   No text -> Option D

4. Quick Check:

   Safe call + Elvis operator returns fallback [OK]

Hint: Safe call with Elvis returns fallback if null [OK]

Common Mistakes:

• Expecting 0 instead of fallback string
• Confusing safe call with normal call
• Thinking code causes error due to null

4. Identify the error in this Kotlin code snippet:

fun main() {
    val number: Int = null
    println(number)
}

medium

A. Variable 'number' is not initialized.

B. Cannot assign null to a non-nullable type Int.

C. Missing semicolon after println statement.

D. Variable 'number' should be declared as var.

Solution

1. Step 1: Check variable type and assignment

   The variable 'number' is declared as non-nullable Int but assigned null, which is not allowed.

2. Step 2: Understand Kotlin null safety rules

   Non-nullable types cannot hold null, so this causes a compilation error.

3. Final Answer:

   Cannot assign null to a non-nullable type Int. -> Option B

4. Quick Check:

   Non-nullable types reject null assignments [OK]

Hint: Non-nullable types cannot be assigned null [OK]

Common Mistakes:

• Thinking semicolon is required
• Assuming var fixes null assignment
• Ignoring Kotlin's null safety

5. You have a nullable variable val input: String?. You want to print its length only if it is not null, otherwise print "Empty input". Which Kotlin code correctly does this?

hard

A. println(input.length ?: "Empty input")

B. println(input!!.length)

C. println(if (input != null) input.length else "Empty input")

D. println(input?.length)

Solution

1. Step 1: Understand nullable variable usage

   Since input can be null, we must check before accessing length.

2. Step 2: Evaluate each option

   println(input.length ?: "Empty input") is invalid because length is Int, not nullable, so Elvis operator on it is wrong.
   println(if (input != null) input.length else "Empty input") safely checks null and prints length or fallback.
   println(input!!.length) uses !! which throws exception if null.
   println(input?.length) prints null if input is null, not the string "Empty input".

3. Final Answer:

   println(if (input != null) input.length else "Empty input") -> Option C

4. Quick Check:

   Explicit null check with if-else for safe access [OK]

Hint: Use if-else to check null before accessing properties [OK]

Common Mistakes:

• Using !! and risking exceptions
• Misusing Elvis operator on non-nullable Int
• Printing null instead of fallback string