Accessing elements safely in Kotlin - Mini Project: Build & Apply

Accessing elements safely

📖 Scenario: You are working on a Kotlin program that manages a list of city names. Sometimes, the list might be empty or shorter than expected. You want to access elements safely without causing errors.

🎯 Goal: Build a Kotlin program that creates a list of cities, sets a safe index to access, retrieves the city safely using Kotlin's safe access methods, and prints the result.

📋 What You'll Learn

Create a list of cities with exact names

Create a variable for the index to access

Use safe access to get the city at the index or a default message

Print the accessed city or the default message

💡 Why This Matters

🌍 Real World

In real apps, lists might be empty or have fewer items than expected. Accessing elements safely prevents crashes and improves user experience.

💼 Career

Knowing how to safely access list elements is important for Kotlin developers working on Android apps or backend services to write robust and error-free code.

1

Create the list of cities

Create a list called cities with these exact city names in order: "Paris", "London", "New York", "Tokyo", "Sydney".

Kotlin

# Create the list called cities with the given city names
# Your code here

Hint

Use listOf to create the list with the exact city names.

2

Set the index to access

Create an integer variable called index and set it to 3.

Kotlin

val cities = listOf("Paris", "London", "New York", "Tokyo", "Sydney")
# Create the variable index and set it to 3
# Your code here

Hint

Use val index = 3 to set the index.

3

Access the city safely

Create a variable called selectedCity that safely gets the city at index from cities. If the index is out of range, set selectedCity to the string "Unknown city". Use Kotlin's safe access methods.

Kotlin

val cities = listOf("Paris", "London", "New York", "Tokyo", "Sydney")
val index = 3
# Safely get the city at index or "Unknown city" if out of range
# Your code here

Hint

Use getOrNull(index) to get the city safely and the Elvis operator ?: to provide a default.

4

Print the selected city

Write a println statement to print the value of selectedCity.

Kotlin

val cities = listOf("Paris", "London", "New York", "Tokyo", "Sydney")
val index = 3
val selectedCity = cities.getOrNull(index) ?: "Unknown city"
# Print the selectedCity
# Your code here

Hint

Use println(selectedCity) to display the city.

Practice

1.

What does the Kotlin expression map[key] ?: "default" do?

easy

A. Removes the key from the map if it exists.

B. Returns the value for key if it exists, otherwise returns "default".

C. Throws an error if key is not in the map.

D. Always returns "default" regardless of key.

Solution

1. Step 1: Understand map access with []

   Accessing map[key] returns the value if key exists, or null if it doesn't.

2. Step 2: Use the Elvis operator ?: for fallback

   The expression ?: "default" means if the left side is null, return "default" instead.

3. Final Answer:

   Returns the value for key if it exists, otherwise returns "default". -> Option B

4. Quick Check:

   map[key] ?: default = safe access with fallback [OK]

Hint: Use ?: to provide a default when key might be missing [OK]

Common Mistakes:

• Thinking it throws an error if key is missing
• Assuming it always returns default
• Confusing with removing keys

2.

Which of the following is the correct Kotlin syntax to safely get the element at index 2 from a list myList, returning -1 if the index is out of bounds?

easy

A. myList.getOrNull(2) ?: -1

B. myList[2] ?: -1

C. myList.get(2) ?: -1

D. myList.getOrElse(2, -1)

Solution

1. Step 1: Understand safe list access methods

   myList.getOrNull(2) returns the element at index 2 or null if out of bounds.

2. Step 2: Use Elvis operator for default

   ?: -1 returns -1 if getOrNull returns null.

3. Final Answer:

   myList.getOrNull(2) ?: -1 -> Option A

4. Quick Check:

   getOrNull + ?: = safe get with fallback [OK]

Hint: Use getOrNull(index) ?: default for safe list access [OK]

Common Mistakes:

• Using myList[2] which throws exception if index invalid
• Using get(2) which throws exception if index invalid
• Confusing getOrElse syntax

3.

What is the output of this Kotlin code?

val map = mapOf("a" to 1, "b" to 2)
println(map["c"] ?: 0)

medium

A. null

B. 2

C. Exception thrown

D. 0

Solution

1. Step 1: Check map access for key "c"

   Key "c" does not exist in the map, so map["c"] returns null.

2. Step 2: Apply Elvis operator

   ?: 0 returns 0 because the left side is null.

3. Final Answer:

   0 -> Option D

4. Quick Check:

   map[missingKey] ?: default = default [OK]

Hint: Missing keys return null, use ?: for default [OK]

Common Mistakes:

• Expecting null to print instead of default
• Thinking exception is thrown
• Confusing existing key values

4.

Identify the error in this Kotlin code snippet and choose the corrected version:

val list = listOf(10, 20, 30)
val value = list[5] ?: -1
println(value)

medium

A. val value = list.get(5) ?: -1

B. val value = list[5]

C. val value = list.getOrNull(5) ?: -1

D. val value = list.getOrElse(5, -1)

Solution

1. Step 1: Understand list access with []

   Using list[5] throws an IndexOutOfBoundsException because index 5 is out of range.

2. Step 2: Use safe access method

   list.getOrNull(5) returns null if index is invalid, so combined with ?: -1 it safely returns -1.

3. Final Answer:

   val value = list.getOrNull(5) ?: -1 -> Option C

4. Quick Check:

   Use getOrNull + ?: to avoid exceptions [OK]

Hint: Replace [] with getOrNull + ?: for safe index access [OK]

Common Mistakes:

• Using [] which throws exception if index invalid
• Using get(5) which throws exception
• Confusing getOrElse syntax

5.

You have a map val scores = mapOf("Alice" to 90, "Bob" to 85) and a list val bonuses = listOf(5, 10). You want to safely get Bob's score and the bonus at index 2, returning 0 if missing, then sum them. Which code correctly does this?

hard

A. val total = (scores["Bob"] ?: 0) + (bonuses.getOrNull(2) ?: 0)

B. val total = (scores.getOrNull("Bob") ?: 0) + (bonuses.getOrNull(2) ?: 0)

C. val total = (scores["Bob"] ?: 0) + (bonuses[2] ?: 0)

D. val total = (scores["Bob"] ?: 0) + bonuses.get(2)

Solution

1. Step 1: Access map value safely

   scores["Bob"] ?: 0 safely gets Bob's score or 0 if missing.

2. Step 2: Access list element safely

   bonuses.getOrNull(2) ?: 0 safely gets bonus at index 2 or 0 if out of bounds.

3. Step 3: Sum the two values

   Adding these two safe values gives the total without errors.

4. Final Answer:

   val total = (scores["Bob"] ?: 0) + (bonuses.getOrNull(2) ?: 0) -> Option A

5. Quick Check:

   Safe map + safe list access with ?: for default [OK]

Hint: Use ?: with map and getOrNull for list to avoid errors [OK]

Common Mistakes:

• Using bonuses[2] which throws exception if index invalid
• Using getOrNull on map which is invalid
• Not providing default fallback