Variables (val, var) and null safety in Android Kotlin - Mini App: Build & Ship

Build: Variable Demo Screen

This screen shows how to declare variables using val and var, and how to handle null safety in Kotlin. It displays variable values and updates them on button clicks.

Target UI

-------------------------
| Variable Demo Screen   |
|-----------------------|
| val name: John        |
| var age: 25           |
| nullable city: null   |
|                       |
| [Change Age] [Set City]|
-------------------------

Display a val variable 'name' initialized to "John"

Display a var variable 'age' initialized to 25

Display a nullable String variable 'city' initialized to null

Add a button 'Change Age' that increases age by 1 when clicked

Add a button 'Set City' that sets city to "New York" when clicked

Show updated values on the screen after button clicks

Use Kotlin null safety features to safely display city or show "Unknown" if null

Starter Code

Android Kotlin

import android.os.Bundle
import android.widget.Button
import android.widget.TextView
import androidx.appcompat.app.AppCompatActivity

class VariableDemoActivity : AppCompatActivity() {
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_variable_demo)

        // TODO: Declare variables here

        // TODO: Initialize UI elements

        // TODO: Set initial text values

        // TODO: Set button click listeners
    }
}

Task 1

Task 2

Task 3

Task 4

Task 5

Task 6

Task 7

Solution

Android Kotlin

import android.os.Bundle
import android.widget.Button
import android.widget.TextView
import androidx.appcompat.app.AppCompatActivity

class VariableDemoActivity : AppCompatActivity() {
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_variable_demo)

        val name: String = "John"
        var age: Int = 25
        var city: String? = null

        val nameTextView = findViewById<TextView>(R.id.nameTextView)
        val ageTextView = findViewById<TextView>(R.id.ageTextView)
        val cityTextView = findViewById<TextView>(R.id.cityTextView)
        val changeAgeButton = findViewById<Button>(R.id.changeAgeButton)
        val setCityButton = findViewById<Button>(R.id.setCityButton)

        nameTextView.text = "val name: $name"
        ageTextView.text = "var age: $age"
        cityTextView.text = "nullable city: ${city ?: "Unknown"}"

        changeAgeButton.setOnClickListener {
            age += 1
            ageTextView.text = "var age: $age"
        }

        setCityButton.setOnClickListener {
            city = "New York"
            cityTextView.text = "nullable city: ${city ?: "Unknown"}"
        }
    }
}

We declare name as val because it does not change. age is var because it changes when the button is clicked. city is a nullable String? initialized to null.

We use the safe call with the Elvis operator ?: to show "Unknown" if city is null.

Buttons update the variables and the UI text accordingly, demonstrating variable mutability and null safety in Kotlin.

Final Result

Completed Screen

-------------------------
| Variable Demo Screen   |
|-----------------------|
| val name: John        |
| var age: 26           |
| nullable city: New York|
|                       |
| [Change Age] [Set City]|
-------------------------

Tapping 'Change Age' increases the age number by 1 and updates the display.

Tapping 'Set City' sets the city to 'New York' and updates the display from 'Unknown' to 'New York'.

Stretch Goal

Add a Reset button that sets age back to 25 and city back to null, updating the UI accordingly.

💡 Hint

Declare a button in the layout and set its click listener to reset variables and update TextViews.

Practice

1. Which keyword should you use in Kotlin for a variable whose value will never change after assignment?

easy

A. let

B. var

C. val

D. const

Solution

1. Step 1: Understand Kotlin variable keywords

   val is used for read-only variables, meaning their value cannot change once assigned.

2. Step 2: Compare with other keywords

   var allows changing the value; const is not a Kotlin keyword for variables; let is from other languages like Swift or JavaScript.

3. Final Answer:

   val -> Option C

4. Quick Check:

   Immutable variable = val [OK]

Hint: Use val for fixed values, var for changeable ones [OK]

Common Mistakes:

• Confusing val and var
• Using const which is not Kotlin variable keyword
• Thinking let is Kotlin keyword

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

easy

A. var name: String? = null

B. var name: String?

C. var name: String = null

D. var name: String = "null"

Solution

1. Step 1: Understand nullable type syntax

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

2. Step 2: Check initialization

   Nullable variables should be initialized with null if no value is assigned yet.

3. Final Answer:

   var name: String? = null -> Option A

4. Quick Check:

   Nullable variable syntax = var name: String? = null [OK]

Hint: Use ? after type and assign null to allow null values [OK]

Common Mistakes:

• Assigning null to non-nullable type
• Forgetting ? for nullable types
• Using string "null" instead of null

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

var count: Int? = 5
count = null
println(count)

medium

A. null

B. 0

C. 5

D. Compilation error

Solution

1. Step 1: Analyze variable declaration

   count is declared as nullable Int (Int?) and initially set to 5.

2. Step 2: Assign null and print

   Then count is assigned null, which is allowed for nullable types, so printing it outputs null.

3. Final Answer:

   null -> Option A

4. Quick Check:

   Nullable variable assigned null prints null [OK]

Hint: Nullable variables can hold null and print it directly [OK]

Common Mistakes:

• Expecting 5 after reassignment
• Thinking null prints 0
• Assuming compilation error for null assignment

4. Identify the error in this Kotlin code snippet:

val message: String = null
println(message)

medium

A. No error, code runs fine

B. val cannot be used with String type

C. println cannot print val variables

D. Cannot assign null to non-nullable type String

Solution

1. Step 1: Check variable type and assignment

   message is declared as non-nullable String but assigned null, which is not allowed.

2. Step 2: Understand Kotlin null safety rules

   Non-nullable types cannot hold null; to allow null, type must be String?.

3. Final Answer:

   Cannot assign null to non-nullable type String -> Option D

4. Quick Check:

   Non-nullable String cannot be null [OK]

Hint: Non-nullable types cannot hold null values [OK]

Common Mistakes:

• Thinking val disallows null but var allows
• Assuming println can't print val
• Ignoring Kotlin's null safety rules

5. You want to create a variable that can change its value but sometimes hold no value at all. Which declaration is correct in Kotlin?

hard

A. val data: Int = null

B. var data: Int? = null

C. val data: Int? = 0

D. var data: Int = null

Solution

1. Step 1: Understand mutability and nullability

   The variable must be changeable, so var is needed, and it must allow null, so type must be nullable with ?.

2. Step 2: Check each option

   val data: Int = null uses val (immutable) and assigns null to non-nullable type (error). var data: Int? = null uses var and nullable type with null initial value (correct). val data: Int? = 0 uses val (immutable) but nullable type with 0 initial value (not changeable). var data: Int = null uses var but non-nullable type assigned null (error).

3. Final Answer:

   var data: Int? = null -> Option B

4. Quick Check:

   Mutable and nullable = var with ? [OK]

Hint: Use var for changeable and ? for nullable types [OK]

Common Mistakes:

• Using val when variable should change
• Assigning null to non-nullable type
• Confusing val and var usage