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.