Bottom navigation bar in Android Kotlin - Mini App: Build & Ship

Build: MainScreen

A screen with a bottom navigation bar to switch between Home, Search, and Profile sections.

Target UI

┌─────────────────────────────┐
│          MainScreen         │
│                             │
│        [Content Area]        │
│                             │
│-----------------------------│
│ Home | Search | Profile     │
└─────────────────────────────┘

Add a BottomNavigationView with three items: Home, Search, Profile

Switch displayed text in the content area based on selected item

Highlight the selected item in the bottom navigation bar

Starter Code

Android Kotlin

package com.example.bottomnav

import android.os.Bundle
import androidx.appcompat.app.AppCompatActivity
import com.google.android.material.bottomnavigation.BottomNavigationView
import android.widget.TextView

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

        val bottomNav = findViewById<BottomNavigationView>(R.id.bottom_navigation)
        val contentText = findViewById<TextView>(R.id.content_text)

        // TODO: Add bottom navigation item selection handling here
    }
}

Task 1

Task 2

Task 3

Solution

Android Kotlin

package com.example.bottomnav

import android.os.Bundle
import androidx.appcompat.app.AppCompatActivity
import com.google.android.material.bottomnavigation.BottomNavigationView
import android.widget.TextView

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

        val bottomNav = findViewById<BottomNavigationView>(R.id.bottom_navigation)
        val contentText = findViewById<TextView>(R.id.content_text)

        bottomNav.setOnItemSelectedListener { item ->
            when (item.itemId) {
                R.id.nav_home -> {
                    contentText.text = "Home Screen"
                    true
                }
                R.id.nav_search -> {
                    contentText.text = "Search Screen"
                    true
                }
                R.id.nav_profile -> {
                    contentText.text = "Profile Screen"
                    true
                }
                else -> false
            }
        }

        // Set default selection
        bottomNav.selectedItemId = R.id.nav_home
    }
}

We added a listener to the BottomNavigationView to detect when the user taps an item. Inside the listener, we check which item was selected using when. We update the TextView to show the corresponding screen name. Returning true means we handled the tap. We also set the default selected item to Home so the screen shows "Home Screen" initially.

Final Result

Completed Screen

┌─────────────────────────────┐
│          MainScreen         │
│                             │
│        Home Screen           │
│                             │
│-----------------------------│
│ Home* | Search | Profile    │
└─────────────────────────────┘

Tapping 'Search' changes content text to 'Search Screen' and highlights Search icon

Tapping 'Profile' changes content text to 'Profile Screen' and highlights Profile icon

Tapping 'Home' returns content text to 'Home Screen' and highlights Home icon

Stretch Goal

Add icons to each bottom navigation item for better visual clarity

💡 Hint

Use vector drawable icons and set them in the menu resource file for BottomNavigationView

Practice

1. What is the main purpose of a BottomNavigationView in an Android app?

easy

A. To let users switch between main sections of the app easily

B. To display a list of notifications

C. To show a full-screen image gallery

D. To handle background tasks

Solution

1. Step 1: Understand BottomNavigationView role

   The BottomNavigationView is designed to provide quick navigation between top-level views in an app.

2. Step 2: Compare options with its purpose

   Options B, C, and D describe unrelated features like notifications, image galleries, or background tasks.

3. Final Answer:

   To let users switch between main sections of the app easily -> Option A

4. Quick Check:

   BottomNavigationView = Easy app section switching [OK]

Hint: BottomNavigationView is for main app section navigation [OK]

Common Mistakes:

• Confusing BottomNavigationView with notification or image display
• Thinking it handles background processes
• Assuming it is a full-screen component

2. Which of the following is the correct way to set a listener for item selection on a BottomNavigationView in Kotlin?

easy

A. bottomNav.setOnClickListener { item -> true }

B. bottomNav.setOnItemSelectedListener { item -> true }

C. bottomNav.setOnItemClickListener { item -> true }

D. bottomNav.setOnItemSelected { item -> true }

Solution

1. Step 1: Recall BottomNavigationView listener method

   The correct method to listen for item selection is setOnItemSelectedListener.

2. Step 2: Check method names in options

   Options A, C, and D use incorrect method names not defined for BottomNavigationView.

3. Final Answer:

   bottomNav.setOnItemSelectedListener { item -> true } -> Option B

4. Quick Check:

   Listener method = setOnItemSelectedListener [OK]

Hint: Use setOnItemSelectedListener for BottomNavigationView [OK]

Common Mistakes:

• Using setOnClickListener instead of setOnItemSelectedListener
• Confusing with setOnItemClickListener (not valid here)
• Misspelling the listener method name

3. Given this Kotlin code snippet for a BottomNavigationView listener:

bottomNav.setOnItemSelectedListener { item ->
  when(item.itemId) {
    R.id.home -> { showHome(); true }
    R.id.search -> { showSearch(); true }
    else -> false
  }
}

What happens when the user taps the "search" item?

medium

A. The app crashes due to missing case

B. The showHome() function is called instead

C. Nothing happens because the listener returns false

D. The showSearch() function is called and the tap is handled

Solution

1. Step 1: Analyze the when block for itemId

   When the itemId is R.id.search, the code calls showSearch() and returns true.

2. Step 2: Understand return value meaning

   Returning true means the tap event is handled properly, so showSearch() runs.

3. Final Answer:

   The showSearch() function is called and the tap is handled -> Option D

4. Quick Check:

   ItemId search triggers showSearch() [OK]

Hint: Return true after handling item to confirm tap handled [OK]

Common Mistakes:

• Assuming else case runs for known items
• Thinking returning false still triggers action
• Confusing itemId values

4. What is wrong with this Kotlin code for BottomNavigationView listener?

bottomNav.setOnItemSelectedListener { item ->
  when(item.itemId) {
    R.id.home -> showHome()
    R.id.profile -> showProfile()
  }
}

medium

A. The listener should be set with setOnClickListener instead

B. Using when instead of if-else is incorrect here

C. Missing return value for the lambda, causing a type mismatch error

D. The itemId values are invalid identifiers

Solution

1. Step 1: Check lambda return type requirement

   The listener expects a Boolean return indicating if the tap was handled.

2. Step 2: Identify missing return statements

   The when cases call functions but do not return true or false, causing a type mismatch.

3. Final Answer:

   Missing return value for the lambda, causing a type mismatch error -> Option C

4. Quick Check:

   Listener lambda must return Boolean [OK]

Hint: Always return true or false in setOnItemSelectedListener lambda [OK]

Common Mistakes:

• Forgetting to return true after handling item
• Using setOnClickListener instead of setOnItemSelectedListener
• Ignoring compiler errors about return type

5. You want to create a BottomNavigationView with 4 items: Home, Search, Profile, and Settings. Which practice should you follow to ensure best usability?

hard

A. Use exactly 4 items and provide clear icons and labels for each

B. Add 6 items to show more options at once

C. Use only icons without labels to save space

D. Place the BottomNavigationView at the top of the screen

Solution

1. Step 1: Recall BottomNavigationView usability guidelines

   Best practice is to have between 3 and 5 items with clear icons and labels.

2. Step 2: Evaluate options against guidelines

   Add 6 items to show more options at once exceeds recommended items; C removes labels which hurts clarity; D places it incorrectly at top.

3. Final Answer:

   Use exactly 4 items and provide clear icons and labels for each -> Option A

4. Quick Check:

   3-5 items with icons and labels = best practice [OK]

Hint: Keep 3-5 items with icons and labels for BottomNavigationView [OK]

Common Mistakes:

• Adding too many items causing clutter
• Removing labels making navigation unclear
• Placing BottomNavigationView at top instead of bottom