Bottom navigation bar in Android Kotlin

Introduction

A bottom navigation bar helps users switch between main sections of an app quickly. It stays at the bottom of the screen for easy access.

When your app has 3 to 5 main sections or pages.

When you want users to switch between sections without going back.

When you want a simple and clear way to navigate on small screens.

When you want to keep navigation visible but not take much space.

When you want to follow common mobile design patterns for easy use.

Syntax

Android Kotlin

val bottomNavigationView = findViewById<BottomNavigationView>(R.id.bottom_navigation)
bottomNavigationView.setOnItemSelectedListener { item ->
    when (item.itemId) {
        R.id.home -> {
            // Handle home click
            true
        }
        R.id.search -> {
            // Handle search click
            true
        }
        else -> false
    }
}

Use BottomNavigationView from the Material Components library.

Set an OnItemSelectedListener to respond when users tap items.

Examples

Basic listener to switch between two sections: home and profile.

Android Kotlin

bottomNavigationView.setOnItemSelectedListener { item ->
    when (item.itemId) {
        R.id.home -> {
            // Show home screen
            true
        }
        R.id.profile -> {
            // Show profile screen
            true
        }
        else -> false
    }
}

XML layout for the bottom navigation bar linking to a menu resource.

Android Kotlin

<com.google.android.material.bottomnavigation.BottomNavigationView
    android:id="@+id/bottom_navigation"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:layout_alignParentBottom="true"
    app:menu="@menu/bottom_nav_menu" />

Sample App

This app shows a bottom navigation bar with three items. When you tap an item, a small message appears at the bottom.

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.Toast

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

        val bottomNavigationView = findViewById<BottomNavigationView>(R.id.bottom_navigation)

        bottomNavigationView.setOnItemSelectedListener { item ->
            when (item.itemId) {
                R.id.home -> {
                    Toast.makeText(this, "Home selected", Toast.LENGTH_SHORT).show()
                    true
                }
                R.id.dashboard -> {
                    Toast.makeText(this, "Dashboard selected", Toast.LENGTH_SHORT).show()
                    true
                }
                R.id.notifications -> {
                    Toast.makeText(this, "Notifications selected", Toast.LENGTH_SHORT).show()
                    true
                }
                else -> false
            }
        }
    }
}

Important Notes

Make sure to add the Material Components library in your build.gradle file.

Define your menu items in a separate XML file under res/menu for easy management.

Use icons and labels that clearly describe each section for better user experience.

Summary

Bottom navigation bars help users switch between main app sections easily.

Use BottomNavigationView with a menu resource and set a listener for item taps.

Keep the number of items between 3 and 5 for best usability.

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