Bottom Navigation Bar in Android

We all have come across apps that have a Bottom Navigation Bar. Some popular examples include Instagram, WhatsApp, etc. In this article, let’s learn how to implement such a functional Bottom Navigation Bar in the Android app. Below is the preview of a sample Bottom Navigation Bar:
Why do we need a Bottom Navigation Bar?
It allows the user to switch to different activities/fragments easily.
It makes the user aware of the different screens available in the app.
The user is able to check which screen are they on at the moment.
The following is an anatomy diagram for the Bottom Navigation Bar:
Steps for Creating Bottom Navigation Bar
Step 1: Create a new Android Studio project
To create a new project in Android Studio please refer to How to Create/Start a New Project in Android Studio. 
Step 2: Adding the dependency to the build.gradle(:app) file
We will be using Android’s Material Design Library so we need to import it in the build.gradle(:app) file. Here’s the dependency we need to add:
implementation ‘com.google.android.material:material:1.2.0’
Step 3: Working with activity_main.xml file
For this example, create a basic app with a FrameLayout and a Bottom Navigation Bar. The  FrameLayout will contain Fragments which will change as the user click on the items in the Bottom Navigation Bar. This is how the activity_main.xml looks like:
XML
<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".MainActivity">
 
    <FrameLayout
        android:id="@+id/flFragment"
        android:layout_width="match_parent"
        android:layout_height="0dp"
        app:layout_constraintBottom_toTopOf="@+id/bottomNavigationView"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintHorizontal_bias="0.5"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent" />
 
    <com.google.android.material.bottomnavigation.BottomNavigationView
        android:id="@+id/bottomNavigationView"
        android:layout_width="match_parent"
        android:layout_height="75dp"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintHorizontal_bias="0.5"
        app:layout_constraintStart_toStartOf="parent"
        app:menu="@menu/bottom_nav_menu"/>
     
</androidx.constraintlayout.widget.ConstraintLayout>
Step 4: Creating a menu for the Bottom Navigation Bar
The Navigation Bar needs to have some items which will create using Menu. To create a Menu, first, create a Menu Directory by clicking on the app -> res(right-click) -> New -> Android Resource Directory and select Menu in the Resource Type. 
To create a Menu Resource File , click on the app -> res -> menu(right-click) -> New -> Menu Resource File and name it bottom_nav_menu. 
Now the user can create as many items as he wants in the bottom_nav_menu.xml file. The user also needs to create an icon for each of these items. To create an icon, click on the app -> res -> drawable(right-click) -> New -> Image Asset.
In the window that opens, the user can name the icon whatever he wants but it should not comprise any uppercase letter. The user can select the icon he wants by searching it and when the user is done, click Next-> Finish. 
Now add these items in the bottom_nav_menu.xml. This is how the bottom_nav_menu.xml file looks like after adding the items:
XML
<?xml version="1.0" encoding="utf-8"?>
<menu xmlns:android="http://schemas.android.com/apk/res/android">
    <item
        android:id="@+id/person"
        android:title="Person"
        android:icon="@drawable/ic_person_foreground"/>
    <item
        android:id="@+id/home"
        android:title="Home"
        android:icon="@drawable/ic_home_foreground"/>
    <item
        android:id="@+id/settings"
        android:title="Settings"
        android:icon="@drawable/ic_settings_foreground"/>
</menu>
Step 5: Changing the Action Bar style
Since we are using Google’s Material Design Library, we need to change the action bar’s style to use the same library otherwise the Bottom Navigation Bar will be black and its items will be invisible. To change it, navigate to styles.xml by clicking on the app -> res -> values -> styles.xml and change the style opening tag as: 
 <style name=”AppTheme” parent=”Theme.MaterialComponents.Light.DarkActionBar”>
This is how the styles.xml file looks like: 
XML
<resources>
    <!-- Base application theme. -->
    <style name="AppTheme" parent="Theme.MaterialComponents.Light.DarkActionBar">
        <!-- Customize your theme here. -->
        <item name="colorPrimary">@color/colorPrimary</item>
        <item name="colorPrimaryDark">@color/colorPrimaryDark</item>
        <item name="colorAccent">@color/colorAccent</item>
    </style>
 
</resources>
Step 6: Creating Fragments to display 
Now that we have our Bottom Navigation Bar, we would want it to be functional by taking us to a different fragment/activity when an item is clicked. In this example, create a fragment for each item and navigate to them whenever a corresponding item is clicked. Since we created three items in the Bottom Navigation Bar, we will be creating three Fragments. To create a Fragment, click on the app(right-click) -> New -> Fragment -> Fragment (Blank). Name the fragment as FirstFragment and the corresponding XML file as fragment_first. To keep things simple, all three of the fragments will just contain a TextView. However, we can tweak this as we want it to be in the app. This is how the fragment_first.xml looks like after adding a TextView:
XML
<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".MainActivity">
 
    <TextView
        android:id="@+id/firstFragment"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Geeks for Geeks"
        android:textColor="#43a047"
        android:textSize="40sp"
        android:textStyle="italic|bold"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintHorizontal_bias="0.5"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent" />
 
</androidx.constraintlayout.widget.ConstraintLayout>
Next, code the FirstFragment to display the fragment_first.xml. For this, delete all the previously written code in FirstFragment and replace it with the below code. The below code just takes the layout we created for our fragment and inflates it. 
Note: If we want our fragment to have any logic or perform any task, we will add that code in our FirstFragment.
Kotlin
import androidx.fragment.app.Fragment
 
class FirstFragment:Fragment(R.layout.fragment_first) {
}
Similarly, create two more fragments for the remaining two items. Below are the fragment_second.xml, SecondFragment, fragment_third.xml, and ThirdFragment files respectively. 
XML
<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".MainActivity">
 
    <TextView
        android:id="@+id/secondFragment"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Data Structures"
        android:textColor="#43a047"
        android:textSize="40sp"
        android:textStyle="italic|bold"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintHorizontal_bias="0.5"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent" />
 
</androidx.constraintlayout.widget.ConstraintLayout>
Step 7: Working with the MainActivity file 
Now we have everything that we need and lastly, we just need to code the MainActivity to connect everything to the application. Here, first, create a function called setCurrentFragment() that takes a Fragment as an argument and sets it in our FrameLayout of activity_main.xml file. Add a click listener to the items of the Bottom Navigation Bar so that we display the corresponding Fragment when an item is clicked. After adding all these codes, the MainActivity looks like this:
Kotlin
import androidx.appcompat.app.AppCompatActivity
import android.os.Bundle
import androidx.fragment.app.Fragment
import kotlinx.android.synthetic.main.activity_main.*
 
class MainActivity : AppCompatActivity() {
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)
 
        val firstFragment=FirstFragment()
        val secondFragment=SecondFragment()
        val thirdFragment=ThirdFragment()
 
        setCurrentFragment(firstFragment)
 
        bottomNavigationView.setOnNavigationItemSelectedListener {
            when(it.itemId){
                R.id.home->setCurrentFragment(firstFragment)
                R.id.person->setCurrentFragment(secondFragment)
                R.id.settings->setCurrentFragment(thirdFragment)
 
            }
            true
        }
 
    }
 
    private fun setCurrentFragment(fragment:Fragment)=
        supportFragmentManager.beginTransaction().apply {
            replace(R.id.flFragment,fragment)
            commit()
        }
     
}
Output:
Video Player
00:00
00:21